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.