The small grain crop amendments used in these tests were air-dried and milled straw residues of mature wheat , barley , oats and triticale plants which were collected from recently harvested fields in the Fresno, California area. All amendments were uniformly incorporated into soil at a concentration of 1.9% , the approximate quantity of stubble residues which would be incorporated into the plow layer of field soil at the end of a cropping cycle in commercial production. Effects of treatments on M. incognita were estimated after 7 days’ incubation using a bioassay procedure, in which treated soil was aired in open plastic bags for 24 h following incubation in bioreactors, then placed in two 10-cm-diam pots per replication. A single plant of a susceptible tomato cultivar was transplanted into each pot and the pots were maintained in a glasshouse at 30°C maximum and 21°C minimum. After 6 weeks’ growth, root systems were excised, washed and an arbitrary gall rating was made by visual examination . Sensitivity of S. rolfsii to treatments was determined by retrieving and surface-disinfesting the 30 sclerotia from each container, then incubating them on potato dextrose agar plates to determine germinability. Effects on P. ultimum were determined by sampling soil from containers, then air-drying and spreading aliquots on a selective agar medium, as described previously . Fungal colonies were identified and enumerated after incubation.Three field studies were conducted at the University of California, Kearney Research and Extension Center, ca 12.5 km southeast of Fresno, California .
The soil type was Hanford fine sandy loam , drying marijuana and experiments were done in conjunction with vegetable crop transplant experiments, according to methodology described previously. In Experiment 1 , raised planting beds, 102 cm between centers, were formed and pre-planting fertilizer was incorporated to a depth of 15 cm. Six rows of sudex were planted on each bed on 6 August, at the rate of 13.6 kg ha−1 . Two drip irrigation lines were placed on the surface of each planting bed and water was applied to field capacity. Following seedling stand establishment, liquid fertilizer was added through the drip system. The green sudex plants were shredded when they reached a height of ca 1.4 m on 24 September, using a tractor-drawn mower. Plots were sprayed 10 days later with a 2% solution of glyphosate, using a CO2-powered backpack sprayer, to preclude plant regrowth. The shredded sudex plants formed a dense mulch layer over the surface of the planting beds. Four replications were prepared for each of the following treatments: plants shredded and sprayed with glyphosate, then left on the soil surface; plants shredded, sprayed, then incorporated in soil with a rototiller; and fallow control . Each plot was 1 m long. All plots were regularly drip irrigated and fertilized every 2 weeks. Vegetable plants were transplanted into the beds as described by Summers et al. . Following the vegetable crop harvests, all weeds from a 0.093 m2 area were harvested on 29 November, dried and weighed. Methodology for Experiments 2 and 3 was similar to that of Experiment 1, but used 152-cm-wide planting beds. Sudex seed was planted on 1 May and 26 July . The plants from Experiment 2 were shredded as before, on 27 June, when green plants were ca 1.8 m tall; and those from Experiment 3 were shredded on 5 September, when plants were ca 2 m tall. Plant stubble regrowth was sprayed 10 days later with 2% glyphosate herbicide, as described above.
Plots in both Experiments 2 and 3 were arranged in randomized complete block design, with six replications each of the following treatments: plants shredded, sprayed with glyphosate, and shoot residues left on the soil surface; shredded plants raked off and placed on a fallow bed that had not previously been seeded with sudex plants shredded, sprayed, then shredded stems manually removed from plots, leaving only the roots plus 3–5 cm of surface stubble ; plants shredded, sprayed, then shoots and roots incorporated into soil with a tractor mounted rototiller, 14 days after shredding; and fallow control . Each plot was 4.5 m long by 1.5 m wide. Prior to planting, two drip tape lines were placed on the surface of each planting bed. Irrigation water and liquid fertilizer were applied weekly through the drip system as before. Forty-three days after sudex shredding , the total weed biomass in Experiment 2 was determined, following tomato harvest, by removing the weeds from 1 m2 of soil surface, selected at random in each plot. Weeds were placed in paper bags, dried for 5 days at 70°C and then weighed. These procedures were repeated 50 days and 57 days after sudex shredding. Similarly, the weed biomass from Experiment 3 was collected on 25 October and 20 December for the first planting, and on 21 December for the second planting, then dried and weighed as before.The laboratory bioreactor experiments conducted in this study showed that amendment of phytopathogen infested field soil, with certain poaceous crop residues at a constant temperature of 23°C, provided mostly significant levels of deleterious activity against M. incognita, P. ultimum and S. rolfsii. Of the amendments tested at 23°C, ‘Yolo’ wheat provided the most consistent activity against M. incognita and S. rolfsii, and triticale the least. When incubated at the higher temperature regimen of 38o /27°C , all amendments demonstrated consistently increased deleterious activity that was statistically indistinguishable, except that triticale residues had the least biocidal activity against P. ultimum. As with most bioactive chemicals, including synthetic pesticides and brassicaceous and alliaceous plant residues , deleterious activity of the tested poaceous amendments increased with increasing soil temperature.
These very consistent results across the various plant taxa tested indicate that, as expected, the volatility and concentration of bioactive chemicals released during plant residue decomposition in soil increases with increasing temperature . Also, given the statistically significant interactions of the [amendment] and [temperature] factorial effects tested with S. rolfsii and P. ultimum , the targeted phytopathogens were shown to incur more harm from simultaneous application of the dissimilar stress sources, i.e., chemical and temperature, than from either stress source alone. These results confirmed the utility of combining plant residue soil amendments with soil heating techniques for improved soil disinfestation. It is commonly assumed that in vitro, bench-top experiments, such as those conducted in bioreactors inthis study, often give more dramatic results than those obtained under similar conditions in a natural environment. Therefore, the field experiments conducted with the sorghum-sudangrass cover crop plants and residues provided strong support for our laboratory study. Over the course of three experiments conducted at different times during the year, the sudex plant residues, particularly the shoot portions, clearly gave a dramatic and long-lasting reduction of both summer and winter annual weed species, regardless of seasonal climate. The deleterious effects were apparent on both broadleaved weeds and grasses, and were similar to those on vegetable transplants grown in the same plots . The consistent and significant inhibition of targeted organisms by certain cultivated grasses demonstrated in these experiments is not surprising, given many previous reports of lethal or inhibitory effects against various plant pests . A portion of the below-ground, inhibitory activity of grass family members results from production of toxic, decomposition compounds . However, the effects of allelochemicals in growing plants can be potent and long-lasting . It is generally accepted that allelopathy results from the release of specific chemicals that influence such factors as seed germination, radicle and hypocotyl elongation, and seedling growth and development . The effect of such chemicals gradually diminishes as they are leached below the root zone by irrigation or rainfall , or microbially degraded following tissue disruption and/or burial in soil . This phenomenon of enhanced degradation was clearly demonstrated by the comparatively milder, and less persistent, deleterious activity of sudex residues when shredded and/or soil-incorporated in the present study. The broad-spectrum, cannabis drying rack biocidal/biostatic activity demonstrated by these agronomically important, poaceous plants presents a challenge to those wishing to maximize their promising pest control potential, without having to worry about subsequent crop phytotoxicity. Clearly, there is a range of allelopathic or biotoxic activity in poaceous plants, and presumably across cultivars of specific taxa as well. Phytotoxicity to subsequent crops may not always occur, or be noticeable in the field if it does. In crop rotations with long fallow periods, or with satisfactory leaching, even rotations into highly bioactive varieties, such as sudex, may present no problems for subsequent crops. In the case of a planned fallow, it may be advantageous to begin the crop-free period with a bioactive, poaceous crop to discourage weed growth and/or reduce populations of soilborne nematodes or fungal propagules. Future efforts to enhance agricultural sustainability will include development of strategies for crop multitasking, i.e., maximizing uses for both harvested and non-harvested portions . Biological and physical alternatives to synthetic chemical soil disinfestation can be important components of crop multi-tasking. However, alternatives that will be attractive to growers for implementation must provide predictable and relatively rapid reductions of pathogen/pest inocula, at reasonable cost, and without harming subsequent crops or soil quality. Development of guidelines for the pesticidal use of cultivated grasses, such as those tested here, as well as other members of the Poaceae, can contribute to these goals.Since the 1930s, the growth of road transportation has exploded in the United States, and about 83% of the land in the United States is within one kilometer of any type of road . Road infrastructure facilitates economic growth and human-social interaction; however, it is also well-known that roads facilitate invasive and weedy plant dispersal .
In most cases, roads act as barriers and filters to block most wildlife species’ movement because of the frequent and intense disturbances; however, roads also act as habitats and conduits for invasive and weedy species adapted to the disturbances . Plants along the roads usually raise safety concerns, such as blocking traffic and affecting drivers’ vision. Additionally, massive road networks have now become a part of the ecosystem. Since roads connect both natural landscapes and agricultural fields, roadside vegetation management is important to minimize the impacts of invasive and weedy species on those ecosystems. In 2021, a study demonstrated that invasive species, including plants and animals, cost North America $1.26 trillion from 1960 to 2017 and $26 billion annually in the 2010s . In 2008, according to the survey by California Invasive Species Council, the economic impact of invasive plant species was estimated at $82 million annually . In addition to the economic impact and safety concerns, invasive and weedy plants can cause ecological damages to the ecosystem, including reduction of species biodiversity, changes inwildfire regime, and water pollution . A changing fire regime can affect local species and human society to a large extent, so managing roadside invasive species is inevitable and necessary. For example, Bromus tectorum and Andropogon gayanu can increase fire intensity and frequency by adding more fuel load to the ecosystem . In general, a successful plant species establishment consists of several factors. Reichard & Hamilton suggested that weedy traits, especially reproductive traits, are the most dominant factors in determining a successful invasion of an ecosystem. In contrast, Parendes & Jones argued that environmental factors or human interferences also partially explained the invasive species’ distribution and dispersal. In the case of roadside habitats, human activities and the environment are also as prominent as weedy traits. The frequent disturbances along the road, including road maintenance and building, create long-lasting bare soil for species colonization. Parendes & Jones reported that locations with intensive disturbances and adequate resources have higher frequencies of exotic plant species. Mills et al. measured and examined the soil properties in two segments from two different highways in Nebraska. The data indicated that roadside soil contained high sodium concentration and high soil compaction, which inhibited the growth of native vegetation . A meta analysis demonstrated that invasive species have higher plasticity than co-occurring native species under many environmental stresses . Besides disturbances, water resources are another factor that increases the invasibility of roadside environments. Roads are considered water collectors and rainfall storage. The surface of roads is designed to have a few degrees of incline to drain excess water during rainfall, and there will be a slide slope with more incline and a ditch to collect the water . A study measured the soil moisture of the ditch along forest roads in 36 samples, averaging 53.5% .