Effets de Dix (10) Variétés de Sésame (Sesamum indicum (L.)) en Association avec Le Niébé (Vigna unguiculata (L.) Walp) dans la Lutte Contre Striga Gesnerioïdes (Willd.) Vatke. au Niger

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 witchweed Striga gesnerioides and the cultivated cowpea: A geographical and historical analysis of their West African distribution points to the prevalence of agro-ecological factors and the parasite’s multilocal evolution potential

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.

Rhizobium inoculants suppress emergence of the weed Striga gesnerioides in cowpea

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 < 0.0001) and emergence (P = 0.005), and cowpea shoot growth (P = 0.016). Cowpea nodulated poorly across host lines, 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 < 0.0001). The G x N interaction was significant in emerged Striga plants (P < 0.0001). Resistant genotypes were free of emerged 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 < 0.0001). Yield response to inoculation was most obvious for resistant genotypes inoculated with the strain IRJ2180A (P = 0.0043). The integrated use of 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.

EVALUATION MULTI-SITE DE LEFFET DU PARASITISME AU STRIGA GESNERIOIDES (WILLD) VATKE SUR LES TRAITS FONCTIONNELS DES VARIETES DE NIEBE

Cowpea(Vigna unguiculata (L.) Walp) is a major legume crop in West African farming systems. Although cowpea is a drought tolerant crop, its production level is very low due to biotic constraints such as Strigagesnerioides parasitism. The present study aims to assess the effect of S. gesnerioides(Willd.) Vatkeon photosynthesis efficiency of cowpea varieties. Multilocal trialwas carried out rural areas during the 2017 rainy season to assess the number of Striga emergence likely to cause a change in photosynthetic efficiency on a panel of 26 cowpea varieties and to determine tolerant varieties. The observations were made on six experimental sites along a south-north climatic gradient representative ofMaradi region agro-ecological zones.The results showed a marked decrease of relative chlorophyll content when the number of visible Strigaemergence exceeded five per plant. Likewise, the LEF * PAR saturation curve is higher when the plants showed no visible attack of Striga. The Striga emergence have declined CRC in varieties CS001 and CS127 and led in rapid saturation in variety CS133.

Assessment of Striga gesnerioides (Willd.) Resistance and Genetic Characterization of Forty-Six Cowpea (Vigna unguiculata (L.) Walp.) Genotypes in Ghana

The parasitic weed, Striga gesnerioides, imposes physiological stress on cowpea (Vigna unguiculata (L.) Walp.) resulting in significant yield loss in the regions of northern Ghana. This warranted identification of resistant cowpeas for sustainable production. The current work aim was to identify Striga-resistant cowpea genotypes and assess their genetic relatedness. Forty-six (46) cowpea genotypes were screened in pots for their reaction to Striga samples obtained from the upper east, upper west, and northern regions of Ghana and validated with C42-2B and 61R-M2 markers involving DNA amplification by PCR assay. Sixteen polymorphic SSR primer pairs were used to assess genetic relatedness among 46 cowpea genotypes. Data were analyzed with PowerMarker V. 3.25 and a dendrogram was generated with MEGA 4. On the whole, 65.2% of the cowpea genotypes had stable resistance to S. gesnerioides from the regions of northern Ghana and 34.8% were susceptible. The C42-2B marker resolved as a single DNA band of 280 bp with segregation efficiency of 80% and 61R-M2 marker as double DNA bands of 320 bp and 380 bp with segregation efficiency of 60% associated with Striga resistance. Sixteen (16) polymorphic SSR primers distinguished all 46 cowpea genotypes into three clusters. Gene diversity ranged from 0.04 to 0.49 with an average of 0.29. The average allele frequency is 0.78, with a mean genetic diversity of 0.29. Polymorphism information content (PIC) varied from 0.08 to 1.00 with an average of 0.55. Therefore, cowpeas with Striga resistance and other desirable traits can be evaluated and released as varieties for farmers to cultivate.