Resistance to cowpea severe mosaic virus, determined by three dosage dependent genes in Vigna unguiculata L. Walp

P1, P2, F1, F2, BC1 and BC2 generations of four resistant × susceptible crosses and three resistant × resistant crosses of cowpea (Vigna unguiculata L. Walp) were screened for resistance to cowpea severe mosaic virus (CPSMV), in an insect protected screen house. The segregation ratio, at maturity, showed a 63 susceptible : 1 resistant ratio in the F2 generation indicating that resistance is governed by three major genes. The backcross tests and the F3 test confirmed this. The intermediate levels of symptoms observed in the F1 generation and the progression of symptom expression in the F2 generation suggested that resistance is gene dosage dependent. The genetic basis of immunity, tolerance, high level of resistance is discussed based on the three gene model. Delayed symptom expression was evident in genotypes with one or two susceptible alleles. The implications of delayed expression of symptoms in selection and breeding for resistance to CPSMV are discussed.[PUBLICATION ABSTRACT]