Management research: Jed Colquhoun, Oregon State University. (541)-737-8868.
Field survey: Tim Butler, Oregon Department of Agriculture. (503)-986-4621.
Seed cleaning: Sabry Elias, Oregon State University Seed Laboratory. (541)-737-4799.
Clover broomrape is a parasitic plant that attaches and draws water and nutrients from several plant species, including red clover. Clover broomrape is a federally listed noxious weed that has quarantine significance to many of Oregon's trading partners. Since 1923, there have been six reports of clover broomrape in Oregon. In 1998, clover broomrape was identified in a single field. In 2000, clover broomrape was found in 15 fields in Northwestern Oregon.
Clover broomrape is an obligate parasite (the presence of a host plant species is required - clover broomrape will not survive on its own) that lacks chlorophyll. It reproduces and spreads only by seed. One broomrape plant produces up to 500,000 dust-like seeds that may be dispersed by wind, machinery, contaminated seed crops, animals, or clothing. Flowering plants that are hand-pulled may still mature and produce viable seed. Seed can remain dormant in the soil for 10 years or more. After germination, clover broomrape attaches to and penetrates the root of the host plant, disrupting nutrient and water transport in the host root system. Parasitism by clover broomrape may reduce host crop yield, and, in heavy infestations, may kill the host plant.
While host parasitism in Oregon is currently limited to red clover, greenhouse studies have identified several other weed and crop species that can be clover broomrape hosts. Clover broomrape germinated and attached to nasturtium, arrowleaf clover, subterranean clover, white clover, sweet pea, lettuce, snap bean, sunflower, and carrot. Weedy host species included spotted catsear (also known as false dandelion, Hypochaeris radicata).
Research is currently underway to identify methods to control clover broomrape in red clover with a variety of strategies, including herbicides and fumigation. In the meantime, the best method to control clover broomrape is to prevent its spread to new fields.
Good sanitation practices, including equipment and footwear, are needed to prevent the spread of clover broomrape. Quaternary ammonium, when in direct contact, will kill clover broomrape seed. This product should be mixed as a 1% solution in water and applied with a hand sprayer to ALL equipment that has entered the contaminated field, including tractor tires, tillage equipment, mowers, swathers, combines, etc. Quaternary ammonium is a strong germicidal detergent that should be handled with caution. As with agricultural pesticides, follow the labeled instructions and MSDS information to minimize potential health risks. Quaternary ammonium is manufactured under a variety of brand names, including Process NPD Germicidal Detergent (Merck & Company Inc.) and Coverage Spray Disinfectant (Calgon Vestal Laboratories). This product is available from commercial cleaning supply dealers.
Management of emerged and flowering clover broomrape
Clover broomrape produces viable seed about three weeks after emergence. Therefore, management of emerged clover broomrape with herbicides is difficult given the short window of opportunity. Heavily infested portions of fields should not be harvested. In preliminary research, glyphosate (2 quarts product/A of 4 lb/gal glyphosate) has effectively controlled clover broomrape and may be used as a non-selective spot treatment in heavily infested areas. Burning of an infested site will destroy plants and seed on the soil surface. In lightly infested areas, remove broomrape plants prior to seed dispersal. Place plants in a sealed bag and dispose of by burning.
Harvesting clover seed in infested fields
In fields with light infestations, harvest those fields last and after any emerged broomrape plants have been removed. Although emerged broomrape plants may have been removed, treat the seed lot as if it was contaminated by clover broomrape seed. Do not mix seed from an infested field with non-infested seed. Place seed in a covered, leak-proof container for transport and storage. Clean potentially infested seed last. Clean all equipment used in handling seed as if it were used in the field (see cleaning of equipment).
Long-term management of infested fields
Clover broomrape management research is currently underway and additional information will be updated as available. In the meantime, the long-term strategy should focus on prevention of spread to new fields, and management of clover broomrape seedbanks in infested fields. Sanitation practices as outlined above will reduce the potential for new infestations. Rotation to false host crops will deplete the clover broomrape seed in the soil by stimulating seed germination without the presence of a host plant. Preliminary research suggests that wheat and sweet corn will stimulate clover broomrape seed germination without host attachment. Fumigation has also been reported to reduce clover broomrape seedbanks, but is currently cost-prohibitive (application and product costs range from $1000 to $1500 per acre) and may cause significant red clover injury.
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| Microscopic view of clover broomrape germinated and attached to lettuce root. | Red clover field heavily infested with clover broomrape. |
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| Emerged clover broomrape flower stalks in red clover. |
