GENETIC ANALYSIS OF RESISTANCE OF COWPEA [Vigna unguiculata (L.) Walp.] TO Alectra vogelii Benth. IN BURKINA FASO
ABSTRACT
Alectra vogelii is a parasitic weed of the Schrophulariaceae family causing severe damage to cowpea in Southern, Eastern and Western Africa. Total yield losses occur in heavily infested fields. In Burkina Faso, A. vogelii has recently been observed to heavily infest fields causing damage. The objectives of the study were to: (i) identify farmers’ varietal preferences and assess their opinion on the parasitic weed A. vogelii; (ii) identify sources of resistance to A. vogelii in cowpea germplasm from Burkina Faso; (iii) determine gene action involved in cowpea resistance to A. vogelii and (iv) determine the genetic inheritance and the allelic relationship of the resistance genes. A participatory rural appraisal (PRA) was conducted in three districts to assess farmers’ awareness of the parasitic weed A. vogelii, and identify their preferred varietal traits. One hundred and twenty cowpea genotypes from INERA-Kamboinse germplasm were screened in a screen house for their response to three ecotypes of A. vogelii. Twenty five genotypes were selected from this list for field assessment. A gene action study was conducted with a 9x9 half diallel cross, the inheritance and the allelic relationship of the resistance genes were studied. The PRA revealed that farmers from western Burkina Faso knew A. vogelii as a parasitic weed better than those from eastern Burkina Faso. A. vogelii was ranked the third most important biotic constraint to cowpea production after Striga gesnerioides and insects. Farmers’ preferred traits were large sized grains with rough texture and white colour. Erect and prostrate short cycle cowpea varieties were also preferred varietal types. Varieties B301, Komcalle, IT98K-205-8, IT99K-573-2-1 and KVX414-22-2 showed resistance to all the three ecotypes of Alectra; these varieties except B301 also possess farmers’ preferred grain qualities. Djouroum local and Pouytenga-9 were the best landraces for Alectra-resistance. Significant genotype by environment interaction with differential reactions of cowpea genotypes to A. vogelii ecotypes were observed. The combining ability study showed significant effects of general combining ability (GCA) for days to flowering, days to Alectra shoot emergence, 100 seeds weight and severity of Alectra damage. Specific combining ability (SCA) was not significant for days to Alectra shoot emergence and 100 seeds weight. Additive gene effects were more important for all the traits except days to flowering suggesting that good selection progress can be achieved with proper choice of parents. Non additive gene effects were also involved in the expression of some parameters. The inheritance and allelic relationship study involved two Alectra resistante genotypes (B301 and KVx414-22-2). The segregation ratios showed that two dominant genes confer the resistance in B301. A single dominant gene is responsible for the resistance in KVx414-22-2. The three genes were all independent. The symbol Rav4 is proposed for the gene for resistance to Alectra vogelii in KVx414-22-2.