ALLELISM AMONG GENES FOR RESISTANCE TO SOYBEAN MOSAIC-VIRUS IN STRAIN-DIFFERENTIAL SOYBEAN CULTIVARS

Five soybean [Glycine max (L.) Merr.] cultivars that have been used to differentiate strain groups of soybean mosaic virus (SMV) were studied to determine the allelic relationships among genes conditioning reaction to SMV. The cultivars PI 96983, 'Ogden', 'York', 'Marshall', and 'Kwanggyo' (PI 406710) are each known to have a single dominant gene conditioning resistance to SMV. These parents were crossed in all possible combinations with each other and with a susceptible cultivar. The crosses were evaluated in the F2 and with F2-derived F3 lines in the field and in the greenhouse for reaction to inoculation with the G1 strain of SMV (SMV-G1). Results confirm that each of the resistant cultivars has a single dominant gene for resistance to SMV-G1, but that dominance is sometimes incomplete. The lack of segregation for susceptibility in F2 and F3 progenies from the resistant x resistant crosses indicates a high probability that the resistance genes in these cultivars are alleles at a common locus. Gene symbols Rsv1y, Rsv1m, and Rsv1k are proposed for the resistance alleles in York, Marshall, and Kwanggyo, respectively. The systemic necrosis reaction that sometimes occurs following SMV-G1 inoculation of segregating populations is highly associated with plants heterozygous for the resistance gene, but may be influenced by environment and genetic background.