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As mentioned above, one of the major bio-safety concerns regarding the field release of GE rice at the scale of commercial production is transgene flow from GE rice varieties to its wild and weedy relatives . The primary bio-safety concern is that transgenes introgressing into the weedy rice populations that grow intermixed with cultivated rice may lead to unwanted environmental or agronomic consequences . The extremely close genetic relationship of weedy rice to cultivated rice, their similar phenology, and their high cross-compatibility suggest that pollen-mediated gene flow from GE rice to weedy rice populations seems highly probable. This hypothesis is reinforced by the fact that the conspecific weed always occurs within or in near the edge of cultivated rice fields . Thus, many studies, descriptive and experimental, have been carried out to measure the extent of gene flow from cultivated rice to weedy rice, both by pollen and by seed. Results from many experimental studies demonstrate that crop-to-weed gene flow in rice occurs at a low per-generation rate . In general, the estimated frequency of pollen-mediated gene flow is low most likely due to the predominantly self-pollination feature and the short life of pollen grains of both cultivated and weedy rice. Low per-generation gene flow is expected to have significant evolutionary effects when it occurs year-after-year . Thus, weed drying rack it is not surprising that descriptive studies have revealed crop-to-weed gene flow is an important evolutionary force in shaping the genetic diversity and structure of weedy rice populations .

Pollen-mediated gene flow can occur in both directions, namely, crop-to-weed and weed-to-crop. But unless some of the harvested grain is replanted, the important maternal source of weedy hybrids will be the weeds. If GE rice is involved in the crop-weed gene flow and introgression, the resulted weedy hybrid progeny will contain one or more transgenes. Such recurrent gene flow and subsequent introgression will play roles in the evolutionary dynamics of the weedy populations. Commonly, antibiotic and herbicide tolerance genes are used as selectable markers in GE rice transformation. They also have been utilized as markers in various experiments to detect the frequencies of rice transgene flow . As shown in Table 1, molecular markers are also used to quantify of crop-to-weed and crop-to-wild gene flow . Gene flow results collected from these experimental studies are the first step for assessing the impact of cropto-weed and crop-to-wild transgene flow in rice. The earliest Oryza crop-to-weed gene flow field experiments are from Sanders et al. who used two herbicide-tolerant rice varieties in independent studies at the Louisiana State University Research Farm in the United States. In these experiments, weedy rice populations were interplanted with the cultivated rice varieties. The gene flow frequencies were estimated by screening seedlings from weedy rice parents with the appropriate herbicide; crop-weed hybrid seedlings would be expected to survive. Extremely few seedlings from weedy rice parents interplanted with the glufosinate-tolerant rice survived glufosinate screening. The few weedy rice plants surviving from the imidazolinone spray were found NOT to be products of natural hybridization. Thus, the researchers estimated that gene flow was extremely low .

Soon after, Estorninos et al. used microsatellite markers to determine the outcrossing rate between the imidazolinone-tolerant rice line “CL 2551” and awnless straw-hulled weedy rice at Stuttgart, Arkansas, USA; the outcrossing rate was estimated to be 0.0–0.05%. After reviewing more than ten published studies, Gealy et al. concluded that rice crop-to-weed gene flow was extremely low but highly variable. Later field experiments supported that conclusion. For example, gene flow frequencies from glufosinate-tolerant GE rice to nearby several weedy rice accessions ranged from 0.0 to 0.5% . Likewise, Shivraina et al. reported a comparable amount of crop-to-weed gene flow in a field experiment that allowed for measuring much greater spatial scale between source and sink plants; frequencies of 0.003– 0.008% crop-to-weed gene flow were recorded. Using a similar transgene marker, Jia et al. investigated the frequency of gene flow from the crop into two weedy rice populations: one in Leizhou of Guangdong province, the other in Yangzhong of Jiangsu province. The estimated gene flow ranged from 0.002% to 0.342%. Sun et al. assessed pollen-mediated gene flow from glufosinate-resistant transgenic hybrid rice to six weedy rice accessions, and found that the frequency of gene flow from GRrice to weedy rice accessions ranged from 0% to 0.47% in different designs of gene flow experimental. To date, the highest frequency of crop-to-weed gene flow was reported by Olguin et al. who studied the transgene flow from indica rice to 58 weedy rice accessions from Costa Rica; it was as high as 2.3%. Notably, Pu et al. also reported insect-mediated pollination in rice, where increased frequency of transgene flow was detected.

As a group, the above studies indicate that – although the gene flow frequency per generation is very low – it also varies considerably under different situations. Given that some gene flow is almost always present when the crop and the weed co-occur and that it will be recurrent if rice is planted in the same location every year, transgene flow and eventual introgression from a GE rice variety to weedy rice populations seems inevitable. It is probably impossible to stop the flow of GE rice transgenes into weedy rice populations without extraordinary efforts . But whether transgene introgression will lead to increased weed problems and/or environmental impacts also depends on any fitness or other evolutionarily significant phenotypic changes in weedy rice individuals that are associated with the introgressed transgene . If gene flow is expected to occur, then such data are next critical step in environmental risk assessment associated with transgene flow . Thus, the following questions need to be considered prior to commercialization of GE where weedy rice is present: What will be the agronomic and ecological consequences of introgressed transgenes in a weedy rice population. Will an introgressed transgene change the fitness and evolutionary dynamics of a weedy population?Measuring the fitness effect of a crop transgene in individuals with a history of crop-weedy/wild hybridization requires one or more field experiments that compare wild/weedy individuals in which the transgene is present and with those in which the transgene is absent. Such individuals can be created with artificial crosses of a transgenic crop line and its nontransgenic counterpart with their weedy relatives as illustrated in Fig. 2. Measurement of the comparative performance of the two types of hybrid progeny or their descendants in the field – such as the survival ratios, competitiveness, and fecundity under specific environmental conditions – reveals the fitness effect to the wild/weedy relative populations associated with the presence of the transgene . The fitness effects of a transgene are expected to be largely determined by the type of transgenes incorporated in wild/weedy population the type of anticipated selection pressure in the environment, rolling bench and the amount of admixture in the transgenic hybrid lineages . Therefore, three types of characteristics should be carefully used when estimating the potential short-term evolutionary dynamics and long-term ecological impacts of a transgene that has been incorporated in weedy/wild populations: possible positive or negative fitness changes associated with a particular transgene when any intended selective pressure is absent ; fitness changes under the intended selective pressure that is related to a particular transgenic trait in the intended specific environment; and fitness effects of a transgene in early and advanced generations of the transgenic hybrid lineage compared with their nontransgenic counterparts, including the pure wild/weedy ancestral types. This approach is in accord with the case-by-case principle for biosafety assessment of genetically engineered crops . We have conducted multiyear common-garden experiments to estimate the fitness effect of insect-resistance transgenes that transformed into cultivated rice and introgressed into weedy and wild rice populations to understand their potential impact. Regarding transgene flow to weedy rice, one of our major foci, we used a number of GE rice lines containing different insect-resistance transgenes to cross with weedy rice populations from different geographic locations. Bt and CpTI transgenic rice lines were developed to deter lepidopteran pests, such as rice stem borers and rice leaf-folders. Several generations of crop-weed hybrid progeny with transgene-present and transgeneabsent lineages were produced to study the long-term fitness effect of the insect-resistance transgenes. We compared an array of fitness-related traits for these hybrid lineages, as well as their crop and weedy progenitor under both natural-insect and low-insect pressure.With regard to herbicide-resistance transgenes, Oard et al. conducted a field experiment and evaluated the seed production, shattering, and dormancy in eight F2 populations produced from controlled crosses of two transgenic, glufosinate-resistant rice lines and four red rice biotypes.

The presence of the transgene was associated with significantly shorter plant height and different maturity in hybrids, compared to those in nontransgenic counterparts. In another study, Wang et al. carried out a multiple-year common garden study on the fitness effects of a transgene that confers tolerance to the herbicide glyphosate through overexpression of 5-enolpyruvoylshikimate-3- phosphate synthase . In a glyphosate-free environment, they found that the transgenic hybrid lineage had higher seed production, greater EPSPS protein levels, tryptophan concentrations, photosynthetic rates, and percent seed germination compared with nontransgenic controls. These results suggest that phenotypic changes associated with such an epsps transgene could result in faster establishment and increased competitive ability of the transgenebearing individuals compared to nontransgenic weedy rice and cultivated rice. Other field studies of weedy rice individuals with introgressed non-GE crop herbicide resistance have shown fitness advantages related with faster germination and taller plants . Gene exchange and the spread of herbicide-resistant alleles in weedy rice suggest that wide-scale adoption of transgenic herbicide-resistance rice as a means to control weedy rice will be limited unless biotechnological solutions could be used to significantly decrease the occurrence of gene flow or otherwise mitigate it . The next of generation GE rice in China is apt to involve multiple transgenes with different intended purposes traits to battle the complex of insect pests of rice and other yield constraints in rice . Such products of crop biotechnology will inevitably require even more complicated experimental approaches for environmental bio-safety assessment with regard to introgressed transgenes in weedy rice populations. More biotic and/or abiotic factors will have to be taken into account to simulate field conditions that may influence the fitness effect of the multiple transgenes employed simultaneously. The environmental bio-safety assessment of the consequences of crop transgene flow to weedy rice populations through artificially created crop-weed hybrid lineages and subsequent fitness testing has proven costly and time consuming , especially with regard to long-term evolutionary impacts. However, our multiple-year field experiment on insectresistant transgenic crop-weed hybrid progeny reveal that the experimental estimation of fitness effects should be sufficiently apparent based on data from hybrid lineage as early as two or three generations posthybridization . Also, it should be sufficient to make some preliminary conclusions on the impacts caused by the insectand herbicide-resistance transgene flow into weedy rice population case-by-case, based on results obtained from above studies. For the insect-resistance transgene, the fitness advantages brought by insect-resistance transgene might be limited due to the fact that weedy plants will be surrounded by insect-resistant plants in a GE rice field, and the extensive commercial cultivation of an insect-resistant GE crop will largely reduce target herbivores in a GE deployed area as previously described by Wu et al. for Bt cotton. As a result, insect-resistance transgenes flow into weedy rice will have a limited evolutionary impact. In contrast, herbicide-resistance transgenes should deserve more attention since the herbicide pressure in GE rice field will always favor the spread and fixation of any herbicide resistance transgene in weedy rice population, unless effective mitigation strategy to be developed to minimize the occurrence of gene flow or reduce the fitness of crop-weed hybrids. The control of weedy rice is challenging because of its unique characteristics, such as strong seed shattering and dormancy, abundant genetic diversity, and mimicry with cultivated rice varieties. Like other conspecific weeds, weedy rice can easily acquire genes/alleles from its cultivated progenitors through cropto-weed gene flow. Some crop alleles can also enhance the fitness of weedy rice, enabling it to adapt to and evolve rapidly in the cultivated rice agro-ecosystem. Although Crop-to-weed gene flow in rice occurs at a relatively low frequency , transgene introgression into weedy population is essentially inevitable because weedy rice and cultivated rice co-occur and year after-year in the same fields.

If humans select crops that grow densely in monospecific stands, are those plants only a few allele changes away from becoming invasives or weeds ? If such is the case, careful and thorough evaluation should accompany the development of new crops designed to answer pressing societal needs, including new bio-energy feedstocks created with the goal of providing a stable domestic energy supply .Escape and establishment of crop species outside of agricultural systems, known as crop ferality, can be a concern if the escaped crop contains novel traits and establishes self-perpetuating populations within agricultural landscapes. A feral population can be found by the dispersal of seeds from the agricultural fields to adjacent habitats such as the roadsides. Human mediated dispersal, such as seed spill from farm machinery and seed transport trucks or hitchhiking on a vehicle, is the main vector for crop seed dispersal out of agricultural fields. Animals can also disperse seeds, but to a lesser extent than humans both in terms of the distance and number of propagule that they can disperse. The persistence of feral populations, similar to any plant population, can be influenced by the spatial and temporal heterogeneity of the habitat. In a heterogeneous landscape, certain sites could be unfavorable for the establishment of feral populations and at these sites the population may only sustain through continuous immigration of propagule from the source sites. Arrival of large number of propagules would increase the survival chance of the population.

While propagule dispersal is a critical first step for the establishment of feral crop populations in natural areas, cannabis dryer certain habitat characteristics such as vegetation density and drainage potential can also influence the establishment of feral populations in these areas. Additionally, seed dormancy and the ability to establish a seedbank would allow the feral populations to persist in natural habitats and enable recovery from stochastic fluctuations of the environment. Sorghum is a major crop in terms of production, ranking fifth worldwide and in the US. As a drought tolerant crop, sorghum can be grown in areas where the extremes of high temperatures and low soil moistures are unsuitable for the production of other row crops such as corn. Similar to many other feral crops, sorghum may have the ability to establish self sustaining populations outside of cultivated fields. The occurrence of feral crops along roadsides might be attributed to seed dispersal and the peculiarity of this habitat such as increased water runoff and low plant community richness; these characteristics may favor the initial establishment of feral sorghum in roadside habitats. The ferality potential in sorghum will be of concern because of sympatric presence of weedy relatives that can outcross with cultivated sorghum. In Southern US, the weedy relatives of S. bicolor include shattercane and johnsongrass. Of these, johnsongrass is known to be the most widely distributed and frequently found relative of sorghum in South Texas . Johnsongrass is one of the most troublesome weeds in the world, capable of spreading by both underground rhizomes and seeds.

Johnsongrass can cause severe yield losses in sorghum and many other crops. Both sorghum and johnsongrass are interfertile and can be hybridized under controlled conditions. Gene flow from sorghum to johnsongrass has also been observed in natural conditions. For sorghum cultivars bred or engineered with adaptive or herbicide resistance traits, ferality can be a concern as it can facilitate the establishment of these traits in the broader environment, causing ecological and/or agronomic issues. The co-occurrence of feral sorghum and johnsongrass in the proximity would increase the chances of cross-pollination between the two species. Pollen-mediated gene flow from johnsongrass to sorghum may enhance ferality in sorghum through de-domestication and the provision of adaptive alleles. The majority of commercial sorghum cultivars grown in the US are hybrids, and cytoplasmic male sterility is used to produce hybrid seeds. Male sterility is a recessive trait and male fertile F1 hybrids are actually in a heterozygous condition for alleles conditioning fertility restoration. Consequently, segregation for male sterility would be expected in progeny from the hybrid. In fact, approximately 25% of the feral sorghum plants established through seed dispersal will be male sterile. In these circumstances, the co-occurrence of feral sorghum and johnsongrass presents an increased chance for outcrossing. Texas is the second largest sorghum producer in the US, closely following Kansas. In South Texas, the majority of sorghum production is concentrated in the Rio Grande Valley, Coastal Bend and Upper Gulf Coast regions. Sorghum grown in this region is frequently transported to Mexico along highways and railroads through the Rio Grande Valley.

Sorghum seed spill along the transportation routes could lead to the establishment of feral sorghum populations in the byways along these routes and in fact, sorghum is seen commonly along the major highways in South Texas. However, no systematic survey has been conducted in the region to document the occurrence of feral sorghum along roadsides and the extent of sympatry with johnsongrass. The objective of this study was to document the prevalence of feral sorghum and johnsongrass along roadside habitats in South Texas and underpin, using GIS and logistic regression models, its association with several anthropogenic and environmental factors.A field survey was conducted along the roadsides of South Texas, from the Rio Grande Valley to the Upper Gulf Coast where grain sorghum cultivation is prevalent. The climate of the survey region is humid subtropical with mild winters and warm summers. Sorghum is planted from mid-February in the Rio Grande Valley to early April in the Upper Gulf Coast regions, with harvest occurring in about four months after planting. Due to the long growing seasons in South Texas, sorghum seed that disperses after harvest will have a chance to germinate and attain reproductive maturity prior to a killing frost during late fall season. In the Rio Grande Valley, frost occurs very rarely and a second sorghum crop can be planted in early August. The survey was conducted during late October- early November 2014 to allow the feral sorghum along the roadsides to establish and mature. However, the survey also included feral sorghum plants that recruited during spring. The survey area was divided into three regions based on distinct environmental conditions: 1) Upper Gulf Coast, from west of Houston, TX to Victoria, TX , 2) Coastal Bend, from Victoria, TX to Kingsville, TX , and 3) Rio Grande Valley, from Kingsville, TX to Brownsville, TX . Survey sites within each region were chosen using a semi-stratified survey methodology, as described by Bagavathiannan and Norsworthy. One hundred survey sites were selected at random within each region and survey routes were optimized using the ITN Converter software on a Google1 map layer. The ITN files were loaded to a GPS device to facilitate navigation to the pre-determined survey sites. In each site, the presence/absence of feral sorghum and johnsongrass was recorded. If feral sorghum was present, observations were carried out on the feral population size and site characteristics within a 25 m strip along the roadside site . If feral sorghum was absent in a pre-determined survey site, the first population found along the route to the next pre-determined site was used for characterization. No specific permissions were required for the activities carried out in this project. Moreover, cannabis growing systems the authors confirm that the field studies did not involve endangered or protected species.At each site where feral sorghum was present, observations were also carried out on the road body-type , micro-topography of the site , vegetation cover of the habitat , and nearby land use type . The micro-topography category ‘road shoulder’ represents the area immediately adjacent to the road margin towards the deepest point of the ditch, ‘field shoulder’ represents the area from the deepest point of the ditch to the field edge. The ‘field edge’ represents the edge of the cultivated field.

The road and field shoulders typically have high vegetation cover and minimally disturbed, whereas the field edges are often tilled and have relatively less vegetation. The co-occurrence of feral sorghum and johnsongrass was defined when both species were present within 50 meters of each other. To understand whether there is a relationship between the presence of feral sorghum and distance to grain sorting facilities, locations of such facilities were recorded during the survey. Data pertaining to the nearby land use type and the presence/absence of feral sorghum and johnsongrass were used for developing a projection model for species distribution, as described below.For each sampled site, the nearest road type was identified using Texas road maps obtained from Texas Natural Resources Information System online database . The road type classifications were county roads, highways, local streets, federal roads, functional classification streets and third-party toll roads. However, the recorded feral sorghum populations were predominantly found in county roads, highways and FC streets and not the other road type categories. As shown in Fig 1, only a sub-region of South Texas, representing the latitudinal and longitudinal limits of anticipated feral sorghum distribution, was included in the analysis. To determine the nearest road type to a given sample site, we first converted the road vector lines to a raster format using the Qgis software . Prior to rasterization, the character strings were converted to numerical values; for example, a highway road type was assigned a numeric value of 1while a county road was given a value of 2, and so forth. For each individual road type, a proximity map was produced which gives the distance of each grid cell on the map from the given road type . The georeferenced sampled sites were then overlaid on these road proximity maps and the raster value of each individual proximity map was appended to these points sequentially. The nearest road type to a given sample site was determined as the road with the smallest proximity value. To determine the nearby land use for the sampled points, we used the very high resolution CropScape database. We drew a buffering circle with a radius of 90 m around each sampling point and then converted the resultant circle polygons to a raster format using the values of CropScape raster layers as the required data field for the conversion. Similar to road type data, nearby land use data are presented using numerical values; for example, a sorghum crop is identified by the value ‘4’ . Using the Zonal Statistical feature of Qgis, the mode was calculated for each circle: mode represents the land cover with the highest frequency for the circle and thus dominant crop adjacent to the sampled sites. The distance of the sampling site to the nearest grain handling facility was calculated using the Matrix Distance feature of Qgis.Using a binary logistic model, we modelled the probability of the presence of feral sorghum as a function of the road type , road body-type , micro-topography of the surveyed site , the nearby land use , presence/ absence of johnsongrass and distance to grain sorting facilities. The above model was fitted using the PROC LOGISTIC procedure of SAS . The ‘odds ratio’ feature of PROC LOGISTIC was used to calculate the odds ratio and test for significance of the differences between the levels of predictors. Prior to analysis, all data were reordered to reduce the number of categories and avoid the effect of ‘quasi-complete separation of data points’. For example, 40 categories were initially recorded for nearby land use but were then reclassified to 16 groups. At 360 sites , the density of the feral sorghum population was recorded on scale of 1 to 5 . The effects of the road type, road body-type, micro-topography of the site, the nearby land use, the vegetation cover and presence/absence of johnsongrass on the scores of feral population density was investigated using PROC GLM of SAS and means were separated using the least significant differences test.A total of 2,077 sites were visited for the presence of feral sorghum and johnsongrass in our survey. Feral sorghum was found in 17% of the sites, whereas johnsongrass was more abundant and found in 45% of the sites visited. To our knowledge, this is the first documented account of the occurrence of sorghum as feral populations outside of cultivated fields. Results from the logistic model showed a significant association between the presence of feral sorghum with the road type and its body-type, micro-topography of the site, nearby land use and the presence/absence of johnsongrass, but showed no relationship with distance to the nearest grain sorting facility .

The authors used the program STRUCTURE to infer the origin of the weedy rice accessions and their possible history of hybridization. STRUCTURE uses a Bayesian approach to examine the relationships of multilocus genotypes of individuals by differences in allele frequency and the nature of linkage disequilibrium. In the United States, the vast majority of cultivars are japonica types, but STRUCTURE analysis assigned almost all of the US weedy accessions to two groups unallied with japonica. Blackhulled weedy rice and a few other accessions were almost identical to domesticated O. sativa indica var. Aus; strawhulled weedy rice and a few other accessions were classified with exoferal ancestry involving hybridization of O. sativa indica and wild O. rufipogon. Clearly, most of the US weedy rice populations evolved from the cultivated species, but it is also clear that evolution did not occur in the United States. These data as well as other historical data suggest that US weedy rice has an Asian origin. Yet another pathway for the origin of weedy rice has been described for its populations in Bhutan . In that country japonica rice cultivars predominate in the highlands while indica cultivars predominate in the lowlands. Ishikawa et al. compared lowland cultivars, highland cultivars, and weedy populations with regards to nine isozyme loci, a chloroplast genome deletion, and four microsatellite loci. They found clear genetic differentiation between japonica and indica cultivars, cannabis drying and at the same time, they found that the weedy populations had genotypes that had both combinations of both japonica-specific alleles and indica-specific alleles.

They report that they did not detect any alleles specific to wild relatives. Thus, their conclusion is that the weedy populations are lineages descended from japonica x indica hybrids. When Gressel named the different evolutionary pathways to ferality, he did not consider intercultivar hybridization. Following his lead, we call this particular pathway for Bhutanese weedy ricde ‘exo-endoferality’ because it is first a case of endoferality because all ancestors are domesticates, but also ‘exo-’ because intertaxon hybridization is a critical evolutionary step. All three of the above comprehensive studies present strong evidence for the origin of the vast majority of the weedy rice populations to be from cultivated rice. Interestingly, the data collected reveals a polyphyletic origin for weedy rice. Polyphylesis is now well-known to play a role in the evolution of many invasive lineages but it is not clear whether it is the rule for domesticate-derived pests. For three of the four discovered pathways, involving direct ancestry from indica and japonica, de-domestication likely occurred via the evolution of shattering due to either mutation or an epistatic recombination event. The most parsimonious pathway for the remaining exoferal lineage detected by Londo and Schaal is for O. rufipogon to have provided the allele or alleles for shattering.In terms of area planted, cereal rye is one of the world’s top 10 grain crops. Volunteer rye has occasionally been a serious agricultural weed problem throughout North America for about 100 years. However, by the early 1960s self-sustaining, naturalized weedy rye populations were identified as increasingly problematic as weeds of cultivated lands and invasives of uncultivated lands in the US states of Washington, Oregon, Idaho, and California.

As a weed of cultivated rye, it was so bad that ‘farmers … abandoned efforts to grow cultivated rye for human consumption’ . Subsequently, weedy rye has spread elsewhere in the western United States and the Canadian province of British Columbia . Western North American weedy rye was originally thought to be a hybrid derivative of cultivated rye and the wild perennial mountain rye [S. strictum C. Presl.]. However, subsequent genetic analysis of several populations of North American weedy rye with 14 allozyme and three microsatellite loci failed to detect any ancestry from S. strictum or any other wild Secale. Overall, the weedy populations are more similar to each other than to any one cultivar. Nonetheless, the invasive populations share a single lineage that apparently evolved directly from one or more cultivars of cereal rye . Just as in the case of rice, cultivated rye is non-shattering and has little dormancy, while its derivative has evolved dispersal by shattering. De-domestication of the non-shattering trait to shattering likely occurred via mutation or perhaps an epistatic recombination event. Interestingly, in this case, both the crop and the feral populations have little seed dormancy. Other traits such as smaller seed, smaller leaves, thinner culms, and delayed flowering have rapidly evolved in this lineage . It is not clear whether all of these traits contribute to its evolution as a plant pest, especially its invasiveness outside of agroecosystems. However, evolution of a change in flowering time relative to an ancestor can be a powerful reproductive isolating mechanism. In this case, it might have evolved under selection to frustrate maladaptive gene flow from the crop to the weedy lineage .Cultivated radish is an important vegetable whose root is consumed worldwide. The wild jointed charlock is a closely related species, separated from the cultigen by a chromosomal translocation and a suite of morphological characters. When the two co-occur in most of the world, spontaneously hybridization occurs to a limited extent, resulting in no more than highly localized hybrid swarms . In contrast, for almost 100 years, hybridization between the two Raphanus taxa in California has been more extensive . In the last 50 years, hybrid-derived wild Raphanus has invaded coastal plains and disturbed inland valleys along the Paci- fic edge of North America from the US state of Oregon south through California to the Mexican state of Baja California . It has also become a troublesome weed for agronomic crops. Experimental work on what is now known as ‘California wild radish’ has confirmed it to be a lineage descended from hybrids of R. sativus and R. raphanistrum. Hegde et al. compared California wild radish populations with cultivars of R. sativus and populations of R. raphanistrum. They used 10 allozyme loci as well as common garden experiments to characterize the three types. The allozyme data revealed that California wild radish populations were in Hardy-Weinberg equilibrium; that is, there was no evidence that pure individuals of the parental taxa had persisted in significant frequencies.

STRUCTURE analysis of the allozyme dataset confirmed that conclusion. STRUCTURE assigned the cultivated radish to one group and the jointed charlock individuals to another group. The individuals from the California wild radish populations were assigned at various levels of hybrid ancestry involving the first two groups. Multivariate analysis of morphological characters measured in their common garden experiments revealed that the standard phenotype of California wild radish is significantly different from both of its progenitors. Interestingly, its bolting date, flowering date, and hypocotyl width are intermediate to its progenitors; its fruit diameter and fruit length are the same as the cultigen; and its fruit weight transgresses both parents! A subsequent common garden experiment showed that in several, drying cannabis contrasting California environments, the hybrid lineage produced both more fruits per plant and more seeds per plant than either progenitor , including specific source cultivars and R. raphanistrum populations as determined from cpDNA analysis . Is there something special about California that permitted this rapid adaptive evolution to proceed in light of the fact that Raphanus hybrids elsewhere have proven to be evolutionary deadends? Another common garden experiment has given a tantalizing result. Synthetic, F4 generation hybrid lineages and their R. raphanistrum progenitors were grown in the field in Michigan and California. The hybrid lineages’ fitness was slightly inferior to R. raphanistrum in Michigan but in California they exhibited 22% greater survival and 270% greater lifetime fecundity .Evolutionary studies on weedy and/or invasive plants that have domesticated ancestors have been useful for detailing the phylogenetic history of such plants. More examples might exist. While accumulating our examples for Table 1, we encountered some cases for which the current evidence is too weak at this time to convincingly support or refute a crop origin for an invasive lineage. These are enumerated in Table 2. Likewise, we encountered examples of domesticated taxa that have become plant pests, but it is not clear whether these have evolved to become pests or are simply ecological opportunists . Consider the case of strawberry guava . The free-living version of this domesticated plant is considered by some to be one of the world’s worst invasive species , but no studies have examined whether the invasive strawberry guava populations are substantially genetically different from their domesticated progenitor. The majority of our entries in Table 1 are examples of remarkably rapid evolution, at least six of our problematic lineages evolved in less than a century. The comparison of progenitors and their wild descendants grown in a common environment reveals differences that may account for the success of the latter.Nonetheless, research on such systems has barely exploited their utility for evolutionary study in comparison with certain other plant pests, such as the large body of integrated ecological, physiological and genetic study employed to understand evolution of invasiveness in North American reed canarygrass by Molofsky and colleagues and references therein. In particular, invasives and weeds descended from domesticated plants are ripe for approaches to tease out the evolutionary pathway to their new lifestyle. How do they differ from their progenitors with respect to their ecological relationships with biotic enemies, that is, herbivores and disease-causing organisms? Are there any differences in their chemical or physical defenses? Genetic and genomic approaches, often used in concert with ethnobotanical data, have been successful in illuminating the evolution of crops from wild species under domestication . These approaches may prove to be equally powerful in investigating evolution in the other direction, the evolution of sustainable feral populations from domesticated species. Let’s consider some of these approaches. Refined cytogenetic tools for studying chromosomal evolution under domestication have expanded to include not only traditional chromosome banding, but also techniques using fluorescent in situ hybridization  and genomic in situ hybridization . Despite the fact that many major crops are cytogenetically well-characterized, we are not aware of any studies that address whether and how chromosomal evolution has occurred under de-domestication. Even if a crop species hasn’t had its genome sequenced, it is likely to be well-mapped. Quantitative trait locus mapping has proven a powerful way to study the domestication-related genes by examining the co-segregation of a trait with markers to determining the number of loci, their chromosomal location, and their relative influence on the expression of that trait. For example, the first maize ‘domestication gene’, teosinte branched1 , was identified by QTL mapping . In the same way, crosses between plant pests and their crop progenitors can be made to examine the genetic basis of key ecological traits that correlate with invasive success . Evolutionary genomic approaches have proven particularly fruitful for identifying the genomic and genetic correlates of crop domestication, in particular, potential adaptive changes . For domesticated taxa that have had their genome sequenced, such as rice and sorghum, comparative evolutionary ecogenomic approaches with their descendants will be able to provide a sweeping view of what genomic changes have occurred in the evolution of invasives and/or weeds relative to their crop ancestor. As genome sequencing become both less expensive and easier to conduct , such approaches will become available for more species, but the descendants of domesticated taxa will still have the advantage of centuries of study. We end with a few intriguing questions based on the simple observation that crops and weeds often have a lot in common ecologically. First, with regard to crops and their weedy derivatives, we note that both grow in exactly the same location, but they are subjected to different selection regimes. How do weedy crop derivatives end up perceiving different selection pressures so that diverge in sympatry? Furthermore, how do they diverge given that they are likely to be swamped by gene flow from the initially more abundant crop? With regards to the latterquestion, it is clear that reproductive isolating barriers must evolve rapidly, perhaps explaining why our list of examples is short . And, at the same time, that would explain why phenological divergence has been noted for all of our examples descended from an outcrossing crop ancestor which would be subject to a rain of cross-compatible pollen, but not for all of those descended from a highly selfing crop ancestor for which relatively short distances should afford reproductive isolation. Second, both crops and weeds are often selected for a life in a disturbed habitat. Both characteristically grow densely in simple communities or even monocultures.

The term “pesticide” indicates a wide range of compounds such as insecticides, fungicides, herbicides, rodenticides, molluscicides, nematicides and plant growth regulators. In Tunisia, weeds and phytopathogenic fungi can cause high yield loss, attaining 80%. Fungi as plant pathogens cause a variety of plant diseases resulting in losses both in food crop production and after harvest. Weeds have been identified as one of the most aggressive agricultural problems, reducing the quality and yield of crop production.In fact, weeds are the main biotic stresses on crop production; they are in competition with crops for natural resources: water, sunlight, space and nutrients. The available literature suggests the substantial yield and economic losses due to weeds. In Tunisia’s cereal production, weeds continue to be an ongoing problem and are one of the limiting factors for growth. To fight against these pests, farmers use synthetic pesticides with negative, harmful effects on the environment and human health. Herbicides are increasingly found in groundwater and surface water due to their extensive use in agricultural systems. In addition, the intensive use of pesticides allows the constant emergence of resistance by all types of organisms. Therefore, biologically active compounds from plants can be used as a potential source of potential fungal, pest and weed control agents. EOs represent a source of compounds with anti-fungal, pest and weed control potential to be used as an alternative to fill the role of synthetic products. Consequently, commercial hydroponic systems for several years there has been a great deal of interest in plant-derived EOs, rich in active compounds which have very important biological properties, as a source of bio-pesticides.

This is the case for the species of the Eucalyptus genus, which is native to Australia, contains approximately 900 species and is a member of the Myrtaceae family. Since 1957, 117 different species of Eucalyptus have been introduced to Tunisia. Eucalyptus species are now widely distributed around the world, owing principally to their advantageous wood properties for the paper industries. Historically, only a few species belonging to this genus have been employed to extract EOs, primarily from the leaves, for application mostly in the cosmetic and pharmaceutical industries. Eucalyptus leaves are rich in EOs, which consist mainly of monoterpenes and sesquiterpenes as well as other secondary metabolites such as phenols, alkaloids, flavonoids and tannins. As well, Eucalyptus extracts have been reported to have herbicidal effects on the seedling growth and germination of many weeds. Hence, the present study aimed to elucidate the chemical composition of the EOs obtained from the leaves of eight Tunisian Eucalyptus species and to investigate their anti-fungal and, for the first time, phytotoxic effects. The anti-fungal activity of the EOs was assessed against four plant pathogenic Fusarium strains, and their herbicidal effects were tested by evaluating their inhibition of the seed germination and seedling growth of three common weeds and three cultivated species.The results showed that the EO yield in E. cladocalyx, grown in the arboretum of Zerniza in the region of Sejnene in northwest Tunisia, was comparable to those obtained from Algeria and Morocco, but much lower than those obtained in another Tunisian site. For E. ovata, the richness of the EO is the same as in those grown in Algeria. E. sargentii showed the highest yield in comparison to other species in our study, but this yield was significantly lower than that obtained from the same species in other regions in Tunisia.

The differences between these yields and those reported in the literature could be attributed to many factors, such as the age of the tree, the climatic conditions, the edaphic conditions and the method of extraction. In total, 41 components were determined, and the highest number were identified in E. ovata EO, while only 17 components were identified in E. angulosa EO. Eucalyptol was the main component in all EOs except in those from E. resinifera and E. diversicolor. These results confirm what has been described by authors who reported that the monoterpene eucalyptol was the dominant component for the main species of the genus Eucalyptus. Kouki et al. reported that eucalyptol was the main component in some Eucalyptus species growing in Tunisia with percentages between 44.9 and 78.1%. Our analysis showed that the α-pinene content takes the second position in some species such as E. ovata, E. sargenti and E. angulosa, which agrees with several previous works. These results were partially in agreement with the data described by an Algerian team showing E. ovata EO contents of 51.2 and 7.8% for 1,8-cineole and α-pinene, respectively, whereas for E. saligna EO, the major component was α-phellandrene followed by p-cyrnene. In Morocco, two studies reported the composition of E. cladocalyx EO; the first identified 24 compounds representing about 81.1% of the EO, with α-pinene , p-cymene and 1,8-cineole as the most prominent components, followed by β-pinene , trans-pinocarveol and α-terpineol. In the second study, Fouad et al. identified 29 different components representing 79% of the total oil. The major components detected were spathulenol and 1,8-cineole , followed by p-cymene . In Tunisia, the work of Ben Hassine et al. described only six compounds in the EO of E. cladocalyx, whose major compounds were 1,8-cineole and α-thujene. Ameur et al. identified 23 compounds including globulol . In our study, the EO of E. cladocalyx was rich in eucalyptol as the major component, followed by β-pinene.

Regarding the EOs of E. diversicolor, the results described by Elaissi et al. partly agree with our conclusions concerning the presence of p-cymene. On the other hand, a Moroccan EO was characterized by the high content of 1,8-cineole and the absence of p-cymene. In vitro tests revealed that all EOs inhibited fungal growth in a dose-dependent way, which agrees with many reports in the literature on the concentration-dependent anti-fungal activity of essential oil. In this study, the EOs tested elicited anti-fungal activity for the four phytopathogenic fungal strains , and this mycelial growth inhibition was variable depending on the concentration and nature of the essential oil. These findings agree with those of previous studies that evaluated the anti-fungal activity against various phytopathogenic fungi of the EOs of some Eucalyptus species with different chemical profiles. Recently, we reported that some EOs inhibited the growth of eight phytopathogenic fungi. E. citriodora Hook. completely inhibited the growth of seven strains belonging to the genus Fusarium at a concentration of 4 µL/mL. Another study showed that EOs from E. oleosa F. Muell. ex Miq. inhibited fungal growth with MICs of 6 µL/mL for five strains of Fusarium. In our work, the EO of E. cladocalyx showed fungicidal activity against all four Fusarium strains tested at low dose, whereas the other EOs were fungicidal only against one or two of the four fungal strains. The EOs of E. ovata at the four concentrations studied did not exhibit fungicidal activity. Consequently, higher concentrations are needed to obtain the MFC. In vitro tests revealed that all EOs inhibited fungal growth in a dose-dependent way, which agrees with many reports in the literature on the concentration-dependent anti-fungal activity of essential oil In particular, the EO of E. cladocalyx showed a remarkable anti-fungal activity against phytopathogenic fungi that have a devastating effect on several species cultivated in Tunisia. This EO is characterized by the abundance of some compounds such as eucalyptol, pinene, trans-pinocarveol and α-terpineol, cannabis racking systems which may explain its anti-fungal power. Several previous works have shown the involvement of 1,8-cineole in the inhibition of phytopathogenic fungi. Based on the literature and according to Table 1, several compounds detected in the oils studied are known to have anti-fungal potential. According to Kim et al. , 16 pure compounds of EOs were tested for their anti-fungal potential; among these compounds were α-pinene, 1,8-cineole and p-cymene, which constitute the major compounds of Eucalyptus tested in our present study. These compounds showed significant anti-fungal potential. This could explain the results of the current study. Similarly, α-pinene, a major compound for the oils studied , is a monoterpene that has been demonstrated to inhibit respiration and ion transport and to act on cell integrity by increasing membrane permeability. In addition, 1,8-cineole, an oxygenated monoterpene and major compound in this study , has been described for its anti-fungal potential against phytopathogenic fungi. Similarly, according to Kim et al. , 1,8-cineole showed a significant inhibition of growth and the production of aflatoxin B1 and aflatoxin B2 of several fungi strains, and these anti-fungal properties were explained by a dramatic downregulation of 1, 8-cineole on the expression of afl E and afl L. This can explain the activities observed in this present study without neglecting the role of other compounds, thus without neglecting the interactions of synergism and antagonism. A research study reported that some components of Eucalyptus EOs such as α-terpineol, terpinolene, and 1,8-cineole are fungitoxic against phytopathogenic fungi. Regarding the anti-fungal mechanism, previous work has reported that the apolar terpenes can penetrate the lipid bilayer of the fungal membrane using their apolar properties. Hence, terpenes induce fungal membrane disruption by increasing the membrane’s permeability. Our report showed that Eucalyptus EOs exhibit herbicidal activity for all species tested, with more noticeable effects on weeds.

In agreement with our findings, previous works showed the herbicidal effects of Eucalyptus EOs against several weeds and crop plants. Thus, some components of Eucalyptus EOs are known for their phytotoxic effects and can be used as natural herbicides. Nevertheless, the phytotoxic activity of Eucalyptus EOs may affect some crops. It is therefore important to develop research to select the EOs with maximum herbicidal activity against weeds while minimizing negative impacts on crop growth. Seven of the eight EOs studied contain appreciable percentages of eucalyptol correlated with significant herbicidal activity. The high levels of eucalyptol, a monoterpene with phytotoxic properties, may partly explain the results obtained. The greatest inhibition was obtained using the EO of E. cladocalyx, which has appreciable levels of 1,8-cineole, α-pinene, β-pinene, trans-pinocarveol and α-terpineol. These findings suggest that 1,8-cineole combined with other terpenes may provide significant phytotoxic effects. According to the chemical composition of the oils of the eight Eucalyptus trees studied , we can notice the presence of several compounds known for their herbicidal activities, such as 1,8-cineole, α-pinene and p-cymene, detected as major compounds of the tested oils. These findings confirm a synergy between the various constituents of EOs for the observed phytotoxic effects. Although some studies have tried to explain the mechanisms of action of the EOs on germination and inhibition of the growth of seedlings, these modes of action remain unclear. Previous reports have suggested a number of effects and hypotheses; the majority of researchers working on this topic agree that EOs have phytotoxic effects that can cause anatomical and physiological changes in plant seedlings leading to accumulation of lipid globules in the cytoplasm, reduction of certain organelles such as mitochondria, inhibition of DNA synthesis or disruption of membranes surrounding mitochondria and nuclei. In this way, the development of natural pesticides and herbicides would help to reduce the negative impact of chemicals, such as the development of resistance in pathogens and parasites and resistant weeds. To this end, bio-pesticides and bioherbicides can be effective, selective, biodegradable and less harmful to the environment and human health. This study reports the potential anti-fungal and herbicidal effects of the essential oils from eight Eucalyptus species.Rice is the major calorie source for a large proportion of the world’s population and is one of the most commonly grown agricultural commodities in the world . California is the second largest rice-growing state in the USA, with around 200,000 ha production, most of which is concentrated in the Sacramento Valley . The majority of California’s rice production consists of short- and medium-grain japonica varieties and a few long-grain indica varieties, including cultivars developed for both the local climate and a continuously-flooded cropping system, where rice is pre-germinated and seeded by airplane onto fields with a 10-15 cm standing flood . The flooded conditions in which California rice is grown favor flood-adapted, competitive grass weeds such as watergrass species Beauv. spp., bearded sprangletop [Leptochloa fusca Kunth ssp. fascicularis N. Snow], and weedy rice f. spontanea Roshev.. The continuously flooded system also promotes sedges such as rice field bulrush [Schoenoplectus mucronatus Palla] and small flower umbrellas edge as well as aquatic broad leaf weeds such as ducksalad [Heteranthera limosa Willd.] and redstems spp..

The monopartite genomic DNA is similar in sequence and genome organization to the DNA-A component of bipartite begomoviruses, with one gene on the v-sense strand encoding the CP, and four genes on the c-sense strand encoding Rep, TrAP, Ren and the C4/AC4 protein. The DNA-B component of the bipartite begomoviruses has two genes, BV1 and BC1, which encodes a nuclear shuttle protein and movement protein, respectively . The sequence of the DNA-B component is typically more divergent than that of the DNA-A component , and also has a hypervariable region . This HVR extends from the 5’ end of the CR to the initiation codon of the BC1 open reading frame and is the most variable region in the genome of bipartite begomoviruses. Begomoviruses also show a phylogeographic distribution, with most bipartite species occurring in the New World and rarely associated with satellites DNAs, and most monopartite species occurring in the Old World and often in association with satellite DNAs that are either required for disease development or have no obvious effect inmodulating disease symptoms . However, there are exception to this, including the introduction of TYLCV into the NW in the early 1990s and the recent identification of indigenous NW monopartite begomoviruses infecting tomato in Peru, Ecuador, Venezuela and Northern Brazil . Begomoviruses have a remarkable flexibility in genome evolution and a long evolutionary history. It is generally believed that the ancestor of modern-day begomoviruses was monopartite and that the bipartite genome likely evolved before the breakup of the supercontinent Pangea because bipartite begomoviruses occur in the NW and OW .

In this scenario, growth rack these viruses have co-evolved with plants for millions of years, i.e., well before the emergence of flowering plants and the domestication of crops . The subsequent diversification and evolution of OW and NW begomoviruses involved genetic mechanisms such as mutation and recombination, which are known as the major contributors to the genetic divergence of viruses . In terms of these mechanisms, recombination has played, and continues to play, an important role in the emergence of begomoviruses. Extensive analysis of begomovirus/satellite sequences have shown that several species of monopartite and bipartite begomoviruses have evolved extensively through recombination . Furthermore, these studies also revealed that recombination hot spots exist in the begomovirus genome, especially the region spans the 5’ end of the AC1 gene, the entire overlapping AC4 gene and the left side of the CR . Begomovirus genomes also showed high level of within-host variation and substitution rates inferred for these viruses are equivalent to those of RNA viruses . More recently, it was shown that mutation is likely the main source of genetic variation for begomovirus genomes . The evolution of begomoviruses also involves pseudore combination . Pseudorecombinants are typically formed between isolates and strains of the same begomovirus species, but in some cases, they can also be formed with DNA components from different species .

Additionally, begomovirus genomes have the capacity to acquire and modify small circular ssDNA components. For example, acquisition of satellite DNAs has played a major role in evolution of OW monopartite begomoviruses, whereas acquisition and modification of the DNA-B components acquired by pseudore combination has played a role bipartite begomoviruses evolution . Finally, begomovirus also evolved locally through the world, resulting in the emergence of multiple begomovirus species that infect one particular host and cause similar symptoms . The emergence and evolution of new begomovirus species has been facilitated by the polyphagous behavior of the whitefly supervector, especially the B. tabaci species MEAM1. Moreover, human activities have led to the long-distance movement of numerous begomoviruses, blurring the geographic separation of OW and NW begomoviruses, e.g., introduction of squash leaf curl virus into the Middle East from the NW, introduction of tomato leaf curl New Delhi virus into the Western Mediterranean Basin from the subcontinent of Asia . However, none of these cases has been as important as the introduction of the invasive TYLCV into the NW during the early 1990s . From there, TYLCV quickly invaded the Southern US, Mexico and the rest of the world . In general, individual begomoviruses tend to have relative narrow host ranges . However, as a group, they can infect a wide range of food and fiber crops, ornamental plants and non-cultivated plants in the OW and NW .

Plants affected include crops such as tomato, cassava, common bean, cotton, cucurbits and pepper; ornamentals such as Abutilon spp. and Lonicera spp. ; and weeds such as Malachra spp., Sida spp., Macroptilium spp. and Malva spp. .In terms of production, tomato has become one of the most important vegetables crops in the world . Tomato is infected by more begomovirus species than any other crop , and this can be attributed to a number of factors. First, production of tomato has grown quickly over the world . Second, domesticated tomato is highly susceptible to begomovirus infection . Third, local evolution has resulted in the emergence of multiple begomoviruses that infect tomato and cause similar symptoms . Together this has accelerated the emergence of numerous tomato-infecting begomoviruses in the NW and OW . The NW begomovirus disease of tomato are mainly caused by bipartite species, although monopartite are becoming more important . Several tomato-infecting begomoviruses have been reported from North and Central America, including tomato mottle virus, pepper golden mosaic virus , pepper huasteco yellow vein virus, chino del tomate virus, tomato golden mottle virus, tomato leaf curl Sinaloa virus , tomato yellow mottle virus , tomato mosaic Havana virus , tomato severe leaf curl virus and the invasive TYLCV . Furthermore, these begomoviruses are often found in mixed infections , which allows for further virus evolution via recombination and pseudore combination . In South America, tomato-infecting begomoviruses emerged in Brazil and other countries after the introduction of the highly polyphagous B. tabaci species MEAM1 in the 90s . The first tomato begomovirus disease in Brazil was cause by the tomato golden mosaic virus , which was reported in the early 1960s . Since then, clone rack at least 14 tomato-infecting begomoviruses have been described and characterized . For example, tomato severe rugose virus is important in central regions, whereas tomato mottle leaf curl virus is more prevalent in north-eastern regions of Brazil . Additionally, since 2010, NW monopartite tomato-infecting monopartite begomoviruses continue to be discovered in BR, Ecuador, Peru and Venezuela . In the OW, tomato yellow leaf curl disease has emerged as one of the most important diseases of tomato . The disease is caused by a complex of species of monopartite and bipartite begomoviruses . Among these viruses, TYLCV and tomato yellow leaf curl Sardinia virus have cause significant crop losses in the Mediterranean Basin . Furthermore, these viruses have been shown to participate in genetic exchanges giving rise to resistance-breaking strains of TYLCV . The bipartite ToLCNDV, the causal agent of tomato leaf curl disease , is one of the most important tomato-infecting begomovirus in the Indian subcontinent . Moreover, in recent years, the virus has expanded its host range and spread into new geographical regions, e.g., North Africa and Southern Europe . Finally, several monopartite tomato-infecting begomoviruses have been described in Asia, including tomato yellow leaf curl China virus, tomato leaf curl China virus, tobacco curly shoot virus, tobacco leaf curl Yunnan virus, tomato leaf curl Guangxi virus, tomato leaf curl Malaysia virus, tobacco leaf curl Japan virus and tomato yellow leaf curl Thailand virus .In addition to infecting crop plants such as tomato, begomoviruses infect a diversity of non-cultivated plants, mostly weeds, in tropical and subtropical regions of the world . Numerous begomoviruses have been reported infecting weed species in the families Asteraceae, Capparaceae, Convolvulaceae, Euphorbiaceae, Fabaceae, Malvaceae, Nyctaginaceae and Solanaceae . Non-cultivated plants have long been believed to serve as a reservoir or alternative hosts for crop-infecting begomoviruses . However, most begomoviruses characterized from weeds are highly adapted to these hosts, and typically infect crop plants less efficiently or not at all . Although there are some exceptions such as tobacco leaf curl Cuba virus infecting Malachra alceifolia in Jamaica , and Nicandra physaloides infected with tomato severe rugose virus in Brazil . Begomoviruses populations in non-cultivated plants are subject to a range of distinct selective pressures compared with those in crop species, as the genetic background of these wildspecies is wider than those of crop species . As a result, begomovirus populations in non-cultivated plants tend to be divergent compared with those of crop-infecting begomoviruses .

Together, this suggest that begomoviruses infecting crops and weeds have coevolved with their hosts over a long period of time, such that the crop-infecting viruses inefficiently infect weeds. More likely, these weeds serve as sources of begomovirus diversity for the emergence of crop-infecting begomoviruses . In this scenario, crop-infecting begomoviruses have originated from ancestral viruses infecting non-cultivated plants, e.g., during spillover events and subsequently evolving and adapting to crops, some of which are highly susceptible, e.g., tomato . Several lines of evidence support this later hypothesis. First, crop-infecting begomoviruses can generally be associated with geographical regions, e.g., begomoviruses that infect crops in Brazil appear to have been indigenous . Second, characterization of some crop-infecting begomoviruses have shown the capacity to infect weeds . Third, the well-established phenomenon of local evolution is consistent with crop-infecting begomoviruses having evolved from weed-infecting progenitors . Together, these findings indicate the importance of weeds in the epidemiology of crop plants and help explains the substantial divergence of weed- and crop-infecting begomoviruses.TYLCV is the most devastating tomato-infecting begomovirus worldwide . Under high whitefly pressure, outbreaks can result in yield losses of up to 100%, especially when plants are infected early in development . Tomato plants infected with TYLCV are stunted, and show abnormal upright grow, and leaves developed upward curling with yellowing, interveinal chlorosis, crumpling and often a bushy appearance . Moreover, flowers on infected plants commonly do not develop and abscise before fruit set, leading to major yield losses, as high as 100%. So far, TYLCV has been reported infecting other solanaceous crops, such as pepper and tobacco; fabaceous crops, such as common bean; and non-cultivated plants, such as Ageratum conyzoides, Chenopodium murale, Cuscuta europaea, Malva parviflora and Sida acuta. However, tomato is by far the most economically important host of this virus, and most of the weeds are symptomless hosts and have low virus titers .Management of TYLCV requires an integrated pest management program that involves measures before, during and after the growing season . Before the growing season, TYLCV can be controlled by using virus- and whitefly-free transplants and propagative stocks and there are now commercially available resistant cultivars generated through conventional breeding . In addition, a variety of transgenic strategies have been used to develop TYLCV-resistant varieties, although these are not currently grow commercially . The use of insecticides to reduce whitefly populations is one of the most important, and commonly used components of a successful IPM approach during the growing season, but monitoring and proper timing of insecticide application is critical . Insecticides such as neonicotenoids and cyazypyr have been used effectively to control the whitefly vector in open fields and greenhouses . Unfortunately, continue usage of insecticides has led to the rapid emergence of insect vector population with resistance to various insecticides . Alternative measures during the growing season also involve roguing of virus-infected plants and row covers and reflective mulches . After the growing season, sanitation, weed management and implementation of a host-free period can be used to reduce the viral inoculumsource and vector populations . Indeed, these approaches have been successfully used to managed TYLCV in the DO .Tomato yellow leaf curl disease can cause devastating losses to tomato production worldwide . The disease is widely distributed in the Old World and New World , and is caused by several begomoviruses, including tomato yellow leaf curl virus . Regardless of the virus involved, symptoms of TYLCD include severe stunting, distorted and upright growth and leaves with upcurling, crumpling, interveinal chlorosis, and yellowing . Tomato is highly susceptible to TYLCV infection and, under high disease pressure , outbreaks can result in substantial yield losses, especially when plants are infected at early stages of development . Because the whitefly vector is difficult to control, an important strategy for disease management is planting of resistant tomato varieties that possess resistance loci introgressed fromwild species .

While water holding capacity and infiltration can be improved with management, soil type is inherent to a location, which is why research should be localized. Increasing soil organic matter will have a greater impact on available water holding capacity in sandy soils than in silty clay loam and silt loam soils . Another focus could be to localize water needs around SGMA Groundwater Sustainability Plan for different subbasins. For instance, some Groundwater Sustainability Agencies will be monitoring water use through satellite evapotranspiration data – therefore, a practical solution could be helping growers to reduce evaporative losses from soil from through mulching and tillage .Based on the IPA, “weed management” is also an area for UCCE to prioritize. Considering this was a top management challenge for respondents, as well, there is a lot more work that needs to be done in weed management for agronomic crops. Currently, CE’s integrated weed management strategies fall under Integrated Pest Management , and much of the research on integrated control methods has been done at the plot and field scale, rather than the management scale . A bibliometric analysis demonstrated that current work in the field of invasion biology, which includes weed control, consists mostly of research related to “knowing” , while research aimed at strategically applying or implementing knowledge is poorly represented . While invasions of a new weed species provide a platform to investigate ecological theories and laws, there is also a direct, practical need to understand possible management interventions . In addition, vertical growing racks the scale of emphasis is rarely at the local level and there is a lack of reporting of costs of management, which is an obstacle to making research on weed control methods useful .

If costs of control are included, they are typically calculated at the experimental scale, which may not accurately reflect management costs . Thus, more localized, applied research in weed management is needed. Contributing to the lack of applied research in weed management is the fact that UCCE has seen a reduction in regional weed control specialists and UC ANR has not hired a weed specialist for agronomic crops since the last specialists’ retirement years ago. Because of its’ importance to agronomic crops clientele, UC ANR should advocate for new advisors and specialists with a background in weed control. Current trends in commodity industry funding reflects the value the nut crop industries see in UCCE. For instance, four staff research associates who joined UCCE scientists in 2020 were funded by the California Walnut Board, the Almond Board of California, and the California Pistachio Research Board – together, they have provided $425,000 to cover annual salaries, benefits, travel, and equipment for the new UCCEstaff . In November 2020, the Almond Board of California also hired a Senior Specialist in Pest Management to focus on pest management and weed control . The lack of funded farm advisors from a centralized agronomy commodity board should be considered in UC ANRs new hires that come from the general fund.Notably, “Greenhouse gas emissions reduction” ranked low in priority in the IPA analysis. While there is recognition of the need to reduce GHGs at the state level – as evidenced by the Global Warming Solutions Act and multiple other climate change adaptation efforts – there seems to be a disconnect between agricultural producers, policymakers, and scientists when it comes to climate change. One hypothesis for why climate change is not seen as a pressing problem is how it is framed and communicated to farmers. The threat of global warming is usually broadly targeted, and the detrimental effects are often intangible. The “psychological distance” associated with climate change impacts that occur further away or well into the future require higher levels of cognitive abstraction . When it is communicated as a global problem that affects everybody, there may be less impetus to act because the problem seems far away and out of the farmer’s control.

For instance, greenhouse gas mitigation is a problem requiring global cooperation to address, while adapting to challenges faced at the local level appeals to a farmer’s self-interest . Therefore, framing climate change in terms of local consequences may motivate actions because personal risks are psychologically close . Farmers face increasing pressure to adopt practices that align with various societal visons of better agriculture, which may overwhelm farm management capacity . State level policy initiatives often fund UC ANR research and extension activities, thereby setting their direction. For example, there is a strong emphasis on Greenhouse Gas emissions reductions in CDFA’s Climate mart Agriculture programs – for the Healthy Soils Program GHG reduction estimations are the main metric of progress.Yet, GHG reduction was ranked as the lowest priority topic by respondents in our survey. This result highlights the tension between issues that are relevant for growers versus policymakers. Water conservation and storage, weed control, and soil health were all high priority topics for survey respondents, and all of these can be addressed by implementation of healthy soils practices. Practices that reduce GHG emissions might also result in benefits that are more tangible to growers, such as increased fertilizer use efficiency and lower input costs. Therefore, policymakers should think about how these programs can measure and display these tangible benefits rather than only focusing on GHG reduction, to inspire greater adoption. If these programs highlighted reduction in weeds and improved water holding capacity of the soil as benefits of healthy soils practices, the outcomes are the same but the emphasis is landowner-centered. The central tenet of a landowner-centered approach is empathy for the landowner wherein the needs, desires, constraints, and experiences of landowners are placed at the forefront .The fact that cover crops fall into the “possible overkill” quadrant of the IPA matrix suggests the importance of adapting practices to specific regions and cropping systems. Cover crops have been proven to help with the top management challenges of our respondents, such as water conservation and storage, soil health, and weed suppression . Yet, respondents overall did not rank them as highly important. Perhaps part of this perception of overkill is framing cover cropping to mean a very specific thing– for instance, the only UC research assessing the costs and benefits of winter cover cropping in California assumes a mix of bell beans, winter peas, and common vetch . The idea behind specific cover crop mixes is important in being able to comparatively quantify their benefits, but the goals of cover cropping can be achieved in other ways. For example, soil erosion can be managed through the maintenance of ground cover on the soil surface, but what that ground cover is can be adapted to what works best in certain climates and cropping systems . Simply keeping roots in the ground can improve water infiltration, feed soil biology, protect surface water quality by reducing runoff, provide competition for weeds, increase soil organic matter, and enhance carbon sequestration. If a grower is planting small grains or forage for agronomic crop production, keeping the ground covered during the winter can achieve some of the benefits of winter cover cropping. In this way, agronomic crop producers may already be “cover cropping”.

There is opportunity here to expand agronomic crop production for those who currently fallow their fields in the winter. Not only will this increase land use efficiency, but it can diversify farm income for those who currently fallow their fields in the winter. If there is only research on a specific way of managing a potentially cost prohibitive mix of cover crop seed, only farmers who find this specific method feasible may adopt this practice. Supporting growers’ autonomy in choosing practices that achieve target principles within the context of their farming system may lead to increased conservation outcomes, since autonomy can lead to integration of conservation behaviors into a landowner’s sense of self and stewardship ethics . Yet, we cannot solely rely on landowner’s stewardship ethics, and must continually question the adaptive capacity of our institutions. Many institutions are designed to pursue narrow or siloed objectives . Within the current research reward system, growing racks in which citation is an indication of impact within academia, there may be a disincentive for scientists to publish applied and local scale research and interdisciplinary research . In many conservation fields, scientific research does not always translate into on-the-ground action, which is known as the “knowing doing gap”. Yet, local scale implementation research tends to draw less attention from the international scientific community . Managers need more applied research, but researchers are more rewarded for publishing basic research . A survey of California managers in 2012 documented that respondents relied little on published research in making management decisions, and for the research that was published, basic research was more than twice as high as that desired by managers’ , who preferred a greater level of applied and interdisciplinary research . In addition, there was a mismatch between researcher and stakeholder priorities for specific topics in basic research and published studies frequently failed tests of relevant as indicated by scale-appropriateness, usability, timeliness, and accessibility . The disconnect between science and management can be characterized as the “research implementation gap”. Managers tend to rely heavily on their own observations and those of their colleagues at other management agencies, rather than scientific research . Conservation planners also rely heavily on experience-based information, rather than evidence-based information from experiments . Therefore, devoting more resources to obtaining management information from experienced practitioners and land managers can greatly increase understanding of factors that contribute to success and failures . Interdisciplinary research that integrates landowners into the scientific process must be employed to solve larger challenges and address clientele concerns. Managers cannot separate their needs from the social and political context in which they work, so research should not either . Focusing on problems in a vacuum when weeds, water, and soil management are all interconnected, is misleading, not to mention a less effective use of limited resources.As the landscape of California agriculture changes, agronomic crop production faces many challenges. Water resources are under threat from changing climate and cropland expansion, there is a lot of uncertainty around the impacts of the Sustainable Groundwater Management Act on water supply, and California has seen large shifts away from agronomic crop production to perennial production. For these reasons, the UC Cooperative Extension Agronomic Program Team conducted a survey of agronomic crops clientele in summer 2020 with the objectives of documenting clientele needs,informing research and extension priorities, and serving as a foundation for future collaborative needs assessment efforts. Results of the survey indicated that water-related issues are of great concern to the agronomic crops community and serve as a primary management challenge. Therefore, UC needs to devote more people and resources to practical solutions for water/irrigation management. Weed management is also a primary management challenge and was identified as a priority area for extension in the Importance Performance Analysis. Currently, there is no statewide weed specialists working in agronomic crops in California, and there are only a few advisors as well. In developing practical solutions for dealing with management challenges, UCCE must balance short-term growers’ interests with long-term education to adapt to future challenges and regulations. Growers tend to want practical, immediate solutions that work at the management scale. In addition, UCCE must work with growers to develop information that integrates practitioner knowledge and is relevant to the realities of agronomic crop production. Based on projected climate change impacts, agricultural systems may have to undergo more transformative changes to remain productive and profitable in the long term . Attempts by UCCE and policymakers to develop solutions within the current framework of our production systems is not a long-term answer and primary management challenges will need to be dealt with again and again if we continue with “business as usual” without focusing on long term adaptation.Barb goatgrass primarily occurs in California, although there are records from Washington, Oregon, and Nevada, as well as from some mid-Atlantic states . Medusahead is widespread in California and the Intermountain West, occupying roughly 2.4 million acres across the western United States . Estimates for the extent of barb goatgrass infestation are not currently available, though it is much less widespread than medusahead. Barb goatgrass is a B-rated noxious weed and medusahead is a C-rated noxious weed in the State of California, meaning that they both cause economic or environmental detriment.

However, the substrate treatments did not affect the marketable fruit yield. Significant differences were noted only on the cull yield. The highest cull yield was observed in the steamed soil with amendments; this was the case at both Mar Vista Berry and Monterey Bay Academy, and it could be attributed to the very low pH and high EC of this substrate. One of the main concerns in soilless strawberry production is the maintenance of a favorable pH, EC and nutrient supply to the growing plants. For most of the sampling periods at the experimental sites, different substrate and soil treatments had significantly different levels of pH, EC, nitrate nitrogen, ammonium nitrogen and available phosphorus. At both sites, the pH of the coir and the peat and perlite treatments was lower in the early sampling periods but increased with time, reaching the targeted value of 5.7 after 3 to 4 months ; this slow rise in pH to the target value was attributed to the high nutrient adsorptive capacity of the soilless substrates. The pH of the amended soil treatments at both sites was generally low at all sampling periods, and the target value was not reached during the production cycle. With the exception of the initial sampling period, the EC of the substrate treatments at Monterey Bay Academy was generally low . In contrast, the EC in the Mar Vista Berry beds was consistently high, grow racks which could be due to the higher amount of salts in the irrigation water. The EC of the steamed soil with amendments treatment at Mar Vista Berry was also consistently high throughout the growing season.

The soilless substrates are low in nutrients; thus, fertilization is one of the key issues in these systems. Surprisingly, the initial nitrate nitrogen of the coir and the peat and perlite mixture was higher at both sites, and the target value of 100 ppm was maintained in the beds through the season except for the latter stages of plant growth . At all sampling periods, the ammonium nitrate was lower than the RABETS target value of 14 ppm . The RABETS target of 30 ppm available phosphorus was maintained in all of the media treatments at both sites .Anaerobic soil disinfestation , a nonchemical alternative to methyl bromide, was developed in Japan and the Netherlands to control soilborne pathogens and nematodes in strawberries and vegetables. Anaerobic soil disinfestation integrates the principles of solarization and flooding in situations where neither method alone is effective or feasible. Anaerobic soil conditions are created by incorporating readily available carbon sources into topsoil, covering the soil with plastic tarp and irrigating to field capacity. The tarp is left in place to maintain soil moisture above field capacity and to sustain anaerobic conditions. Anaerobic decomposers respire using the added carbon, which results in a buildup of anaerobic byproducts that are toxic to pathogens . These byproducts degrade rapidly once the tarp is removed or holes are punched through the tarp for planting. Studies were conducted during 2008 to 2011 in an attempt to optimize anaerobic soil disinfestation for California strawberry and Florida vegetable production systems. Overall, it was very effective in suppressing Verticillium dahliae in soils, and it resulted in 85% to 100% of the marketable fruit yield observed with fumigated controls in coastal California strawberries when 9 tons per acre of rice bran was preplant incorporated and 3 to 4 acre-inches of irrigation was applied in sandy loam to clay loam soils .

In the semitropical climate of Florida, when composted broiler litter and heavy black strap molasses were incorporated as substrate, anaerobic soil disinfestation treatments provided good control of nutsedge and excellent control of grasses, broadleaf weeds, Phytophthora capsici and Fusarium oxysporum f. sp. lycopersici . In the cooler conditions of the Central Coast, however, anaerobic soil disinfestation may not provide effective control of many weed species . To ensure consistency of pest suppression across varying locations, the effects of soil temperatures and treatment length and the mechanisms of pest suppression by anaerobic soil disinfestation are being further elucidated. Its integration with other non-fumigant approaches may also have promise. For example, a combination of anaerobic soil disinfestation and mustard seed meal application is currently being tested .Heat treatment with steam can be used for soil sterilization or pasteurization . Studies have shown that most plant pathogens, insects and weeds will die when moist soils are heated to temperatures higher than 150°F for 30 minutes . The duration and amount of steam needed to raise the soil temperature to 150°F depend on various soil factors, including texture, type and moisture content. Minuto et al. found that soil could be heated most rapidly at a moisture content between 8.5% and 12% in a sandy loam and between 6% and 7% in a sandy soil. Steam applied to field soil that raised the temperature to 158°F for 20 minutes resulted in weed control comparable to methyl bromide . In addition to pest control, an advantage of steaming is that it lacks the negative environmental and worker health issues associated with chemical fumigants. Some have reported that steaming has little or no lasting negative impact on soil quality or soil microbial communities as opposed to the known potential impact of methyl bromide fumigation on both soil quality and microbes .

Other studies have reported a more significant change in soil microbial activity due to steam sterilization . Differences among steam studies may be related to duration of steam application and soil temperatures attained during steam treatments as well as the soil organic matter content. Steam has also been shown to increase crop growth and yields . Previous work found that strawberry fruit yields from steam-treated soils were similar to those from soils fumigated with methyl bromide plus chloropicrin .Natural products such as mustard seed are being evaluated as bio-fumigants. Recent studies found that mustard seed meal amendment can suppress root infection by Rhizoctonia solani . We have been testing mustard seed meal in strawberry beds at rates of 500 to 4,000 pounds per acre incorporated into the soil. Mustard meal alone does not consistently produce high fruit yields or control weeds . One possible method to enhance solarization is to use combinations of mustard meal, chloropicrin, planting racks and metam sodium treatments . By heating the soil with solarization or steam, the pest control activity of metam sodium, chloropicrin or mustard meal may be higher than at ambient soil temperatures.A field study was conducted at Monterey Bay Academy from October 2010 to September 2011 to evaluate anaerobic soil disinfestation and steam with and without mustard seed meal application prior to planting strawberry beds. Treatments included a control; Pic-Clor 60 at 300 pounds per acre as a standard; mustard seed meal at 3,000 pounds per acre; anaerobic soil disinfestation with rice bran at 9 tons per acre; anaerobic soil disinfestation with rice bran at 7.5 tons per acre and mustard seed meal at 3,000 pounds per acre; steam; and steam plus mustard seed meal at 3,000 pounds per acre.The trial was arranged in a randomized complete block design with four replicates. Anaerobic soil disinfestation was initiated Oct. 7 to create a saturated condition. The plots were maintained above field capacity with intermittently applied irrigation water from Oct. 8 to Nov. 3, 2010. Steam was applied via spike injection from a stationary steam generator for a sufficient time to raise the soil temperature to 158˚F for 20 minutes on Oct. 13 and 14, 2010. Weed densities were measured in 25-square-foot sample areas covered with clear tarp, on Dec. 15, 2010, Jan. 21, Feb. 23 and April 6, 2011. Strawberry fruit was harvested weekly from April 28 to Sept. 15, 2011. Fruit was sorted as marketable and cull at each harvest date. Data were subjected to analysis of variance and means were separated using Fisher’s protected LSD. Trial results. Overall, the steam treatment and the steam treatment with mustard seed meal were as effective as Pic-Clor 60 in providing weed control . Anaerobic soil disinfestation plus rice bran suppressed weed densities, but it was less effective than Pic-Clor 60. No strawberry plant injury was observed in any of the treatments . Marketable yields data collected from April 28 to Sept. 15, 2011, indicate that strawberry fruit yields in the steam treatments and the anaerobic soil disinfestation treatments were comparable to those in the Pic-Clor 60 application .

These data, along with data from our prior studies, show that steam is as effective as chemical fumigation; and that anaerobic soil disinfestation also produces yields equivalent to Pic-Clor 60 but may need to be combined with herbicide use in severely weed-infested sites. The costs of the anaerobic soil disinfestation treatments with rice bran, and with rice bran plus mustard seed meal, were $1,632 and $3,093 per acre, respectively, including material, spreading, incorporation and irrigation . The cost of steam was $10,440 per acre, compared to $1900 per acre for Pic-Clor 60. Therefore, although the yields and gross revenues were comparable across treatments, the net returns after treatment and harvest costs were highest for the Pic-Clor treatment, followed by the anaerobic soil disinfestation with rice bran. The lowest net revenue was for the steam plus mustard seed meal treatments due to the high cost of the steam treatment. The cost data showed a critical need for more-efficient steam injection systems before steam can be adopted commercially. Recent advances with steam application equipment can reduce the cost of steam treatment to less than $5,500 per acre with the potential for further cost reductions . Since 2011 we have used an automatic mobile steam applicator in our research, which lowers the labor costs relative to those reported here by approximately 50% to 70%. It mixes steam with soil, allowing soil to be heated from 60˚F to 160˚F in 90 seconds — much more rapidly than the steam application system used here .The phase-out of methyl bromide has proven to be a daunting task for the California strawberry industry. Not only are strawberry producers faced with the likelihood that methyl bromide will no longer be available to them by 2015, but they also must deal with increasing regulatory stringency on the use of all soil fumigants. While fumigants face an uncertain future in California, barrier films can help trap fumigants in the soil and reduce the likelihood of environmental or health impacts associated with fumigants in the atmosphere. It appears very likely in the near future that barrier films will be the only type of film approved for use with fumigants in California.Potential methods of strawberry production that do not use fumigants include growing plants in substrates and using steam treatments or anaerobic soil disinfestation. All of these systems are being evaluated on a much larger scale, from 1 to 10 acres, with different soil types, to determine commercial feasibility and cost effectiveness. It is not likely, nor is it desirable from a pest management perspective, that one non-fumigant system will dominate on a large percentage of the strawberry acreage. Multiple production systems, using fumigants and non-fumigants, would allow producers to rotate treatments to suppress soil pests.Agronomic crops are the basis of the world’s food and fiber production systems . In California, agronomic crops include small grains, rice, corn, beans, oilseed, cotton, and forages, and represent a significant share of irrigated acreage in the Central Valley . Agronomic crops were planted on an average of 4 million acres annually from 2000- 2020, occupying more land than the categories of fruit and nuts or vegetables and melons and generating a total of $4.3 billion . Yet, the agricultural landscape is changing in California due to water scarcity, economic challenges, competition for land, weed pressure, and new regulatory requirements related to water and nutrient management. Since 2000, acreage planted to agronomic crops has declined by more than 100,000 acres per year, with a corresponding shift towards high value perennial crops such as almonds, pistachios, and walnuts . Given these changes, there is a need to better understand the concerns and management challenges of growers and others working in agronomic crop production. As part of the University of California’s Division of Agriculture and Natural Resources , Cooperative Extension is responsible for agricultural research, education, and outreach throughout the state. The mission of Cooperative Extension has always been to solve practical problems and disseminate useful information to its stakeholders .

Deer, rabbits, and other herbivores do not readily graze brooms, possibly due to the bitter taste of the stems and the availability of more palatable forage . However, grazing does occur in other parts of the world, where goats control Scotch broom.Using fire to control Scotch and French brooms has had varied results. Some researchers suggest frequent prescribed fires to encourage regeneration and deplete the seed bank over time. Cooler fires can encourage seed germination, followed by prescribed fires that kill the young seedlings before they generate seed . Soil temperatures from 130°F to 300°F in moist conditions have been shown to stimulate seed germination . A hot fire produced by hand-cutting mature plants, allowing the cut material to dry, and then burning in spring effectively controlled French broom re-spouts but had little effect on germination . Hot fires that generate soil temperatures over 300°F killed Scotch broom seed . Obtaining soil temperatures at this high temperature and deep enough to effectively deplete the seed bank is difficult to achieve safely.Many herbicides are effective on broom. The concentration, timing, and method of application determine which herbicide and method of application are most appropriate. Since herbicide formulations and recommendations are subject to change, commercial grow racks check with your local county agricultural commissioner or pest control adviser for current recommendations.

Spray the plant with a solution of 1.5 to 2 percent a.i. glyphosate mixed in water until the plant is thoroughly wet. Apply this mixture just as the flowers are blooming for most effective control. Painting the cambium region of cut broom stumps with a glyphosate or triclopyr solution at 50 percent a.i in water can also be effective. Stump treatments are most effective when applied within a few minutes of cutting.Processing tomatoes are important to the California agricultural economy; in 2018, California accounted for over 90% of the 12 million tons of tomatoes grown in the United States . Some of the most potentially damaging pests of tomato include the weedy broomrapes , which have recently made an appearance in several California tomato fields after a 40-year hiatus. While broomrape is not currently at levels that can impact yield, presence in a field causes a large economic loss to growers because of the weed’s status as a quarantine pest. The establishment and spread of broomrape in California tomato production regions could cause severe consequences for individual growers and the entire tomato industry. Broomrapes are obligate root parasitic plants that can cause devastating damage to tomatoes and many other economically important broadleaf crops.

These weeds use a modified root, called a haustorium, to fuse into a host plant root and extract nutrients and water. This greatly reduces productivity and sometimes kills the host. Globally, seven broomrape species have been identified that can cause damage to crops. Ofthese, small broomrape , Louisiana broomrape , Egyptian broomrape and branched broomrape are known to be economically important pests in the United States . Tomato is highly susceptible to both branched broomrape and Egyptian broomrape. Branched broomrape is currently classified in California as an “A” pest. An “A” pest is an organism of known economic importance subject to state-enforced action involving “eradication, quarantine regulation, containment, rejection, or other holding action” . The discovery of branched broomrape in a commercial tomato field leads to quarantine and crop destruction without harvest; processers will not accept a load of tomatoes from an infested field. Egyptian broomrape, which, like branched broomrape, has been detected in some California tomato fields , is listed as a “Q” species. “Q” species have a temporary “A” classification pending determination of permanent rating by the state. Though Egyptian broomrape is currently considered less of a threat to California tomato crops than branched broomrape, Egyptian broomrape is also highly destructive. Studies in Israel showed that at high infestation levels , Egyptian broomrape can cause processing tomato yield losses as high as 70% .

In Chile and Israel, annual economic losses in tomato due to Egyptian broomrapes have been estimated at $5 and $200 million, respectively . Globally, branched broomrape is one of the most damaging and widespread of the weedy broomrape species, infesting nearly 6 million acres of broadleaf crops across Asia, the Mediterranean basin and North Africa . Branched broomrape infests a wide range of crops including tomato, cabbage, potato, eggplant, carrot, pepper, beans, celery, peanut and sunflower . A broomrape-parasitized plant suffers growth and yield reduction, and death can result in cases of severe infestation. Yield reduction can be significant depending on the level of infestation, susceptibility of the host and environmental conditions . Growers have reported up to 80% tomato crop loss due to branched broomrape in Chile . This is highly concerning given the similarity in tomato production systems and broomrape species with California.Branched broomrape was first documented in Europe in the 17th century , and is now present in 24 countries in Europe, North and South America, Africa and Asia . Most of the countries or locations where branched broomrape is reported have a Mediterranean climate, with warm-dry summers and rainy winters . In the United States, branched broomrape was first reported in 1890 and, since then, over 150 occurrences have been documented . Reports of branched broomrape in the United States have been increasing, from seven occurrences in 2015 to 65 in 2019 , and it has been documented in Texas, Virginia, South Carolina, Illinois, New Jersey, Tennessee, Kentucky, Alabama and California . In California, branched broomrape was first seen in Butte County and later in Alameda County .Eventually it spread to other counties in California, including Colusa, Sacramento, San Benito, Santa Clara, San Joaquin, Ventura and Yolo . A severe infestation of branched broomrape in the Sacramento Valley in 1959 prompted an intervention that involved soil fumigation with methyl bromide; this was as an industry-led effort funded through a legislative marketing order program . The effort, which lasted from 1973 to 1982 and cost over $1.5 million , involved research, intensive field surveys and fumigation of infested fields and equipment to target the soil seedbank. As a result of those endeavors, branched broomrape became a less significant problem. Recently, however, this parasitic weed has been detected in tomato fields in Yolo, Solano and San Joaquin counties . The cause of the re-emergence of branched broomrape remains unclear, although re-introduction or recurrence from long-dormant seed in the soil and subsequent spread are the most likely explanations. The re-emergence of this species in California is of concern to the processing tomato industry for many reasons: the demonstrated global vulnerability of tomato to branched broomrape parasitism; the similarity of California’s climate to the species’ native climate; repeated cultivation of processing tomato in the same fields; the cultivation of a wide range of hosts besides tomato in California; intensive agricultural practices that could rapidly spread broomrape seeds to uninfested fields; the plant’s prolific production of tiny seeds that can easily disperse via machinery and irrigation water in the highly mechanized and irrigated cropping systems of California; the ability of seeds to persist in the absence of hosts due to seed longevity of more than 20 years; the difficulty of using conventional means of weed control, dry racks for weed such as cultivation and contact herbicides, because so much of the plant’s lifespan occurs underground; the lack of some known important management tools because they are not yet registered or tested in California; and regulatory and environmental challenges with soil fumigation practices.Branched broomrape is a holoparasite, meaning that it obtains all its nutrients from the host. Seed germination depends on the presence of a suitable host plant and on prevailing environmental conditions. Seeds need to undergo a pre-conditioning period in the form of warm stratification before they can germinate .

The pre-conditioning period requires moist and warm environmental conditions from 5 to 21 days. The conditioned seed then can germinate in response to a signaling compound released from the host plant root . If conditions remain conducive, multiple flushes of germination can occur within a single season ; however, in the absence of stimulants, these preconditioned seeds re-enter dormancy. As the environment becomes drier, the seed’s ability to germinate gradually reduces. After germination, the radicle of the broomrape seedling grows a few millimeters in length and attaches to the host plant . If it fails to attach to a host within a few days, the radicle exhausts its food reserves and dies . Following attachment to the host plant, the radicle develops into a specialized modified root called a haustorium, a plant organ common to all parasitic plants . The haustorium fuses into the vascular system of the host root and serves as the bridge for extraction of nutrients and water from the transport systems of the host . Once connected to a host plant, broomrape grows rapidly, forming a tubercle — a storage organ for nutrients and water extracted from the host— underground . Multiple shoots develop from the tubercle and emerge above the soil surface, then grow to stalks from 6 inches to 12 inches in height . The shoots, wrapped with alternate bracts, completely lack leaves and chlorophyll. Prior to flowering, young plants look like yellowish spikes . Flowering begins within 3 to 7 days after a broomrape shoot emerges above the soil surface . Branched broomrape flowers are spike-like, irregular, bisexual and usually pale white to purple in color. The petals of the flower are merged, tubular and have an upper and lower lip . The carpels are usually united to form a single chamber on the upper part of the flower; this chamber matures as a capsule with thousands of very tiny seeds, each smaller than a grain of sand . Seed production can occur within 14 days after flowering. A mature broomrape plant can produce hundreds of thousands of tan or brown-colored seeds, which can remain dormant and viable for many years in soil. The entire life cycle, from seed germination to seed production, takes place within the March-to-August growing season of processing tomatoes in California.Effective control of broomrapes is difficult, largely due to its unique biology and complex life cycle. As indicated above, most of the broomrape life cycle occurs below the soil surface, which makes it difficult to detect and control before it causes damage to the host plant. The short time period between emergence and seed dispersal also makes detection and control difficult, while the absence of chlorophyll and photosynthesis limits potential herbicide target sites and complicates chemical management. The tiny, hard-to-detect and abundant seeds, and the ability of the seeds to remain viable for decades, promotes the spread and persistence of branched broomrape in crop production systems. Thus, effective management of broomrape will require a long-term, integrated approach that involves sound understanding of the biology of the parasitic weed and the dissemination of information about management practices to all stakeholders.Early detection and awareness of a new infestation, rapid reporting of the infestation to the local agricultural commissioner, proper removal of the branched broomrape plants, and management of the seedbank are crucial steps for successful containment and eradication of this parasite. Preventing the spread of branched broomrape is the most important component of the integrated approach to managing the weed. A current containment approach used in California is based on a quarantine regulation that places a recently infested field on hold for a period of at least 2 years; in subsequent years, only rotational crops approved by the local agricultural commissioner may be cultivated in the field. Upon detection of a new infestation, all branched broomrape plants should be removed carefully , ideally before they produce seeds. However, because of variability in the plant’s growth stages , seed production might already have occurred by the time they are detected. The application of broad-spectrum herbicides at this stage, although likely to kill both the host plant and parasite, is less likely to affect the seeds. Therefore, the plants should be pulled and placed in plastic bags to minimize seed addition to the seedbank. The bags, tightly sealed, can be left under the sun for a few days to promote the degradation of seeds. The plant materials can also be burned or destroyed by autoclave. Weed seeds are often dispersed among fields by human activities, such as the transportation of contaminated farm produce , the movement of contaminated vehicles and implements, and the spreading of contaminated soil and manure.

The person may be unable to repay the loan when the loan interest is too high or inflated. In some cases, the person’s work is sufficient to cover the interest but not the principle. Individuals become trapped by debt when it is passed on to future generations. The International Labor Organization Convention 182 describes bonded labor as one of the worst forms of child labor. Forced child labor in the production of Ganesh bidis from Mangalore, India, prompted the U.S. Customs Department to issue a ban on the import of the Indian bidis to the U.S. Debt servitude originates from labor arrangements between landless farmers and landholding farmers, and between farmers and tobacco companies. In the landless farmer-landholding farmer arrangement, a landless farmer agrees to grow tobacco on land provided by the landlord. The landless farmer agrees to sell tobacco to the landlord who agrees to provide on loan inputs such as seeds, fertilizers, hoes, watering cans, and plastic sheeting. At the end of the tobacco-growing season, the landlord deducts the input prices from earnings of the farmer. The labor arrangement is unequal and favors the landowning farmer. Prices for seeds, chemicals are often higher than retail prices, increasing the likelihood that tobacco farmers actually lose money. In Kenya 90% of tobacco farmers sign contracts without a clear understanding of the contract language, and 80% of tobacco farmers lose money. In the contract arrangement, the landlord sets tobacco prices and the agreement is oral, growing racks making it virtually impossible for farmers to find remedies when they have been treated unfairly. In India, 60 million children work full time and one million children are in bonded servitude.

Children, mostly girls, as young as 4 years old, are in bonded labor in the bidi sector, some working 10 hours a day and still attending to domestic chores and sometimes experiencing physical abuse from their employers. In Africa, evidence of tobacco farming bonded labor exists in Nigeria, Tanzania, and Uganda. Anna White with Global Partnerships for Tobacco Control in Essential Action in Washington, D.C., reported that a tobacco farmer in Nigeria did not earn a profit in four years and explained that indebtedness to BAT prevented him from ending tobacco farming. In Tanzania, tobacco farmers require pesticides purchased on loan from global leaf companies, perpetuating farmers’ entrapment in a cycle of poverty. John Waluye, a Tanzanian environmental journalist, in the 2003 documentary film “Smoke Sacrifice: Blue Haze-Forest Raze,” said that tobacco farmers in Tanzania are “slaves of tobacco” due to debts to U.S. leaf companies, who try to reduce the price of tobacco. Many tobacco farmers in Uganda receivelow earnings from tobacco, experience food insecurity, and continue to grow tobacco because of debts to tobacco companies. Honduras and Brazil have evidence of tobacco industry bonded labor. In Honduras tobacco farm workers experienced extreme dependency and near servitude in their relationships with farm authorities.42 Tobacco farmers in Brazil experience debt servitude through direct contracts with global tobacco companies that manipulate leaf classification and provide farm inputs at inflated prices on loan. Jauri, a tobacco farmer in Brazil, said, “Tobacco demands a lot of work, but makes you very little money. If the companies started paying more for the tobacco that would, of course, change things. If we could, we’d change business. But first we’ve got to pay our bills.”1 Cecilia, a tobacco farmer in Brazil said, “Tobacco growing is like slave labor. It’s worse. A slave gets food and doesn’t have to go to work on an empty stomach. We suffer a lot, producing this crop.”1Child labor in tobacco farming is a human rights issue.

The United Nations Convention on the Rights of the Child includes principles that protect children from exploitation. 192 of 194 countries have ratified the UN Convention on the Rights of the Child. The Minimum Age Convention 138 was adopted by the United Nations in 1973 to establish a minimum age at which children can work. 142 countries have ratified Convention 138. The World Forms of Child Labor Convention 182 was adopted in 1999 and ratified by 157 countries, including the U.S. Categories of child labour to be abolished are labour performed by a child who is under a minimum age specified in national legislation for that kind of work, labour that jeopardizes the physical, mental or moral well-being of a child, known as hazardous work, and the unconditional worst forms of child labour, which are internationally defined as slavery, trafficking, debt bondage and other forms of forced labour, forced recruitment for use in armed conflict, prostitution and pornography, and illicit activities. Tobacco-related child labor persists due to lack of enforcement mechanisms and weak national labor laws. Many cases of child labor often go unreported because tobacco farm families fear retaliation from farm authorities or families are unfamiliar with child labor violations. Tobacco farmers and their families experience inadequate labor inspection services. Labor inspection services, if they exist at all, are poorly funded, inadequately staffed and trained, and suffer from the lack of specialized technical advice. In Africa, where BAT and other tobacco companies obtain low cost tobacco, child labor exists in countries such as Malawi, Kenya, and Nigeria. In Malawi, 78,000 children as young as 5 years old in tobacco families clear fields, harvest tobacco, and perform a range of potentially hazardous tasks. A study of 50 farmers in Kenya revealed that children are involved in tobacco growing in virtually all farms. In Uganda, children from tobacco families are kept from school and sent to fields to weed, water, string and sew bunches of tobacco leaves together for drying in flue-curing barns. In Nigeria, school age children harvest and help to cure tobacco, earning little or no money and are denied education. Tobacco industry funded studies reported that in Mozambique 80% of tobacco families used their children as young as 6 years old on tobacco farms,61 and in Zambia over 6,000 children work on tobacco farms and perform tasks such as lifting heavy loads, spraying chemicals, and working excessively long hours. In India 225,000 children work in the bidi industry. In a study in 10 blocks in Malda District in Bengal State, Indian researchers reported that 6,100 children with an average age of 10 years old perform bidi production tasks. Child workers in the bidi industry suffer from poor psychosocial development, addition to tobacco, and sever punishment for infractions committed while working. In Indonesia the majority of child work occurs in rural areas and child labor is common on tobacco plantations.66 In a tobacco industry funded study of 100 child laborers in Indonesia, researchers reported that 78 children work in tobacco fields,  children experienced work related accidents, and 24 children had beentreated poorly by their parents or farm authorities. Children, mostly girls, cultivate tobacco and, if they are paid at all, earn US$0.60 a day, well below the legal minimum wage. In the Americas, tobacco-related child labor is a problem in countries such as Mexico, Honduras, Argentina, and Brazil. In Mexico, children as young as 5 years old assist their parents in harvesting, threading and hanging tobacco leaves on tobacco plantations. In a study of 171 migrant working children in Nayarit State, Mexico, growing weed vertically researchers reported that 56 children were exposed to unacceptable levels of pesticides. In Honduras, children under 15 without protective clothing dip their hands and arms into bags of pesticides to fill small cap containers and apply pesticides to tobacco plants. In Kentucky, U.S., children as young as 10 years old drive tractors for transporting equipment, hauling crops, and loading hay on average ten days a year on farms. Tobacco industry funded studies reported child labor in Fiji and the Philippines. Eighteen percent of children of tobacco farm families in Fiji missed school due to harvesting, and 12% of children on tobacco farms used backpack sprayers with toxic chemicals and carrying capacities heavier than believed safe. In the Philippines, a study funded by Philip Morris International reported that 16% of children is engaged in economic activity and that participation of children in tobacco production is a common feature in tobacco growing regions. Child workers plow, weed, cultivate leaf and assist adults in chemical spraying in tobacco fields in the Philippines. In Brazil, where child labor emerged with the development of the tobacco industry,46 200,000 farm families cultivate tobacco and many families make their children work in fields, exposing children to toxic chemicals, nicotine, snake bites, and tobacco loads to carry that are far beyond their capacities.

According to the report “Brazil: Child Labor Rampant in theTobacco Industry,” children as young as 6 years old tie tobacco bunches. The global tobacco farming industry is comprised of cigarette manufacturers such as BAT, Philip Morris, and Japan Tobacco, and leaf buying companies such as Universal Corporation and Alliance One International. BAT and Universal exemplify the farming dimension of the tobacco industry. In addition to being a cigarette manufacturer, BAT is the third largest global leaf buyer . BAT obtains through its own vertically integrated operations 65% of its tobacco through direct contracts with 280,000 farmers in developing countries. BAT uses$40 million worth of tobacco each week.88 Universal Corporation sells tobacco through prearranged contracts with five companies that purchase 80% of Universal’s leaf . Universal has 56 subsidiaries, including Santa Cruz do Sul, Rio Grande do Sul, Brazil; Universal Leaf in Guntur, Andhra Pradesh, India; and Tanzania Leaf Tobacco Company in Morogoro, Tanzania. In Brazil, Tanzania, and other non-auction markets, Universal operates direct contracts with tobacco farmers. India has an auction and direct contract arrangements with tobacco farmers and Universal. The concentration of a few powerful tobacco companies in the global economy provides companies with control over leaf prices, markets and governments. Companies have exerted monopoly and monopsony power in tobacco sectors in Bangladesh, Brazil, Malawi, and other developing countries. In Bangladesh, tobacco companies “operate like a cartel, sharing among themselves all market-related information.” Tobacco companies’ cartel and collusion over prices at auction depresses tobacco prices in Malawi. A 2005 study by Malawi’s AntiCorruption Bureau concluded that Limbe Leaf and Alliance One operate a tobacco cartel and collude with each other, reducing competition and decreasing prices at auction. Limbe Leaf and Alliance One privately agree on percentages of tobacco that each company is supposed to buy each day at auction, cautioning each other when either of them purchased more than the percentage allotted to them. Global tobacco companies have a monopoly on the leaf procurement system, as well as the marketing and distribution of tobacco products. Companies determine what price they pay farmers, and therefore the pay and conditions of field workers. This system with layers of subcontracts is designed to avoid responsibility for what happens down the tobacco leaf commodity chain. With their economic and political influence, tobacco companies could increase prices and pay living wages. Companies have decided to ignore tobacco farmer poverty and health insecurity in the drive for greater profits. Tobacco companies through monopoly power in leaf buying contribute to an imperfect global tobacco market, where companies’ buying practices defy economic laws of supply and demand.Tobacco farmers use trees to process tobacco in flue-curing barns that require wood fuel. Tobacco families use wood for cooking and heating purposes. As forests are depleted, women and children have to travel even greater distances to obtain firewood, adding pressure to work routines focused on domestic chores and tobacco cultivation. Many families are unable to get firewood and turn to charcoal, so the charcoal producers cut even more forest. The money spent on fuel further erodes the income of families to buy food. Desperate to survive, tobacco farmers expand production into the forest. Tobacco-related deforestation destroys vegetative cover that contributes to soil erosion, flooding and famine, and contributes to global warming . In Yunnan Province, one of China’s most important tobacco growing areas, soil erosion is as a major environmental problem. The decline of flue-cured production is associated with a process of gradual reforestation. According to researcher Bryan Farrell, deforestation “affects the atmosphere, by raising the level of carbon dioxide emissions responsible for global warming. Scientists affiliated with the climate research group Global Canopy Program in England have reported that the 51 million acres cut down every year account for nearly 25 percent of heat-trapping.” In Tanzania some tobacco farmers stop farming intermittently due to changing climate conditions.

The water-seeded production system in California is a common method to suppress weedy grasses and non-aquatic weed species. In California, pregerminated rice seed is air-seeded onto fields with a 10- to 15-cm standing flood and the fields are typically maintained continuously flooded throughout the growing season . The California rice cropping system is again unique because of its presence near growing urban communities and a variety of neighboring high value crops. Surface water used for rice production is mainly derived from reservoirs that capture water in the Cascade Mountain Range and Sierra Nevada from the Sacramento River and the Feather River, respectively . There is potential for contamination of drinking water and water for wildlife by herbicide use in California rice fields, which has historically been documented with the rice herbicides thiobencarb and molinate . Production lands further away from the water sources will also use drainage water downstream as irrigation . Many neighboring crops can be susceptible to pesticide residues at low concentrations and this can be of concern if herbicide residues are present in the irrigation water . Historically, regulatory agencies and the California rice industry have collaborated to implement successful programs to manage and reduce off-target pesticide effects by mandating report of pesticide use, monitoring water quality, indoor grow table and water-holding periods after chemical applications . Pesticide use reporting and monitoring encourage stewardship of chemical use among agencies and applicators .

Water-holding periods prevent the pesticide active ingredient from becoming runoff in the tail water and contaminating non-target areas and organisms. The water-holding period can differ among pesticides based on their physico-chemical properties and degradation pathways . Therefore, it is important to understand the behavior of herbicide active ingredients in the water-seeded system to successfully characterize them in support of sustainable stewardship and efficacious use of chemicals. Herbicide products can be developed in various formulations to assist with weed control, for instance, to achieve longer soil residual activity, reduce crop injury, affect dissipation or forapplicator safety . Formulation is also suggested to influence the potential of the active ingredient to contaminate surface waters . Pendimethalin is a mitotic inhibiting herbicide from the dinitroaniline chemistry, it is a selective pre-emergent that ceases seedling growth shortly after germination of susceptible plants . Physico-chemical properties of pendimethalin are presented in Table 1. Pendimethalin has been proposed for use in water-seeded rice, since it controlled herbicide-resistant grass populations and if labeled would provide an additional tool for management over herbicide-resistant grasses in California rice. However, there has been no work characterizing pendimethalin’s behavior in water from a water-seeded rice field. It is hypothesized, based on the physico-chemical properties, that pendimethalin will not persist in surface water, however, product formulation could affect dissipation in water. Therefore, the objectives of this study were to evaluate the dissipation behavior of pendimethalin across three formulations in rice flood water after an application in a water-seeded rice field.

A field study was carried out at the Rice Experiment Station in Biggs, CA . Because of scrupulous quality assurance for each experimental unit to meet regulatory standards, which led to extensive costs associated with the analysis and labor, the study was only conducted in 2021 with three replications. Individual plots were arranged in a randomized complete block design across the field. Soils at the site are characterized as EsquonNeerdobe , silty clay, made up of 27% sand, 39% silt, and 34% clay, with a pH of 5.1, and 2.8% organic matter. Irrigation waters at the research site on average have a pH of 7.81 and electrical conductivity of 0.12 ds/m. Individual 3- m wide by 6-m long plots surrounded by 2.2-m wide shared levees were made to prevent contamination from adjacent treatments. Water temperature, when delivered from the irrigation canal, can average as low as 13°C, and in the field, it is recommended for the water to not be below 18°C for appropriate rice growth and development . Irrigation water was first delivered on June 2, 2021 into a warming field basin, where it circulated before traveling to the field basin with the plots. To move water inside each individual plot, 5-cm diameter by 1.5-cm length single bend aluminum siphon irrigation tubes were placed over the 2.2-m wide levees. The plots were flooded to 4-inch by June 4, 2021 and maintained at that depth for the duration of the study. ‘M-206’ rice was air-seeded at a rate of 170 kg ha-1 onto the field with a standing flood on June 5, 2021.Rice flood water was sampled at 1, 3, 5, 10 and 15 days after treatment application for each plot and replication separately. At each individual plot, a composite water sample was collected with a glass beaker from four areas in each plot near the center and quickly homogenized in a ~1-L plastic container . Then, 3 oz were poured in a 4-oz tight seal jar and placed in storage at 0°C immediately until delivered inside the lab within four hours. For each individual plot, new containers were used to sample each time. In the lab, water samples were cleaned and 50 mL were allocated from the filtered sample and placed in storage at -20°C until analysis. Daily temperature, relative humidity and solar radiation data were obtained from the California Irrigation Management Information System , Biggs, CA weather station number 244 .

Liquid-liquid extraction methods were modified from USEPA . High pressure liquid chromatography tandem mass spectrometry was employed to analyze for residue in water samples. A standard for pendimethalin, were obtained as a reference to quantify residue in samples. The recovery in water samples was on average 79%. See supplementary material for details on method. Data analysis were performed using R v4.1.2 . Linear regression analysis and analysis of variance was used to determine associations on the concentrations across formulations, rates and sampling time with LMERTEST R package . Means separation with Tukey’s honestly significant difference at α=0.05 was then used where appropriate with EMMEANS R package . The data was log transformed to fulfill homogeneityand linearity requirements for a linear regression .There were differences in concentrations recovered from water samples across rates , sampling time , and formulation by sampling time . At 1 DAT sampling, the EC had the highest concentrations at 73.0 parts per billion  averaged over rates . The CS and EC formulations maintained similar concentrations throughout sampling times after the 1 DAT . The GR maintained the greatest concentrations at 10 and 15 DAT compared to the CS and EC . The differences in dissipation across formulations could be attributed to the formulation properties. The EC is constructed of an oil-water-emulsion with organic solvents, drying rack cannabis while the CS encapsulates the active ingredient in layers of water-soluble polymers . As an oilbased formulation, the EC would make pendimethalin persist in suspension on the water at higher concentrations early on because of the inactive carriers being not water soluble. The encapsulating polymers in the CS would allow the compound to be water soluble and extend the amount of time the compound is suspended in water . These characteristics can explain the higher concentrations early on from the EC formulation compared to the other two formulations. GR herbicide formulations tend to have the active ingredient adsorbed to inert material, allowing slow and continuous release of the active ingredient . This characteristic of the GR formulation may help explain the increases of concentration in water three days after the application of the 3.4 kg ha-1 rate . The delayed increase in concentration was rate dependent, however. Similarly, Ngim and Crosby observed formulation affected dissipation of the insecticide fipronil in water-seeded rice, with the granule formulation being most persistent. A GR pendimethalin application onto a water-seeded rice field may need a longer waterholding period than the liquid formulations. Dissipation generally followed first-order kinetics . The GR demonstrated halflives up to 6.9 days. The CS had half-lives three to four days less than GR and the EC had halflives nearly seven days less . The average daily temperature for the duration of the study was 25°C with a low of 16°C and high of 34°C. Daily solar radiation averaged 346 Watts m2 with a low of 341 Watts m2 and high of 366 Watts m2 .

Relative humidity averaged at 50% with a low of 30% and high of 80%. These are the typical conditions during the early rice growing season in California and are important to note as factors that can affect the pendimethalin degradation. Half-lives of pendimethalin in water were reduced in this study probably due to greater degradation occurring in a field environment stimulated by microorganisms, photolysis degradation and partitioning onto organic sediments from the soil . Pendimethalin residue half-lives in water have been previously reported at 12.7 and 13.7 days afteran application of an EC pendimethalin formulation at 0.5 parts per million and 1.0 ppm , respectively, onto irrigation canal water . Degradation pathways can be inferred based on the physico-chemical properties of pendimethalin. The pendimethalin molecule is not high water soluble, non-ionizable and not hydrolyzed in water and possesses a high affinity for organic matter ; therefore, sediment partition is most likely the significant degradation pathway. Partitioning of pendimethalin onto sediment in water/sediment investigations in dark demonstrated to be within 0.4 to 1.6 days for 50% allocation onto sediments . Pendimethalin is moderately volatile and volatilization is an important dissipation pathway in dry and moist soil, however, as soil moisture increases over soil field capacity, volatilization decreases due to lower movement of the vapor phase in wetter soils . Solar radiation was high in the study area and can be a significant degradation pathway. Both photolysis and sediment partitioning are most likely the important pathways of pendimethalin degradation. While this study negates the pendimethalin metabolites, it is important to note there are three metabolites that can form in water . Nevertheless, the pendimethalin residues in the water indicate the importance of holding flood water in the field after an application to allow the herbicide molecule to settle on the soil surface when applied onto a flooded rice field.The US EPA has recorded an observed maximum level of pendimethalin in surface water at 17.6 ppb, probably contaminated by spray drift, and expressed the risk of pendimethalin contaminating surface waters to be less than 2% . While there is no water quality criteria level for pendimethalin, residues of pendimethalin have been observed in surface water tributaries near agricultural regions with concentrations up to 0.02 ppb . Additionally, pendimethalin residues as low as 30.0 ppb in soil have shown to cause injury on tomato , a common crop grown near California rice fields . Despite observed concentrations above these levels from the EC and CS formulations early on, pendimethalin dissipated quickly below levels of concern . Apart from preventing potential herbicide runoff, water-holding periods can be useful for increasing herbicide efficacy. Some pesticides currently used need the water for activation or to evenly distribute in the field and holding water in the field is common practice for California growers when using granule pesticides in rice . The concentrations observed from this study also suggest pendimethalin could benefit from a water-holding period to increase the efficacy when applied onto the flood. However, an increase in efficacy can also develop greater rice crop injury and should be balanced through application rates and timings. The rates used in this study were the typical use rates in dry-seeded rice, which are known to provide adequate weed control. This study did not focus on weed control but ongoing work is examining this aspect to enable efficacious and safe use of pendimethalin for water-seeded rice. Pendimethalin did not persist to levels of concern in the surface-water of a water-seeded rice field and was detected at very low concentrations, in general. The results from this study can assist regulatory agencies and registrants in articulating a water-holding period for pendimethalin in water-seeded rice, which can help prevent potential contamination to municipal drinking waters, prevent damage to downstream high value crops and ensure efficacious use, therefore, promoting responsible stewardship of chemical use in California rice.In the lab, water samples were cleaned from debris by periodically pouring the 90 mL sample through a funnel with filter paper of 11 µm Whatman 1 of 90 mm diameter outlining the inside the funnel’s wall.