SOYBEAN MOSAIC-VIRUS INCIDENCE LEVEL AND INFECTION TIME - INTERACTIONEFFECTS ON SOYBEAN

Soybean mosaic virus (SMV) infection can reduce soybean [Glycine mau ( L.) Merr.] yield and seed quality, but the magnitude of these reductio ns depends on several factors including the soybean developmental stag e at infection and the incidence of infection. While most research has involved 100% infection at a defined growth stage, natural SMV infect ions are usually not 100% and occur at different times in different ye ars. The objectives were to better define the relationship between inc idence level and timing of infection and to determine the best approac h for evaluating potential yield losses due to SMV infection. The effe cts of SMV on soybean seed yield and quality in relation to (i) the in cidence level of SMV infection, (ii) the soybean growth stage at infec tion time, and (iii) the interaction between these were investigated u sing blends with different proportions of one pair of isolines, 'Willi ams' (susceptible to SMV) and L78-379 (resistant to SMV). The effects of incidence level of SMV infection on yield, percentage of seed trans mission and seed mottling were all linear in both early and late plant ing environments. A greater reduction in yield, and higher percentages of seed transmission and seed coat mottling resulted from infection o f soybean by SMV at or before flowering compared to infection after fl owering. When soybean plants were infected by SMV after flowering, neg ative effects were small or negligible. The interaction between incide nce level of infection and inoculation stage was highly significant. W hen data from inoculation after flowering were dropped, the interactio n between incidence of infection and inoculation stage was nonsignific ant. Large's critical stage model is appropriate for the assessment of soybean yield losses due to SMV infection, and this critical stage is Growth Stage R-1.