Broad-based root-knot nematode resistance identified in cowpea gene-pool two

Cowpea (Vigna unguiculata L. Walp) is an affordable source of protein and strategic legume crop for food security in Africa and other developing regions; however, damage from infection by root-knot nematodes (RKN) suppresses cowpea yield. The deployment through breeding of resistance gene Rk in cowpea cultivars has provided protection to cowpea growers worldwide for many years. However, occurrence of more aggressive nematode isolates threatens the effectiveness of this monogenic resistance. A cowpea germplasm collection of 48 genotypes representing the cowpea gene-pool from Eastern and Southern Africa (cowpea has two major pools of genetic resources – Western Africa and Eastern/Southern Africa) was screened in replicated experiments under field, greenhouse and controlled-growth conditions to identify resistance to RKN, to determine the spectrum of resistance to RKN, the relative virulence (VI) among RKN species and isolates, and the relationship between root-galling (RG) and egg-mass production (EM). Analysis of variance of data for RG and EM per root system identified seven genotypes with broad-based resistance to Meloidogyne javanica (Mj), avirulent (Avr-Mi), and virulent (Mi) M. incognita isolates. Two of the 48 genotypes exhibited specific resistance to both Mi isolates. Most of the genotypes were resistant to Avr-Mi indicating predominance of Rk gene in the collection. Based on RG data, both Mj (VI = 50%) and Mi (VI = 42%) were fourfold more virulent than Avr-Mi (VI = 12%). Resistant genotypes had more effective resistance than the Rk-based resistance in cowpea genotype CB46 against Mj and Mi. Root-galling was correlated across isolates (Avr-Mi/Mj: r = 0.72; Mi/Mj: r = 0.98), and RG was correlated with EM (r = 0.60), indicating resistance to RG and EM is under control by the same genetic factors. These new sources of resistance identified in cowpea gene-pool two provide valuable target traits for breeders to improve cowpea production on RKN-infested fields.