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Striga gesnerioides (Willd.) Vatke Bas.: Buchnera gesnerioides Willd. Perennial herb, parasite of roots on various dicot species, in wooded savannah, grassy savannah, open forest, and thicket. Uses: medicinal., Published as part of Monteiro, Francisca, Costa, Esperança Da, Kissanga, Raquel, Costa, José Carlos & Catarino, Luís, 2022, An annotated checklist of the vascular flora of Quiçama National Park, Angola, pp. 1-67 in Phytotaxa 557 (1) on page 49, DOI: 10.11646/phytotaxa.557.1.1, http://zenodo.org/record/6985699

Striga gesnerioides is one of the major constraints to cowpea production in West Africa. In Burkina Faso, Striga is a national phenomenon despite the control measures proposed by research. This study aims to collect farmers' perceptions of S. gesnerioides. Thus, a survey was conducted among cowpea producers in 15 localities across the four agroclimatic zones of Burkina Faso. Data collected were related to farmers' ability to distinguish S. gesnerioides from Striga hermonthica, their perceptions of yield losses, local control strategies they use and the social utility of Striga. The study revealed a good knowledge of the pest by farmers. Farmers identified S. gesnerioides by its small height, its bushy growth habit and the haustorium that distinguishes it. They attributed 20 to 100% yield losses to the effects of the parasite. The most widely used indigenous control method is by manually uprooting the plants. Despite of its harmfulness to cowpeas, S. gesnerioides is used in traditional medicine to treat diabetes and curing animal’s wounds and urinary problems. Key words: Farmers’ perceptions, ethnobotanical survey, Striga gesnerioides, use.

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.

International audience; Striga is among the most noxious parasitic plant genera causing yield losses to staple crops in the semiarid and subhumid areas of Africa and Asia. Understanding the factors that affect the current distribution of Striga infestation is critical for developing integrated control strategies. This study attempts to elucidate the soil properties that are related to and that affect the plot-level infestation and land-scape level distribution of Striga gesnerioides (Willd.) Vatke in Niger. At the plot scale, 27 cowpea varieties were sown in randomised blocks of 30 plots in 14 fields, and their soils were sampled and analysed. S. gesnerioides infestations were recorded in the fields over 2 years. These records were obtained with georeferenced data, and soil properties were extracted from global databases. At the plot scale, the variance in S. gesnerioides infestations were affected by available phosphorus (29.17%), organic carbon (16.61%), pH (15.15%), nitrogen (13.13%), and sand contents (10.12%) and varied among the cowpea varieties. This led to the identification of cowpea varieties with consistent high levels of resistance (varieties: CS030 and CS095). At the landscape scale, soil moisture is the key factor explaining 48.6% of S. gesnerioides distribution, followed by silt content (28.1%), sand content (13.7%), and bulk density (6.2%). These results showed a clear dichotomy in the interactions between the occurrence of S. gesnerioides and soil properties from the plot to landscape scales. These results are valuable for understanding the ecology of S. gesnerioides and can support the development of integrated control strategies.

The increasing severity of Striga gesnerioides attacks on cowpea across West Africa has been related to its prolificity, seed mobility and longevity, and adaptation to aridity, in a context of agricultural intensification. To understand this fast extension, we analyzed (1) the distributions of the crop and the witchweed with ecological niche modeling and multivariate climate analysis, and (2) the chronological information available from collections and the literature. The ecoclimatic envelope of S. gesnerioides attacks on cowpea is the same as on wild hosts. Consistently, the modeled distribution of cowpea infestations is closely similar to the simple superposition of the parasite model (involving all hosts) and the crop model. Striga gesnerioides infestations are restricted to the driest component of the cultivated cowpea ecoclimatic niche, corresponding to the Sahelian and Sudano-Sahelian belts and the Dahomey gap. Thus, the parasite distribution, determined by its own requirements, does not constrain cowpea cultivation under Guinean climates. The spatial and temporal distributions of S. gesnerioides field infestations are consistent with an earlier impact on cowpea production in eastern West Africa, related itself to a similar trend in cowpea cultivation intensification from Niger, Nigeria and Benin to Burkina Faso and Ghana. Mali and Senegal were affected later, and literature reports of Senegalese strains of S. gesnerioides from the wild developing virulence on cowpea offer a model for the diffusion of witchweed parasitism by multilocal evolution, through host-driven selection, instead of epidemic diffusion. A contrario, in Côte d’Ivoire, cowpea is much less widespread, so the parasite has remained confined to the wild compartment. Thus, both historical and ecogeographic analyses refute the vision of S. gesnerioides as an invader. Instead, they point to the increasing importance and intensification of the crop, and the consequent loss of biodiversity, as the main drivers of the extension and diversification of its crop-specific strains.

Cowpea is a grain legume of major importance in sub-Saharan Africa where it is cultivated by smallholder farmers on poor soils and production is often constrained by the parasitic weed Striga gesnerioides. Experiments were conducted to assess the potential of rhizobium inoculation in mitigating Striga infestation and increasing cowpea productivity. We tested under basal P application and artificial S. gesnerioides inoculation the impact of cowpea genotypes (G) (nine Striga-resistant and 11 Striga–susceptible genotypes) and bradyrhizobium inoculation (N) (two bradyrhizobium strains USDA3384 and IRJ2180A, and uninoculated control) on Striga dynamics and cowpea yield. Additional treatments included N supplied as urea (with and without), and no input (i.e., soil inherent N and P) that served as negative check. A first experiment was carried out in potted sterile soils in the screen house excluding addition of N-fertilizers. Significant G x N interactions were observed in counts of nodule (P = 0.012), Striga attachment (P Striga counts were the lowest for resistant varieties with no emerged plants. Rhizobial inoculants depressed Striga counts with consistent differences found across cowpea genotypes. Inoculation with IRJ2180A performed the best against Striga attachment in resistant genotypes, and its emergence in susceptible genotypes. In the field trial, nodule numbers were lowest in cowpea without inputs (P Striga plants (P Striga while for susceptible ones, Striga emergence was the highest without any input addition. Significant G x N interaction was observed in cowpea grain yield (P Striga-resistant cowpea lines and elite bradyrhizobium inoculant under moderate application of P-based fertilizer could be a promising approach for mitigating Striga infestation and increasing productivity.

Cultural and biological control measures have failed to provide satisfactory management of Striga gesnerioides, a major biotic constraint to cowpea production in sub-Saharan Africa. Field trials were conducted to evaluate the efficacy of imazaquin seed treatment (at 0.00, 0.06, and 0.24 kg a.i. ha−1) on Striga gesnerioides control and the growth of seven cowpea genotypes (Achishiru, IT97K-1263, IT97K-390-2, IT98K-133-1-1, TVU-1283, TVU-1542, and TVU-1908) at the Teaching and Research Farm of the Faculty of Agriculture of Bayero University, Kano, Nigeria, during the rainy seasons of 2010 and 2011. Treating the seeds with imazaquin at 0.24 kg a.i. ha−1 produced the lowest number of striga stand−1. Across herbicide rates, Achishiru had most S. gesnerioides; IT97K-1263 had the fewest. The results indicated that the untreated control recorded the highest number. The untreated TVU-1283 produced the fewest branches; Achishiru produced the least dry matter while IT97K-1263 had the most branches and dry matter in 2010 and 2011 rainy seasons. Application of 0.24 kg a.i. ha−1 to IT97K-390-2 consistently produced the highest branches and dry matter; Achishiru produced the fewest branches and least dry matter in 2010 and 2011 seasons. Treating IT97K-390-2, IT98K-133-1-1, and TVU-1542 with imazaquin at 0.24 kg a.i. ha−1 provided an effective means of managing striga in cowpea and enhanced the growth characters of these genotypes.

Striga gesnerioïdes est une plante parasite qui cause des dégâts importants sur le niébé et les méthodes de lutte testées restent insatisfaisantes. L’étude vise à mettre au point, une référence de technique d’association sésame-niébé pour contribuer à lutter efficacement contre Striga gesnerioïdes. À cet effet, un dispositif en Split-Plot est utilisé sur un terrain naturellement infesté par Striga gesnerioïdes. Trois comptages à des périodes différentes (42, 56 et 70 jours après semis) du nombre de plants de Striga gesnerioïdes émergés par poquet de niébé ont été réalisés. Ainsi, en association dans le même poquet (sésame + niébé), les variétés de sésame EF 146, HC 110, ICN 130, HB 168, Vgr 156 et EF 147 sont efficaces pour atténuer l’émergence de striga. En alternance de poquets (niébé et sésame), les variétés de sésame EF 146, HC 110, ICN 137, EF 147 et 38-1-7 sont efficaces contre striga. Et les variétés de sésame EF 146, Vgr 156, HC 108 et 38-1-7 ont montré leurs efficacités en alternance de lignes avec le niébé. EF 146 est la seule variété de sésame efficace dans toutes les trois mesures de protections. Ces résultats pourraient être vulgarisés au niveau des producteurs de niébé pour booster sa production au niveau national, voir mondial. Striga gesnerioïdes is a parasitic plant that causes significant damage to cowpea and the control methods tested remain unsatisfactory. The study aims to develop a technical reference of sesame-cowpea association to contribute to the effective control of Striga gesnerioïdes. For this purpose, a Split-Plot device is used on land naturally infested by Striga gesnerioïdes. Three counts at different periods (42, 56 and 70 days) of the number of emerged Striga gesnerioïdes plants per cowpea were made. Thus, in combination in the same plot (sesame + cowpea), the sesame varieties EF 146,HC 110, ICN 130, HB 168, Vgr 156 and EF 147 are effective in mitigating Striga emergence. In alternating rows (cowpea and sesame), the sesame varieties EF 146, HC 110, ICN 137, EF 147 and 38-1-7 were effective against striga. And the sesame varieties EF 146, Vgr 156, HC 108 and 38-1-7 showed their efficacy in alternating rows with cowpea. EF 146 is the only sesame variety that is effective in all three protective measures. These results could be disseminated to cowpea producers to boost its production at the national and even global level.

The parasitic weed, Striga gesnerioides , is a major constraint to cowpea production in sub-Saharan Africa. It causes significant yield reductions of cowpea, especially in dry areas. The objective of this study was to evaluate the response of 80 genotypes to Striga gesnerioides under natural infestation. The results showed significant variations in the resistance of cowpea lines to Striga; lines IT93K-693-2, IT99K-573-1-1 and IT98K-205-8 being free from Striga infestation; while lines 2491-171, 2472-154 and Suvita-2 supported few Striga shoots. The other lines supported more and varied numbers of emerged Striga shoots. The reduction of yield due to Striga infestation was more pronounced for the susceptible genotypes as compared to the resistant and tolerant lines. The high level of resistance observed in some breeding lines can be exploited in breeding cowpea for resistance to Striga . Principal Component Analysis (PCA) and clustering grouped the genotypes in three main clusters as follow: (i) high yielding and tolerant to Striga (ii) moderate yielding and resistant (iii) low-yielding and susceptible. Key words: Parasitic weed, Striga resistance

Background: The parasitic plant Striga gesnerioides (Willd) Vatke is becoming a threat to cowpea production in the three northern regions of Ghana (Derived Savanna, Southern Guinea Savanna and Northern Guinea Savanna. The objective of this study was to determine the extent of yield loss due to Striga in F3 progenies and their parental lines. A total of twelve cowpea genotypes were evaluated to ascertain the level of damage by S. gesnerioides. Results: The results indicated varietal differences with respect to Striga susceptibility in the genotypes studied. The susceptible varieties (Asomdwee and Hewale) showed an array of emerged Striga shoots. The F3 progenies (s52, s37, s147, s272) also exhibited the same trend. The resistance genotypes (IT99K-573-1-1 and GH3684), however, were totally devoid of Striga shoots. Similarly, some of the F3 progenies (r246, r286, r282 and r69) were completely without Striga shoot. Striga height was higher on susceptible genotypes than those that are moderately resistance. The results obtained from this study revealed a percentage reduction in pods per plant range between 45.12 and 49.53 %; grain yield reduction ranged between 78.22 and 87.17% on susceptible genotypes which indicated that Striga had greater influence on yield. Fodder yield was also significantly reduced between 70.59 and 73.03% in susceptible genotypes. There was a strong negative correlation (r = -0.724) between grain yield and days to Striga emergence. A significant negative correlation (r = -0.762) was also found between Striga shoot per plant and grain yield. Conclusion: Striga infestation reduced pods per plant to 49%, grain yield loss to 87% and fodder yield loss to 73%. This study has revealed potential F3 genotypes resistant to S. gesnerioides under pot experiment. The implication is that if susceptible genotypes are grown the farmer stand the chance of loosen everything to the parasite.

Witch weed (Striga gesnerioides) is a root parasitic weed of tobacco and cowpea in Sub- Saharan Africa. It is very difficult to control and host plant resistance is the most promising effective method affordable to small-scale farmers. This study was done to screen local tobacco genotypes for tolerance and resistance to Striga gesnerioides infection. Laboratory screening experiments were conducted during the 2016/17 season using fifteen local tobacco genotypes. The experiments were done using an Agar gel technique at the University of Zimbabwe, with treatments replicated four times in a Randomised Complete Block Design (RCBD). All tobacco genotypes significantly (p

Cowpea (Vigna unguiculata L.) is the most important food and forage legume in the African Sahel providing essential income and protein nutrition to millions of farmers. While most cowpea cultivars are susceptible to the root parasitic weed Striga gesnerioides, cultivar B301 is resistant to all known parasite races except SG4z. When challenged by races SG4 and SG3, the roots of B301 display a hypersensitive response (HR) at the site of parasite attachment followed by death of the invading parasite. In contrast, no visible response occurs in B301 roots parasitized by SG4z and the parasite successfully penetrates the host root cortex, forms vascular connections, and grows to maturity. This study seeks to clarify the molecular mechanism underlying the virulence of S. gesnerioides overcoming host resistance. In order to identify potential parasite components that specifically suppress or elicit host defense responses, I have carried out transcriptome profiling using high throughput RNA-seq analysis on the tissues isolated from parasite haustoria during compatible and incompatible response of resistant (B301) and susceptible (Blackeye) cowpea cultivars with three races of S. gesnerioides (SG3, SG4 and SG4z). Comparative transcriptomics and in silico analysis revealed the first S. gesnerioides transcriptome and several candidate effectors that display differential expression between individual races, that may contribute to race-specific virulence. To clarify the molecular mechanism underlying the hypervirulence of SG4z, I molecularly characterized a leucine-rich receptor (LRR)-protein kinase (PK) homolog dubbed SHR4z (Suppressor of Host Resistance 4z) that is highly expressed in SG4z haustoria and secreted into the host root. I found that overexpression of SHR4z in transgenic B301 roots leads to suppression of HR elicitation and loss of host innate immunity by targeting a host BTB-BACK domain containing ubiquitin E3 ligase homolog (VuPOB1). Subsequent silencing VuPOB1 expression in transgenic B301 roots lowers the frequency of hypersensitive response (HR) while overexpression of VuPOB1 results in decreased parasitism by SG4z suggesting VuPOB1 functions as a positive regulator of HR and plant innate immunity. Finally, to explore host effect on Striga virulence, I examined global expression changes of the parasite during compatible and incompatible interactions with cowpea. I found that resistant hosts trend to induce the expression of disease resistant genes in SG3 and SG4 relative to susceptible hosts but suppress the expression of genes responsible for development. Comparative analysis on the expression profile of SG4z revealed that, even though SG4z triggers the same response from B301 and Blackeye, several candidate effectors still display differential expression at the interactions with different hosts. These candidates may contribute to the suppression of host resistance by SG4z. Overall, this thesis study provides new insight into the role of secreted effectors as part of the strategy used by parasitic weeds to overcome host immunity and complete their life-cycles. It could potentially contribute to the development of novel strategies for controlling Striga and other parasitic weeds thereby enhancing crop productivity and food security globally.

There have been reports of breakdown of striga resistance in previously resistant cowpea cultivars in Burkina Faso. This could be attributed to new striga races emerging or to an increase in the aggressiveness of current striga races. Therefore, cowpea genotypes were evaluated in fields infested with S. gesnerioides at three striga hot spots in Burkina Faso and in pots under artificial infestation with striga races SR 1, SR 5 and SR Kp to identify new, adapted and striga-resistant sources. Cowpea genotypes showed differential reactions for striga resistance over sites and for striga races in pot experiments, indicating differences in the races involved, and SR Kp was reported as a new race. Resistant sources conferring site-specific or multiple striga-race resistance were identified. Genotypes 58-57, Sanga 2, IT84S-2049, IT98K-205-8, IT93K-693-2, KVx771-10, KVx775-33-2, KVx61-1, Gorom local, Mouride and Melakh conferred resistance to all three striga races. These genotypes are potential donor parents for breeding new, adapted and striga-resistant genotypes. Cowpea landraces including Moussa local and Niaogo local with farmers' preferred traits were susceptible and need improvement for striga resistance.

Striga gesnerioides is a root parasitic weed of economic significance to cowpea (Vigna unguiculata) crops in Western Africa. Seeds of the parasite germinate in response to cowpea root exudates. Germination stimulants for the seeds were isolated from the hydroponic culture filtrate of cowpea, and their structures were unambiguously determined as (-)-(3aR,4R,8bR,2'R)-ent-2'-epi-orobanchol and (+)-(3aR,4R,8bR,2'R)-ent-2'-epi-orobanchyl acetate, on the basis of mass, CD, and (1)H NMR spectra; optical rotatory power; and chromatographic behavior on HPLC. The alcohol was first isolated and identified from the cowpea root exudates, and the acetate may be the same compound that had been previously isolated from the exudates and designated as alectrol. Identity of the stimulants produced by cowpea to those produced by red clover (Trifolium pratense) was confirmed.

Le niebe est une denree de base en Afrique subsaharienne. L’objectif de ce travail est d’identifier des lignees a haut rendement, tolerantes a la secheresse et resistantes a la race 1 de S. gesnerioides . Des lignees issues de la selection recurrente et des varietes temoins ont ete evaluees au Centre Ouest (condition maximale d’humidite) et au Nord (condition de stress hydrique) du Burkina Faso en 2014. Tous les genotypes ont montre une resistance a la race 1 de Striga et plusieurs lignees ont reussi a produire plus de 1300 kg/ha sur les deux sites. L’Analyse en Composantes Principales realisee sur la base des caracteres quantitatifs a permis d’identifier deux groupes. Le groupe 1 est compose par des individus precoces a rendement modere (Nafi, Gorom local, KVX˗61˗1 et Komcalle). Le groupe 2 est constitue par les genotypes extra precoces a rendement modere (C2˗6˗3, C2˗18˗2, C2˗27˗2, IT97K-499-35 et Yiis yande). © 2017 International Formulae Group. All rights reserved. Mots cles: Niebe, Striga gesnerioides , stress hydrique, rendement English Title: Evaluation of 15 genotypes of rain-fed cowpea [ Vigna unguiculata (L.) Walp.] In northern and central-western Burkina Faso English Abstract Cowpea is a staple food in sub-Saharan Africa. The objective of this work is to identify high yielding, drought tolerant and race 1 resistant lines of S. gesnerioides . The breeding lines of including some varieties were evaluated in field trial under drought stressed and non-stressed conditions in 2014. Results show that the breeding lines showed complete resistance to race 1 of Striga and many improved varieties are capable of producing over 1300 kg grain/ha. Biplot displays indicated that the genotypes could be grouped into two categories according to their cycle and yielding ability as indicated below: moderate yielding-early maturing (Nafi, Gorom local, KVX˗61˗1 et Komcalle) and moderate yielding extra-early maturing (C2˗6˗3, C2˗18˗2, C2˗27˗2, IT97K-499-35 et Yiis yande). © 2017 International Formulae Group. All rights reserved. Keywords: Cowpea, Striga gesnerioides , drought, yield

Drought and weed parasite Striga gesnerioides(Willd.)Vatke, are major constraints to cowpea production in sub-Saharan Africa. The purpose of this study was to examine the response of four cowpea (Vigna unguiculata(L.)Walp.)cultivars, VITOCO, VITA5, IT87D-10-10, and IT98K589-2 to water deficit, to a weed parasite S. gesnerioides and to determine the effect of this weed parasite on these cowpea cultivars under water deficit conditions. Pot experiment was conducted in a greenhouse condition under artificial infestation at the Agronomic Experimentation Station of Lomé. The experimental design was a split-split-plot design with three replications of two levels of S. gesnerioides;(presence and absence) in main plot, four water regimes; (periodic watering of 5, 10, 15 and 20 days) in sub-plot, and the cultivars in sub sub-plot. Data were collected on yield components (number of seed per pod, number of pod per plant and seeds weight per plant). Data collected on parasitic plantdate of emergence and its rate of emergence per pot. Water deficit tolerance was evaluated by the rank summation index (RSI).The results showed that the water régimes of 10 and 15 days had significant depressive effect on all yield components. The number of S. gesnerioides emerged per pot ranged from 10.20 (VITOCO) to 48.60 (IT98K589-2). In addition, the infestation of S. gesnerioides showed a significant reduction of all yield components evaluated in all tested cultivars. Therefore, no cultivar proved to be resistant, nor tolerant to S. gesnerioides.The overall rankings of the evaluated cultivars in terms of growth and yield parameters studied revealed that the cultivar IT98K-589-2 showed the best adaptive response in drought condition. The results also revealed that, the significant negative effect of water deficit on the seeds weight per plant is exacerbated by the S. gesnerioides infestation.

Cowpea is an important food crop in the dry savannas of West and Central Africa because of its high protein content. Yields are, however, considerably reduced by the parasitic angiosperm Striga gesnerioides. Field trials over 2 years in two localities in northeast Nigeria evaluated the performance of diverse cowpea varieties under conditions of natural infestation by Striga. Grain yield was higher in Tilla than in Damboa where the higher Striga infestation may be caused by the lower rainfall and the sandy nature of the soils. Two varieties (IT97K-499-35 and IT90K-82-2) were confirmed to be resistant to Striga. Yield gain from IT97K-499-35 over the local variety ranged from 30% in Tilla in the northern Guinea savanna to 126% in Damboa in the Sudan savanna. The local variety Borno Brown and two improved varieties were found to be as susceptible to Striga as the susceptible control, TVX-3236, and therefore may not be recommended for cultivation in areas where Striga infestation is a problem. One variety, rep...

Alectrol was first isolated from root exudates of cowpea (Vigna unguiculata), a genuine host of a root parasitic weed Striga gesnerioides, as a germination stimulant for seeds of the parasite. The proposed structure, an isomer of strigol, was disproven by chemical synthesis. Recently, another structure, namely orobanchyl acetate, was proposed. Surprisingly, however, the synthetic compound having this proposed structure for alectrol was not active in inducing germination of S. gesnerioides seeds although it was active toward seeds of other root parasitic weeds such as S. hermonthica and Orobanche minor. Detailed studies on 1H NMR, mass and CD spectra of naturally occurring alectrol, re-isolated from cowpea root exudates, revealed that the genuine structure of the germination stimulant is not orobanchyl acetate but its stereoisomer ent-2′-epi-orobanchyl acetate. Accordingly, the structure of natural orobanchol was revised to ent-2′-epi-orobanchol 12 years after a tentative structure of orobanchol was proposed. Strict stereochemical requirements of strigolactones for germination induction of S. gesnerioides seeds, authentic samples of synthetic strigolactones and advanced analytical instruments made the structural assignment possible, thus ending a 20 years controversy concerning the true structure of alectrol.

Striga gesnerioides is a root hemiparasite that primarily parasitizes dicotyledonous species, including cowpea ( Vigna unguiculata L.) and other legumes. Based on the differential resistance response of various cultivars, landraces, and breeding lines, it has been proposed that several distinct races of cowpea-parasitic S. gesnerioides exist in West Africa. In this study, we used amplified fragment length polymorphism profile analysis to examine the genetic variability within and among populations of cowpea-parasitic S. gesnerioides within the suspected distribution range of a particular race, and statistical clustering methods to define the phenetic relationships of the various races in West Africa. Our data indicate that genetic variability within and among populations of each of the previously recognized races of cowpea-parasitic S. gesnerioides is extremely low. On the basis of genotypic profile and host differential resistance responses, 2 previously unknown races were identified. Of the 7 races now identifiable, races SG1 (from Burkina Faso) and SG5 (from Cameroon) are the most closely related, and SG4 (from Benin) and SG3 (from Niger/Nigeria) are the most divergent. SG6, a new race of the parasite identified in Senegal, was found to be the most genetically similar to SG4 from Benin. We also demonstrate that a hypervirulent isolate of the S. gesnerioides from Zakpota, in the Republic of Benin, is genotypically distinct from other populations of SG4, thereby warranting designation as a separate race, which we called SG4z. To further support our race classification scheme, we identified a group of molecular markers that effectively discriminate each of the various races. Finally, we show that an isolate (designated SG4i) of the wild legume Indigofera hirsuta –parasitic S. gesnerioides is genetically distinct and significantly diverged from the various races of cowpea-parasitic S. gesnerioides. Our data suggest that both geographic isolation and host-driven selection are critical factors defining race formation in S. gesnerioides in West Africa.

Cowpea (Vigna unguiculata (L.) Walp.) is an important legume crop in subsaharian Africa. However, its cultivation is threatened by the parasitic weed Striga gesnerioides (Willd.) Vatke, which causes considerable yield losses. Striga gesnerioides is an obligate parasite with low photosynthetic activity. Its seeds germinate in response to specific germination stimulants exuded by host roots. After germination, an haustorium is formed through differentiation of the radicular apex. A vascular connection is subsequently established, allowing the parasitic weed to absorb water and nutrients that are essential for its development. Control of S. gesnerioides is difficult to achieve because of the intimate association between the parasitic weed and its host. The use of resistant cowpea cultivars, in combination with appropriate cultural practices, is probably the easiest and most effective method to control S. gesnerioides and achieve long term reduction of its seed bank in the soil.Key words: germination, interaction, control methods, cultivar resistance, Striga gesnerioides, Vigna unguiculata.

Pot and laboratory experiments were performed in order to elucidate the genetics of resistance of three cowpea (Vigna unguiculata (L.) Walp.) resistant sources to two biotypes of the parasitic weed Striga gesnerioides (Willd.) Vatke (witchweed). B301, Suvita-2 and IT82D-849 were crossed to susceptible cultivars. Roots of each plant of parental, F1, F2, BC1 and BC2 populations were washed free of soil and examined closely for S. gesnerioides attachment. Data on resistant and susceptible cowpea plants were analyzed using the chi-square test to ascertain the goodness of fit to different genetic ratios. Segregation patterns indicate monogenic dominant inheritance of resistance to both S. gesnerioides biotypes in B301. Suvita-2, which is susceptible to the biotype from Maradi, Niger, showed segregation patterns that indicate monogenic dominant inheritance of resistance to S. gesnerioides biotype from Cinzana, Mali. Results also indicate monogenic dominant inheritance of resistance to S. gesnerioides biotype from Cinzana in IT82D-849. However, the inheritance of resistance to the biotype from Maradi would be monogenic and recessive in this line. These results suggest that the resistance of all three sources would be easy to transfer to promising breeding lines.