Cowpea witchweed [Striga gesnerioides (Willd.) Vatke] is a primary constraint of cowpea [Vigna unguiculata (L.) Walp.] production in West Africa. Previously, seven S. gesnerioides races were classified based upon host specificity and genotypic profiling. Because race number and distribution are dynamic systems influenced by gene flow, genetic drift, and natural selection, a thorough investigation of S. gesnerioides diversity and the effectiveness of known sources of resistance in cowpea is needed to develop varieties with durable and broad-spectrum Striga resistance. In this study, we screened seven cowpea lines against 58 unique S. gesnerioides populations collected from across nine West African countries. Individuals from 10 S. gesnerioides populations were genotyped with simple sequence repeat (SSR) markers. We identified six races of S. gesnerioides based on their parasitism of the seven cowpea lines with known differential resistance genotypes. No cowpea line was resistant to all 58 Striga populations and none of the Striga populations were able to overcome the resistance of all seven lines. A novel race, SG6, of the parasite collected from Kudu, Nigeria, was found to overcome more cowpea resistance genes than any previously reported race. SSR analysis indicates that Striga populations are highly differentiated and genetic relatedness generally corresponds with geographic proximity rather than their host compatibility. Due to the dearth of broad-spectrum resistance found among Striga-resistant cowpea lines, there exists a need to stack multiple Striga resistance genes in order to confer broad-spectrum and durable resistance.

Field dodder is an important weed in alfalfa grown for seed, and stringent control is required to keep the alfalfa seed from becoming contaminated with dodder seed. Pendimethalin has been the primary tool used to control dodder in alfalfa seed production for more than 25 yr. Flumioxazin was recently registered in alfalfa seed and forage crops, but its activity on field dodder was unknown. Control of field dodder with flumioxazin and pendimethalin was compared in greenhouse trials in a loamy sand soil. The number of emerged dodder seedlings able to twine on a simulated host were counted weekly for a 4-wk period following herbicide application. Flumioxazin applied at the alfalfa field use rate of 0.14 kg ai ha−1 completely controlled dodder over the initial 4-wk period. Dodder either failed to emerge or emerged and died without twining on the simulated host in the 4-wk period. When dodder was reseeded after 4 wk in flumioxazin-treated soil, the number of twined dodder seedlings was reduced by 56% in one trial but was unaffected in a second trial compared with nontreated checks. In the initial 4-wk period following herbicide application, flumioxazin controlled field dodder similar to pendimethalin applied at 2.2 and 4.4 kg ai ha−1 in both trials. However, after additional dodder seed was planted at 4 wk after treatment, dodder was suppressed more by pendimethalin than flumioxazin in the additional 4-wk period. Flumioxazin offers alfalfa seed and forage producers a new mode of action to manage early-emerging field dodder.

Laboratory experiments were carried out under aseptic conditions to study the effect of nitrogen applied as ammonium nitrate at 0, 20,40,60,80 and 100 ppm (wt per vol) in either distilled H2O or modified Hoagland's solution on germination and radicle length of branched broomrape in association with growing seedlings of different crops. As nitrogen rate increased, seed germination and radicle length of branched broomrape decreased linearly in flax, lentil, pepper, tomato, and wheat in modified Hoagland's solution. Similar responses to nitrogen application were found with distilled water. The parasite's seed germination was considerably lower in the media with wheat than with other crops, and lower still in the absence of crop seedings.

The dispersion of ethylene in the soil after application from a backpack ethylene applicator (BPEA) was determined using a bioassay of witchweed [Striga asiatica (L.) Kuntze # STRLU] seed germination. Witchweed seed were buried in packets in ridged seedbeds under field conditions. Ethylene dispersion was better when injected into the furrow than into the ridge top. Injections 30 cm deep were more effective in causing seed germination than injections 15 cm deep. Seed germination induced by ethylene injection into the furrow 30 cm deep was 13% more than that obtained from the ridge top injection 15 cm deep. Seed germination as high as 90% was obtained at soil depths to 75 cm at a horizontal distance of 100 cm from the point of ethylene injection. Ethylene diffused up to 300 cm horizontally and to 75 cm vertically to cause at least 45% seed germination in a field of Lakeland soil (Typic Quartzisamment). The BPEA was found to be an effective application tool in witchweed control operations.

A mathematical model of crenate broomrape populations in broad bean as affected by cropping frequency and planting dates in the absence of crenate broomrape control practices was constructed using previously reported data. In consecutive broad bean cropping, broomrape populations reached a maximum infection severity (D) of 62, 47, and 30 emerged broomrape m−2 for early (mid-October), intermediate (mid-November), and late (mid-December) planting dates, respectively. The maximum D values were reached earlier as planting dates were brought forward, taking from 4 to 6 yr, starting from very low initial infections (D ≤ 0.2 emerged broomrape m−2). If broad bean was cropped every 3 yr, 15, 21, and 27 yr were needed, respectively, according to the model, to reach the maximum D for the three planting dates considered. A sensitivity analysis was conducted to determine the effect of changing the values of the main demographic parameters in broomrape life cycle (germination, attachment, and seed loss) on the output of the model under different management strategies (planting dates and cropping frequency). Generally, an increase in seed attachment and a decrease in seed loss affected broomrape population dynamics. Between the two processes evaluated, the time taken to reach the maximum infection severity (D) was less sensitive than the maximum broomrape population values. Model predictions were validated using results from long-term field studies at the late planting date sown every year. Simulated values generated good predictions (R2 = 0.82).

Newly seeded alfalfa tolerated glyphosate applied from the 8-trifoliolate leaf stage (17 cm tall) to the pre-bud stage (34 cm tall) at 75 or 150 g ae ha-1. Injury symptoms were moderate and temporary. Glyphosate at 300 g ha-1 suppressed alfalfa severely and killed 30% of the plants when applied at the smallest (3-trifoliolate) stage of alfalfa growth. Alfalfa responded similarly to glyphosate and SC-0224.

Witchweed (Striga asiatica Lour. ♯4 STRLU) seed remained viable for 6 yr under open-shelf laboratory conditions and for 14 yr when deep buried under field conditions. Seed kept in normal atmosphere of the laboratory for < 1 through 6 yr had the following germination percentages: 2, 95, 95, 94, 83, 38, and 7. Newly harvested seed will not germinate because of postmaturity ripening requirements. There were no viable seed after 7 yr of shelf storage in the laboratory. Seed buried in the field gave 60 to 90% germination the first season after production. Annual recovery and testing showed a gradual reduction in viability to 10% at year 14 for the deepest buried seed; shallower buried seed expired sooner. No germination occurred thereafter.

In greenhouse studies, the American strain of Striga gesnerioides (Willd.) Vatke, indigo witchweed, was potted with a wide range of potential hosts, including some known to be parasitized by this species elsewhere, and with agricultural and ornamental crops of importance in the southern United States. The host range of the indigo witchweed was very limited and no major crop species were seriously affected. Based on these host studies, it appears that indigo witchweed poses little danger to American agriculture. Other strains of the parasite were likewise shown to be host specific.

Established alfalfa treated with glyphosate at 75 or 150 g ae/ha (rates known to control dodder) during the vegetative or bud stage showed symptoms of narrowed leaves and mildly chlorotic new growth, and yield of one cutting of alfalfa usually was reduced by 10 to 20%. Glyphosate, applied in the vegetative stage, affected the treated foliage more than postharvest regrowth. Applied at bud stage, the regrowth that followed harvest of treated foliage was affected more. Symptoms and effect on yield were greater at 300 g ae/ha, but alfalfa always recovered. Applying glyphosate at 75 or 150 g ae/ha in each of two or three sequential cuttings showed no cumulative effect. Glyphosate and SC-0224 had similar effects on alfalfa and suppressed alfalfa much less than would a severe infestation of dodder.

A heavy, uncontrolled infestation of field dodder and largeseed dodder suppressed newly seeded alfalfa severely and killed many of the plants. Glyphosate at 75 or 150 g ae ha-1, sprayed over the top of the dodder-infested alfalfa, stopped the growth of dodder and did not injure alfalfa significantly. Treated dodder began to grow normally again 2 to 3 wk after treatment. This regrowth injured the alfalfa severely in spite of earlier control. Glyphosate at 75 or 150 g ha-1 applied two or three times controlled dodder effectively and protected the alfalfa. A rate of 40 g ha-1 was not effective.

Ethylene-stimulated germination of witchweed [Striga lutea Lour. = S. asiatica (L.) O. Ktze.] seed first occurred after 13, 10, 6, 3, and 2 days of conditioning in moist sand at day/night temperatures of 20/14, 23/17, 26/20, 29/23, and 32/26 C, respectively. Maximum germination percentages in these regimes were 0.5, 3, 20, 24, and 37%, respectively. No germination occurred at 17/11 C. Witchweed seed survived in sand frozen for 7 weeks at −7 or −15 C and subsequently germinated in response to ethylene or in the presence of corn (Zea mays L., ‘DeKalb B73 × Mo.17H′) roots. The parasites emerged from the soil and flowered when maintained at 29/23 C after the termination of the freezing treatments. In other experiments, witchweed parasitized corn and/or sorghum [Sorghum bicolor (L.) Moench ‘DeKalb E-59+′] root systems in a sandy loam under 26/17, 26/20, 29/20, 32/23, and 32/26 C day/night regimes. Witchweed emerged from the soil with 26/20, 29/20, 32/23, and 32/26 C and flowered with 26/20, 32/23, and 32/26 C day/night regimes. Underground development and subsequent emergence of the parasites were substantially reduced with day/night temperatures below 29/20 C. Winter soil temperatures and growing season soil and air temperatures are unlikely to limit the spread of witchweed into important corn- and sorghum-producing areas of the United States.

Broomrapes are parasitic angiosperms that live on the roots of many important crop plants. We have conducted a series of in vitro experiments to investigate the direct effects of nitrogen fertilizers on broomrape seed germination and early development. These experiments confirm that nitrogen in the ammonium form is more inhibitory than nitrate, but we conclude that it is the elongation of the seedling radicle that is primarily inhibited by ammonium, rather than the seed germination itself. Reductions in radicle length were observed when ammonium solutions were applied during either preconditioning or germination periods. For germinating seeds, exposures to ammonium sulfate of 4 to 8 h (depending on the species) reduced radicle elongation by half, indicating a relatively rapid inhibition. Broomrape species varied in sensitivity to ammonium, with Orobanche aegyptiaca being the most tolerant, followed by O. crenata, O. ramosa, O. cernua, and finally O. minor as compared in the presence of 5 mM ammonium. However, all broomrape species were much more sensitive to ammonium toxicity than were host crop species under similar treatment conditions.

The effects of acid scarification, prechilling, summer irrigation, and root extract stimulant from faba bean plants grown during different seasons on crenate broomrape seed dormancy and germination were determined. Immersion of 6- to 12-mo old seed in 0.5 to 5 N H2SO4 solution from 1 to 20 min increased germination by 20%. Similarly, seed pretreatment at temperatures between −4 and 6 C increased germination, regardless of the pretreatment duration. Results suggest that the hard seed coat is an important dormancy mechanism in newly formed crenate broomrape seeds. Summer irrigation did not substantially change the germination capacity of crenate broomrape seeds. The germination stimulant activity of the root extract from faba bean plants grown during different seasons varied considerably, decreasing in this order: autumn > winter > spring > summer. This suggests an ecological adaptation of crenate broomrape to its main host crop.

Ethylene gas stimulated seed germination of the pearl millet [Pennisetum americanum (L.) K. Schum] strain of Striga hermonthica (Del.) Benth. # STRHE) in two soil types in the North Kordofan Region of Sudan. Soils were the noncracking clay pediplain or ‘Gardud’, in Arabic, and the stabilized sand locally called ‘Qoz’. Ethylene at 1.5 kg ai/ha induced greater seed germination (suicidal seed germination) in the Gardud soil than in the Qoz soil. However, ethylene at 3.0 kg ai/ha was better on Qoz than Gardud soil. The effect of injection depth of ethylene was dependent on soil type. Ethylene was more effective when injected at a 15-cm depth in the Gardud soil and at a 30-cm depth in the Qoz soil. The viable population of S. hermonthica seed was reduced by suicidal seed germination at soil depth to 60 cm and to a horizontal distance of 25 cm from the point of ethylene injection. Ethylene-induced germination reduced seed population by 67 and 34% in Gardud and Qoz soils, respectively. A single injection of ethylene diffused up to 300 cm horizontally and up to 60 cm vertically and caused 12.6 and 8.5% reduction in viable 5. hermonthica seed in Gardud and Qoz soils, respectively. The potential of ethylene gas as an effective tool in striga control operations in a pediplain and stabilized sand soil such as is found in N. Kordofan is dependent upon the availability of the gas and the injection apparatus and the economics of its application.

For effective full-season control of dodder in alfalfa produced for seed, herbicides applied to soil in late winter or early spring should remain active through June. Pendimethalin or prodiamine at 2.2 kg ai ha-1 applied in mid March controlled 97 to 100% of dodder through May and 92 to 100% through June. At 3.4 kg ha-1, both controlled 96 to 100% of dodder through June each year, and control of 95 or 96% persisted through July in 1 of 3 yr. In the greenhouse, trifluralin was only about 1/8 as active against dodder as pendimethalin and prodiamine. In the field, trifluralin at 2.2 kg ai ha-1 controlled dodder satisfactorily through May in 3 of 4 yr and through June in 1 of 4 yr. At 4.5 kg ha-1, trifluralin controlled 99% of dodder through May in each of 3 yr; control was satisfactory through June in 2 of 3 yr. Oryzalin was less effective than the other three dinitroaniline herbicides. A standard herbicide, chlorpropham at 6.7 kg ai ha-1, controlled 97 to 100% of dodder through May, but control usually did not persist through June. Another standard herbicide, DCPA at 11.2 kg ai ha-1, controlled 97 or 98% of dodder through June in 2 of 3 yr. Pendimethalin and prodiamine at 3.4 and 6.7 kg ai ha-1 did not affect the yield or quality of seed produced by established alfalfa. Heavy sprinkler irrigation (15 cm of water) after application did not diminish dodder control from pendimethalin or prodiamine.

Witchweed, [Striga asiatica (L.) Kuntze5 STRLU], an important parasitic weed throughout much of the old world, occurs as a recent (30 yr) introduction in a limited region in North and South Carolina. Reproduction in this population has been shown to occur primarily through self-pollination of flowers. Genetic diversity in two populations from this region was assessed, utilizing starch gel electrophoresis and staining of enzymes. An average of 64 individuals was observed to be monomorphic at an estimated minimum of 32 genetic loci coding 18 enzyme systems. This lack of polymorphism is attributable to the probable founding of the population by one or a few individuals coupled with the predominantly autogamous breeding system.

Based on an herbarium survey, clover broomrape (Orobanche minor J. E. Smith) was found to have an 116 yr history in the United States. It has been collected 98 times in a total of 32 counties in 12 states and the District of Columbia, and it appears to have become established in seaports and along railroads. It most commonly parasitizes red and white clover, but also burley tobacco, carrots, and several species of ornamental shrubs and herbs. Distribution of clover broomrape indicates either a pattern of repeated introductions from foreign sources, or a persistence in areas where the parasite has become established. While not presently a significant threat, the potential for damage to crops of economic importance cannot be ruled out. The flowers are autogamous.

Five broadleaf crops, alfalfa, peanut, soybean, tobacco, and tomato, were tested for their susceptibility to three broomrape species, branched, Egyptian and crenate, in the greenhouse. All five crop species were parasitized by one or more broomrape species. Peanut plants were most susceptible and soybean plants were least susceptible to parasitism by broomrape. Egyptian and branched broomrapes had a wider host range affecting plants in both the Solanaceae and Fabaceae. Crenate broomrape mainly infected plants belonging to Fabaceae, except for the few attachments to tomato and tobacco. Apparently, this is the first report on parasitism of peanut plants by branched broomrape and that of peanut and alfalfa plants by crenate broomrape. Parasitism of soybean plants by any of the broomrape species also was observed for the first time in these experiments. These results indicate that broomrapes, although not a major problem on any of the crops in the United States, could parasitize several major broadleaf crops should the parasite become established.

Glyphosate and SC-0224, at rates that control dodder (75 to 150 g ae ba–1), injured flowers, caused flowers to abscise, and sometimes reduced seed yields when applied to alfalfa when buds or blooms were present. Glyphosate at 75 or 150 g ha–1 did not injure flowers, impair seed set, or reduce yield or quality of seed when applied to alfalfa foliage before the bud stage.

Small broomrape is an annual, parasitic weed that was discovered recently in Oregon's red clover seed production system. Field experiments were conducted in 2002 and 2003 at two locations to evaluate 10 herbicide treatments applied after small broomrape emergence in red clover. Bentazon, bromoxynil, glyphosate, imazamox, imazamox plus bentazon, imazethapyr, MCPA, and pendimethalin were evaluated. Small broomrape density, small broomrape seed viability after treatment, and clover injury and seed yield were quantified. Small broomrape control with imazamox, glyphosate, and imazamox plus bentazon treatments was greater than the nontreated check in both years. However, imazamox and imazamox plus bentazon treatments were the only herbicide treatments that consistently exhibited a high level of crop safety, reduced small broomrape density, and did not reduce red clover yield. Herbicide treatments did not prevent production of viable small broomrape seeds. Future research is needed to develop control options that will prevent red clover yield loss and viable small broomrape seed production when applied before small broomrape emergence.