INHERITANCE IN SOYBEAN OF RESISTANT AND NECROTIC REACTIONS TO SOYBEAN MOSAIC-VIRUS STRAINS

Five soybean [Glycine max (L.) Merr.] cultivars, PI 96983,'Ogden', 'York', 'Marshall', and 'Kwanggyo', were studied to determine the inheritance of their resistant (R) or necrotic (N) reactions to soybean mosaic virus (SMV strains G4, G5, G6, G7, and G7A. Each parent was crossed reciprocally with a susceptible (S) cultivar ('Essex' or 'Lee 68' to determine the number of genes for resistance or necrosis. The R parents were also crossed with each other to test the allelism of the genes conditioning the R or N reaction. All the F2 populations from N x S crosses segregated in a 3:1 ratio with necrosis being dominant to susceptibility. The absence of S segregants in F2 populations of N x N and R x R crosses and the lack of segregation in S x S crosses indicate that the single dominant genes in the five non-susceptible parents are alleles at a common locus. All R x S crosses produced a combination of R, N, and S plants and gave a good fit to a 3(R + N):1S ratio. Two different SMV-strain x resistance-gene interactions were observed to produce the N reaction: (i) alleles which are necrotic to specific strains in the homozygous state are dominant to alleles which are resistant or susceptible to the same strain, (ii) alleles which are resistant in the homozygous state to a strain often exhibit necrosis when they occur in a heterozygote with a susceptible allele.