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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 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.

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.

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.

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.

Background Cowpea, Vigna unguiculata L. Walp., is one of the most important food and forage legumes in the semi-arid tropics. While most domesticated forms of cowpea are susceptible to the root parasitic weed Striga gesnerioides, several cultivars have been identified that show race-specific resistance. Cowpea cultivar B301 contains the RSG3-301 gene for resistance to S. gesnerioides race SG3, but is susceptible to race SG4z. When challenged by SG3, roots of cultivar B301 develop a strong resistance response characterized by a hypersensitive reaction and cell death at the site of parasite attachment. In contrast, no visible response occurs in B301 roots parasitized by SG4z. Results Gene expression in the roots of the cowpea cultivar B301 during compatible (susceptible) and incompatible (resistant) interactions with S. gesnerioides races SG4z and SG3, respectively, were investigated at the early (6 days post-inoculation (dpi)) and late (13 dpi) stages of the resistance response using a Nimblegen custom design cowpea microarray. A total of 111 genes were differentially expressed in B301 roots at 6 dpi; this number increased to 2102 genes at 13 dpi. At 13 dpi, a total of 1944 genes were differentially expressed during compatible (susceptible) interactions of B301 with SG4z. Genes and pathways involved in signal transduction, programmed cell death and apoptosis, and defense response to biotic and abiotic stress were differentially expressed in the early resistance response; at the later time point, enrichment was primarily for defense-related gene expression, and genes encoding components of lignifications and secondary wall formation. In compatible interactions (B301 – SG4z), multiple defense pathways were repressed, including those involved in lignin biosynthesis and secondary cell wall modifications, while cellular transport processes for nitrogen and sulfur were increased. Conclusion Distinct changes in global gene expression profiles occur in host roots following successful and unsuccessful attempted parasitism by Striga. Induction of specific defense related genes and pathways defines components of a unique resistance mechanism. Some genes and pathways up-regulated in the host resistance response to SG3 are repressed in the susceptible interactions, suggesting that the parasite is targeting specific components of the host’s defense. These results add to our understanding of plant-parasite interactions and the evolution of resistance to parasitic weeds.

A large-scale continuous system for recovery of root exudates from millet and cowpea was developed and tested extensively. Large quantities of germination stimulants were exuded into the medium flowing through the plant trays, which could be demonstrated by germination of seeds of three species of angiospermous parasitic plants, Alectra vogelii, Striga gesneriodes, and witchweed. Recovery of root exudates was affected by adsorption on the macroreticular polymer resin XAD-4 followed by desorption with methanol. Furthermore, indications were found that germination inhibitors are released into the medium. Subsequent testing confirmed the quantitative release of the germination stimulants as well as a large number of other compounds. Nomencla- ture: Alectra vogelii Benth.; cowpea, Vigna unguiculata (L.) Walp. 'Saunders Upright'; millet, Sorghum vulgare (L.) 'Haygrazer'; Striga gesnerioides (Willd.) Vatke #3 STRGE; witchweed, Striga asiatica (L.) Kuntze # STRLU.

An in vitro growth system was used to determine the virulence of two samples of Striga gesnerioides from Zakpota in southern Benin. Cowpea variety B301, previously considered resistant to all races of S. gesnerioides, was susceptible to both samples of the parasite. Two other cowpea varieties, 58–57 and IT81D-994, were totally resistant. Resistance in 58–57 was associated with a hypersensitive necrosis of infected roots, whilst IT81D-994 supported production of small S. gesnerioides tubercles with stems which failed to develop. Striga gesnerioides from southern Benin is the fourth race of the parasite to be identified, and the first with virulence on variety B301. The implications for breeding cowpeas with resistance to S. gesnerioides are discussed.

A germination stimulant, alectrol, for the seeds of the angiospermous root parasites Alectra vogelii Benth. and Striga gesnerioides (Willd.) Vatke has been isolated from root exudates of its genuine host plant Vigna unguiculata Walp cv Saunders Upright. Its spectroscopic data lead to a chemical structure closely related to (+)-strigol. The compound is more active than strigol in stimulating seeds of the respective parasites, Alectra vogelii and Striga gesnerioides.

Resistant cowpea varieties to Striga gesnerioides were grown on striga sick plots at Maiduguri, Northern Nigeria, in order to check their response to S. gesnerioides in various regions in West Africa. Under field conditions all the varieties including highly resistant varieties were all attacked by Striga at both 9 and 12 weeks after sowing. However, B-301, Sutiva-2, IT82D-479-1 and IT82D-849 varieties had significantly very low Striga infestation compared with KVX 183-1 variety. These varieties also show higher cowpea yield compared with other highly infested varieties. But the pot experiment shows that B-301 variety is highly-resistant to 5. gesnerioides as no Striga germination was observed. Therefore, it is strongly believed that under field conditions Merrimia emarginata a weed host of S. gesnerioides which was common on the experimental site might have contributed to the presence of S. gesnerioides in some of the highly resistant varieties such as B-301.

Summary: An in-vitro growth system for investigating the behaviour of S. gesnerioides on cowpea has been developed. The roots of young cowpea plants were spread over glass fibre filter paper in a shallow plastic tray, and a known number of germinated Striga seeds were placed on the cowpea root surface. This allowed the infection process of the parasite to be quantified. Good access to the host-parasite association enabled the entire infection process to be easily monitored and tissue removed for cytological studies. Observations revealed for the first time that contact of S. gesnerioides radicles with host roots stimulated the development of radicular hairs and swelling of the tip of these radicles. Furthermore, penetration of cowpea roots by Striga was shown to be localized and to cause very little damage to the host roots. Swollen parasite tubercles developed on the surface of the roots only after the host stele had been penetrated by invading Striga cells. This growth system is suitable for in-vestigating the mechanisms of host resistance. and also for rapidly screening cowpea germplasm for new sources of resistance to the Parasite. Resistant cowpea cultivars offer the best potential solution for control of this Parasite. Resume: Un systeme de culture in vitro pour etudier le comportement de S. gesnerioides vis a vis du niebe a ete mis au point. Les racincs de jeunes plantes de la dolique de C. sont repandre au dessus d'un papier filtre en fibres de verre dans un plateau peu profond de matiere plastique et un nombre connu de graines de Striga germees est place sur la surface de racines du niebe. Ceci permet de quantifier le schema d'infeslalion par le parasite. Le bon acces a la relation hote-parasite permet que le processus entier d'infection soil aisement surveille et les tissus preleves pour des etudes cytologiqucs. Ces observations ont montre pour la premiere fois que le contact des radicules de S. gesnerioides avec les racines holes stimulc le developpement d'un chevelu radiculaire et un renflement du bout de ces radicules. Ensuite la penetralion des racines du niebe par le Striga apparail localisee et cause de tres petits dommages aux racines holes. Les tubercules renfles du parasile ne se developpent a la surface des racines qu'apres que le centre de l'hote a ete penetre par les cellules envahissanles de Striga. Ce systeme de culture est approprie pour l'etude des mecamsmes de resistance de I'hote et pour cribler rapidement des plasmidcs du niebe comme nouvelles sources de resistance au parasite. Les varietes resistantes du niebe sont la meilleure solution pour lutter contre ce parasite. Zusammenfassung: Es wurde ein Pflanzenwuchssystem zur in-vitro-Untersuchung des Verhaltens von Striga gesnerioides an der Kuhbohne entwickelt. Die Wurzeln junger Kuhbohnen-Pflanzen wurden in einer flachen Plastikschale auf Glasfaserfilterpapier ausgelegt, und eine bestimmte Anzahl gekeimter Striga-Samen wurde auf die Oberflache der Wurzeln gelegt. Auf diese Weise lies sich der Infektionsprozes quantifizieren. Durch den einfachen Zugang zu den Wirt-Parasit-Verbindungen konnte der gesamte Infektionsprozes leicht beobachtet und Gewebe fur zytologische Untersuchungen entnommen werden. Es konnte zum ersten Mai beobachtet werden, das durch den Kontakt der Keimwur-zeln von Striga gesnerioides mit den Wirts-wurzeln die Entwicklung von Wurzelhaaren und an den Keimwurzein ein Anschwellen der Spitze stimuliert wurde. Auserdem zeigte sich. das beim Durchdringen der Kuhbohnen-Wurzeln durch Striga an den Wirtswurzeln nur ein sehr kleiner Schaden verursacht wird. Die gesch-wollenen Tuberkeln des Schmarotzers auf der Wurzeloberflache entwickelten sich erst, nachdem der Zentralzylinder der Wirtswurzeln von den Striga-Haustorien durchwachsen worden war. Dieses Pflanzenwuchssystem erlaubt die angemessene Untersuchung des Mechanismus der Resistenz der Wirtspflanze und eine schnelles Screening resistenten Zell-materials der Kuhbohne zur Gewinnung neuen resistenten Materials gegen diese parasitische Pflanze. Resistente Kuhbohnen-Sorten bieten die beste Voraussetzung fur die Bekampfung dieses Schmarotzers.

Significant efforts have been made to develop cowpea (Vigna unguiculata (L.) Walp.) varieties resistant to Striga gesnerioides in Burkina Faso. Despite these efforts, the resistant genotypes developed still express differential responses to Striga gesnerioides in different zones of the country. This suggests existence of intraspecific variability within the parasite. The objective of this study was to assess the intraspecific variability of cowpea genotypes to Striga gesnerioides infection in Burkina Faso. Ten cowpea varieties were screened over two consecutive years, under artificial infestation with 30 ecotypes of Striga seeds at Kamboinsé research station, in a screenhouse in Burkina Faso. Cowpea varieties used included Moussa local, Komsaré and KVx404-8-1, which are susceptible to all Striga ecotypes; and varieties B301, IT93K-693-2 and IT82D-849, which are free from all Striga ecotypes infestation. Cowpea varieties Tiligré, 524B, local Gorom and Niizwè had specific reactions depending on the ecotypes. The study highlights the existence of varietal specificities according to the geographical origin of Striga gesnerioides seeds. The structuring of the intraspecific diversity showed five biotypes, of which three were clearly identified as SG1, SG5 and SG Kp races; and two biotypes could not be identified. Although this study did not allow for a clear determination of the racial affiliation of the two new biotypes, it offers the possibility of developing new strategies to control Striga by focusing on the selection of resistant varieties based on regional specificities of Striga races in each agricultural zone.