BREEDING FOR APHID (Aphis craccivora Koch) RESISTANCE IN COWPEA [Vigna unguiculata L. (Walp.)] GENOTYPES IN BURKINA FASO
Cowpea is a leguminous crop that produces dry grains used for human and animal nutrition. It is one of the most important staple crops grown in the three ecological zones of Burkina Faso and could play an important role in the fight against rural poverty. It has potential of becoming industrial crop. However, several biotic and abiotic stresses impose serious threats to cowpea production. Aphis Craccivora Koch. has been described as one of the most important pests of cowpea. It is also a major vector of plants viruses. This study was conducted with the aim to contribute to an increased cowpea productivity by developing varieties with resistance to aphids. A participatory rural appraisal (PRA) was conducted to determine cowpea production constraints and management practices adopted by cowpea growers in controlling them. The PRA used a mixed method consisting in collecting both qualitative and quantitative data. To do so, a focus group discussions (FGD) was undertaken in each of the three agro-ecological zones and a survey in five (5) other districts. A total of 481 cowpea growers were interviewed, among them 219 women and 262 men. Results showed that water scarcity, damages by insect pests, lack of training and challenges in the marketing of cowpea production are currently the main constraints to cowpea production. Results also established that cowpea growers are aware of aphids as cowpea pest but have limited knowledge on the biology and incidence of pests in their fields. Local resistant varieties locations have been recorded and a prospection would help to collect these ecotypes as additional sources of resistance to aphids. Investigation was also made to determine the inheritance pattern of aphids' resistance in cowpea genotypes and to identify Single Sequence Repeat (SSR) markers linked to genes involved in the resistance to aphids in cowpea. Thus, F2 populations from two couples of parents (Tiligre x KVx295-124-2-99 and Tiligre x IT97K-556-6) were screened with aphids to determine the mode of inheritance to aphids resistance in cowpea.
The two populations were also screened with SSR to identify polymorphic markers and then markers linked to genes involved in cowpea resistance to aphids. Results showed that the ability to survive under aphids infestation in Kvx295-2-124-99 and IT97K556-6 is characterized by oligogenic inheritance. Two duplicated genes were involved in the ability of Kvx295-2-124-99 and IT97K556-6 to survive under aphids infestation. The results also showed that the two genes are not allelic. Markers MA61 and MA70 were found to be linked to one of the genes involved in the ability to survive under aphids infestation respectively in Kvx295-2-124-99 and IT97K556-6. Half diallel crosses and combining ability study of three traits (the survival rate, the degree of infestation and the yield loss under aphids stress) were undertaken in two locations: Kamboinse in the sudano- Sahelian and Farakoba in the sudanian zone to determine the nature of genes action in the resistance to aphids in cowpea. Results showed that the ability to survive under aphids pressure is under control of non-additive genes action with partial dominance and presence of epistasis. In the expression of the degree of infestation, both additive and non-additive genes were involved with over dominance. Additive and non-additive genes action were involved in yield loss control with presence of over dominance and without epistasis. Results from the two locations established that these two environments were not significantly different. However, the genotype by environment interaction (GxE), the GCA by environment interaction (GCAxE) and the SCA by environment interaction (SCAxE) were significant. Excepted the yield loss that had high to moderate broad and narrow sense heritability, the two remaining traits (the survival rate and the degree of infestation) recorded moderate to low broad and narrow sense heritability.