Small Broomrape Biology and Management in the Pacific Northwest

EM 8884-E • Reprinted August 2008

SMALL BROOMRAPE BIOLOGY AND MANAGEMENT IN THE PACIFIC NORTHWEST

R.D. Lins and J.B. Colquhoun

Small broomrape (Orobanche important rotational crop in grass Flowers usually are purple, minor Sm.) is a parasitic weed seed production systems. but they can be white or yellow. that recently has become trouble- Numerous fl orets are arranged some in red clover seed produc- Description along the stalk and open within tion in Oregon. It was identifi ed Small broomrape is an her- 1 week of plant emergence. Leaves in a single red clover seed produc- baceous, fl eshy annual that is a look like small, triangular scales. tion fi eld in 1998, and the number member of the Orobanchaceae of infestations increased to 15 by (broomrape) family. Its easily Biology and impact 2000 and 22 by 2001. identifi ed orange or brown tuber- In order to germinate, small A small broomrape infestation cles, or “spiders,” are attached broomrape seed must receive a can cause severe crop destruction, to host plant roots (Figure 1). chemical signal from a potential yield loss, and seed contamina- Emerged fl owering stalks are 5 to host plant. Germination does not tion. Small broomrape is a feder- 20 inches tall and completely lack occur without a potential host pres- ally prohibited noxious weed and chlorophyll, often resembling ent, and seeds can remain viable in is restricted from export as a seed purple asparagus (Figure 2). soil for as long as 50 years. contaminant. Red clover is an

Figure 1. Small broomrape tubercles or “spiders” Figure 2. Emerged small broomrape stalks in a red attached to host plant roots. clover seed production ﬁ eld.

R.D. Lins, former graduate research assistant, and Jed Colquhoun, former Extension weed specialist, Oregon State University. After germination, an organ called the haustorium penetrates the root of the host plant. Once attached, nutrients and water are translocated from the host to the small broomrape tubercle. In Oregon, small broomrape plants remain below ground, accumulating resources from host plants, for about 5 months (January to May). Host plants can be severely drained of resources, and crop failure may result. Next, the ﬂ owering stalk emerges, and viable seed is produced in as little as 3 weeks after emergence. The complete life cycle of small broomrape is shown in Figure 3. Small broomrape development and the number of plants attached Figure 3. Small broomrape life cycle. to hosts are strongly associated with the accumulation of growing degree days (GDD). GDD are accumulated heat units that drive many biological processes. GDD can be used to predict parasitic growth stage (Figure 4). A small broomrape plant can produce as many as 1 million dustlike seeds (Figure 5), which are difﬁ cult to remove from crop seed. Small broomrape seed can be transported via water, wind, animals, contaminated crop mate- rial, machinery, etc. In addition to red clover, small broomrape parasitism has been

documented on other Oregon Small broomrape (number per host plant) host per (number broomrape Small

Small broomrape plant size (mm) GDD (°F)

Figure 4. A model for small broomrape size and number per red clover plant in relation to growing degree days (GDD). Figure 5. Red clover seed (left) and small broomrape seed (right). crops such as alfalfa, carrot, small broomrape development agency for rules and restrictions crimson clover, and white clover data can be accessed at http:// before burning. (Table 1). Common weeds such pnwpest.org/cgi-bin/ddmodel. as prickly lettuce, spotted catsear, pl?clm Crop selection Host crops: and wild carrot also can be hosts. Host plants This extensive host range enables Cutting/mowing such as red clover are common small broomrape to produce seed Cutting or mowing emerged cover crops. They can be used in many environments, even when small broomrape plants and as trap crops to reduce the small a nonhost crop is grown on an removing them from the site (fol- broomrape seed bank on infested infested site. This makes manage- lowed by equipment sanitation) sites. Fall-planted host crops stim- ment of host plants essential. effectively limits seed produc- ulate small broomrape germination. However, it is too late in the tion and attachment during winter parasite’s life cycle to prevent and early spring. Killing the cover Management loss of resources from the host crop with herbicides in the spring Small broomrape is a rela- plant. Mowing or cutting small damages the weed directly and tively new and unconventional broomrape more than 4 weeks halts the fl ow of resources to the pest in Oregon agriculture. In after emergence will spread parasite. recent years, much research has mature seed and contaminate Nonhost crops: Planting been conducted to develop effec- equipment. nonhost crops effectively limits tive best management practices small broomrape seed production. (BMPs). These practices include Manual removal However, seed will persist in the sanitation, cutting or mowing, Pulling plants is effective only soil if it is not signaled to germi- manual removal, burning, and if fl owering stalks are removed nate, and control of weedy hosts herbicide applications. from the site. Uprooted fl owering in nonhost crops is critical. stalks can produce viable seed. Prevention/early detection False host crops: Certain plant Small broomrape seed remains Burning species, called false hosts, release viable in the soil for many years; A well-timed burn effectively exudates that stimulate parasitic therefore, prevention of introduc- limits seed production, but it does germination but not attachment. tion to new sites is the best long- not prevent belowground growth Parasites not attached to host roots term control method. Movement or damage to the host plant, nor die because they have no source of small broomrape seed in crop does it signifi cantly reduce the of nutrients and water. Several seed and on farming implements amount of viable seed in the soil. common Oregon crops, such as is a common way to start new Consult the appropriate regulatory fescue and wheat, are false hosts populations. Best management practices for prevention include: Table 1. Small broomrape hosts, nonhosts, and false hosts in Oregon. The • Planting certifi ed weed-free approximate percentage of small broomrape germination induced by false hosts crop seed in greenhouse experiments is shown in parentheses. • Disinfecting machinery, tools, Host plants Nonhost plants False host plants and workers before leaving infested sites Alfalfa Curly dock Barley (2) • Scouting potentially infested Arrowleaf clover Sugarbeet Bentgrass (8) fi elds regularly in the spring Carrot Corn, fi eld and sweet (9, 10) based on an accumulated GDD Celery Fescue, fi ne and tall (1, 11) model. Small broomrape is Crimson clover Flax (40) easily identifi ed in the fi eld. Lettuce Oat (7) Attachment to host plant Prickly lettuce Orchardgrass (9) roots begins at approximately Red clover Ryegrass, annual (3) 800 GDD, and emergence Spotted catsear Ryegrass, perennial (1) begins at approximately White clover Snap bean (16) 2,000 GDD. Local accumu- Wild carrot Sugar pea (19) lated GDD and associated Wheat (70) for small broomrape. They can GDD reduce the small broomrape seed bank while producing a market- able commodity. Growth chamber and green- Small broomrape emergence house experiments have shown that most winter wheat variet- ies grown in Oregon (except ‘Connie’) deplete the small Herbicide application window broomrape soil seed bank (by as much as 70 percent). In addition, red clover grown in infested plots previously planted to wheat Small broomrape showed less small broomrape attachment attachment than did red clover in plots where no false host had been planted. Wheat also can be intercropped with red clover. This system allows wheat to be harvested Figure 6. Growing degree days (GDD) in three red clover growing seasons in a in the red clover establishment fi eld near Portland, Oregon based on soil temperature 2 inches under the soil sur- year, while reducing the potential face. Vertical and horizontal lines link the observation date and GDD when small parasite infestation. The following broomrape was observed at each stage. year, red clover seed, free of small broomrape, can be harvested. demonstrated that when herbi- shoot emergence occurs at approxi- cides active on small broomrape mately 2,000 GDD. Thus, the window Herbicides are applied to host plant foli- for herbicide application to host plants Recent research has identifi ed age, they are translocated to the is approximately 1,000 to 1,900 GDD. herbicide options for removal of parasite, causing injury and death. small broomrape from red clover. Soil-applied herbicides have lim- These products are in the herbi- Glossary ited activity on small broomrape. cide registration process. Consult Exudates—Compounds given off A predictive model for small the latest edition of the Pacifi c by plant roots. broomrape parasitism in red clo- Northwest Weed Management Growing degree days (GDD)—Heat ver has been developed to assist Handbook for current options. units accumulated from January 1 with herbicide application timing Glyphosate (several trade each year. GDD are calculated (Figure 6). The model is based on names) effectively controlled daily as: the strong relationship between small broomrape in noncrop and max daily temp (°F) – min daily temp (°F) GDD and parasite size that was – 32 cover crop removal applications. 2 observed in climatic conditions Glyphosate is a nonselective Base temperature is 32°F for realistic to Oregon. GDD can herbicide that injures or kills most small broomrape GDD. accurately predict stage of small plant species, so it cannot be used Haustorium—A specialized rootlike broomrape development, while in red clover. organ used by parasitic plants to the calendar date of developmen- Glyphosate and other tested draw water and nutrients from host tal stages varies from year to year. herbicides were most effective plants. According to the model, small when applied after parasitic Tubercle or spider—Underground broomrape attachment occurs at attachment, but before fl ower small broomrape growth form. approximately 800 GDD, and stalk emergence. Research has

© 2005 Oregon State University. This publication was produced and distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Extension work is a cooperative program of Oregon State University, the U.S. Department of Agriculture, and Oregon counties. Oregon State University Extension Service offers educational programs, activities, and materials—without discrimination based on race, color, religion, sex, sexual orientation, national origin, age, marital status, disability, or disabled veteran or Vietnam-era veteran status. Oregon State University Extension Service is an Equal Opportunity Employer. Published January 2005, reprinted August 2008.