Soybean mosaic virus (SMV) and the SMV resistance gene (Rsv₁): Influence on Phomopsis spp. seed infection in an aphid free environment

Infection of soybean [Glycine max (L.) Merr.] plants with Soybean mosaic virus (SMV) has been reported to enhance Phomopsis spp. infection, which reduces seed quality. The timing and incidence of SMV infection depends largely upon the level of primary inoculum and aphid-activity. Two field experiments were conducted in aphid-free environments, to examine the influence of (i) SMV-infection, and (ii) SMV-resistance alleles of the Rsv₁ gene, on the incidence of Phomopsis spp. seed infection. In the first experiment, mock inoculated (potassium phosphate buffer) SMV-susceptible cultivars (Clark and Williams), and their SMV-resistant isolines (L78-434 and L78-379, with dominant Rsv₁ allele conferring resistance to SMV strains G1-G6), showed low levels (<10%) of Phomopsis spp. seed infection. In contrast, susceptible cultivars mechanically inoculated with SMV (G2 strain, V8 stage) exhibited a 3- to 8-fold increase in the incidence Phomopsis spp. seed infection. In the second experiment, mock inoculation of the susceptible cultivar, Clark, and two SMV-resistant lines (10-rsv_r^y and 18-rsv_r^y, with recessive rsv₁^y allele conferring resistance to SMV strains G1-G3), resulted in <20% Phomopsis spp. seed infection. In contrast, those plants mechanically inoculated with SMV (strain G6, V8 stage) had significantly higher levels of Phomopsis spp. seed infection (52 to 78%). It is concluded that the lower incidence of Phomopsis spp. seed infection in SMV-resistant plants was not due to the SMV resistance alleles of the Rsv₁ gene per se, but rather due to the absence of SMV infection. Thus, the use of SMV-resistant varieties prevented/reduced SMV and Phomopsis spp. seed infection.