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.

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.

Four synthetic multiring analogs of strigol (0.1 mM) were effective as germination stimulants of dormant, unchilled seeds of shepherdspurse (Capsella bursa-pastoris (L.) Medik. # CAPBP). The effect was concentration dependent and exceeded that produced by 0.1 mM gibberellic acid (GA). The analog treatments resulted in normal, unpigmented seedlings after 7 days, whereas GA treatment led to distorted, nongeotropic, yellow seedlings. The strigol analogs were also more effective in inducing shepherdspurse germination than 10 mM nitrate or 5.0 mM thiourea (ineffective in the dark) and 5.0 mM thiourea plus 4.5 mM ascorbate (germination stimulated, but seedling growth, inhibited). Chilling at 4 C for 11 days reduced the effectiveness of the strigol analogs, but not that of gibberellic acid and thiourea plus ascorbate. Ascorbate acts synergistically with thiourea in promoting shepherdspurse germination. Strigol and epistrigol had no effect on either chilled or unchilled shepherdspurse seeds. The stimulation of dormant shepherdspurse seeds by the synthetic strigol analogs is the first demonstration that these compounds have bioregulatory activity in dormant seeds of genera other than Striga.

Increasing plant density of grain sorghum [Sorghum bicolor (L.) Moench.] from 57100 to 114300 plants/ha had no significant effect on grain yields, but significantly increased Striga hermonthica (Del.) ‘Benth’ # STRHE shoot yields by 49% in S. hermonthica infested soil of the Sudan. Addition of urea at 43, 86, and 172 kg N/ha increased grain yields by 14, 25, and 45%, respectively, above the control and S. hermonthica shoot yields were reduced by 14, 21, and 35%, respectively, below the control. Although average grain yields in 1981 were 77% less than in 1980, S. hermonthica shoot yields were significantly increased by about sixfold. Grain sorghum yields of pure stands were not consistent over both years. In 1980, the cultivar ‘Ariana’ outyielded ‘Tetron’, but in 1981 the reverse occurred. However, in both years, the 50:50 mixture of both cultivars gave intermediate grain yields. While pure stands of Tetron’ sustained the lowest infestations of S. hermonthica and ‘Ariana’ had the greatest, the mixture had intermediate infestations which were on average 31% less than those of ‘Ariana’. Thus, the results suggest that the cultivation of equiproportional mixtures of relatively resistant and susceptible grain sorghum varieties, coupled with the use of high rates of nitrogenous fertilizer and wider inter- and intra-row spacing, may be a better cultural practice under dense striga infestations.

Twelve analogs of the strigol D-ring (butenolides) were evaluated at 0.1 mM as germination regulators of 26 weed and crop seeds. In general, the butenolides either inhibited or did not affect monocot germination, although the hydroxybutenolide stimulated germination of dormant cheat and nondormant perennial ryegrass and sorghum seeds. The response rate for dicots was greater but still primarily inhibitory. Dormant lettuce seeds (both light-sensitive and light-insensitive), Palmer amaranth, and redroot pigweed showed several significant responses, both inhibitory and stimulatory. Some of the butenolides that had not been previously assayed against witchweed were tested as 10-4 to 10-11 M dilution series. The allyloxybutenolide demonstrated activity comparable to strigol (30 to 60% germination at 10-6 to 10-11 M); the sec-butyloxybutenolide was somewhat less active (20 to 40% germination) but still effective at 10-9 M. In general, the butenolide analogs of strigol were less effective in stimulating seed germination than the multiring analogs.

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.

The witchweeds, members of the genus Striga, are noxious and persistent pests in farmers' fields and serious constraints to crop productivity throughout Africa, India, and Southeast Asia. Among the primary hosts for Striga are the major cereals (maize, sorghum, rice, and millet) and grain legumes (cowpea) that are important food staples worldwide. The negative impact of parasitic plants on crop productivity increases globally each year, and their potential for affecting domestic agriculture looms larger as the movement of seed resources expands on a global scale. At the present time there is a limited understanding of how Striga and other parasitic plants select a suitable host and overcome the innate defense responses of the host in order to complete their life-cycle. In the grasses most reported resistance to Striga appears to be polygenic with a large genotype by environment interaction. In contrast, resistance to S. gesnerioides in cowpea is conferred by single dominant genes functioning in a race-specific manner suggesting that a gene-for-gene mechanism similar to effector-triggered immunity (ETI) described in other host–pathogen interactions is likely operating in these parasite-host associations. A hallmark of ETI is the direct or indirect recognition of parasite-derived avirulence (Avr) factors and other effectors that interfere with plant innate immunity by host sensors (or R proteins) leading to activation of defense responses. The recent cloning and functional characterization of a race-specific R gene from cowpea encoding a canonical coiled-coil (CC)-nucleotide binding site (NBS)-leucine-rich repeat (LRR) type R-protein opens the door for further exploration of the mechanism of host resistance and provides a focal point for studies aimed at uncovering the molecular and genetic factors underlying parasite virulence and host selection. The potential for the development of novel strategies for parasite control and eradication based on parasite virulence factors is discussed.