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

Five soybean [Glycine max (L.) Merr.] cultivars, PI 96983,'Ogden', 'Yo rk', 'Marshall', and 'Kwanggyo', were studied to determine the inherit ance of their resistant (R) or necrotic (N) reactions to soybean mosai c virus (SMV strains G4, G5, G6, G7, and G7A. Each parent was crossed reciprocally with a susceptible (S) cultivar ('Essex' or 'Lee 68' to d etermine the number of genes for resistance or necrosis. The R parents were also crossed with each other to test the allelism of the genes c onditioning the R or N reaction. All the F2 populations from N x S cro sses segregated in a 3:1 ratio with necrosis being dominant to suscept ibility. 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 all eles 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 diffe rent SMV-strain x resistance-gene interactions were observed to produc e the N reaction: (i) alleles which are necrotic to specific strains i n the homozygous state are dominant to alleles which are resistant or susceptible to the same strain, (ii) alleles which are resistant in th e homozygous state to a strain often exhibit necrosis when they occur in a heterozygote with a susceptible allele.