Soybean mosaic virus incidence level and infection time: Interaction effects on soybean

Soybean mosaic virus (SMV) infection can reduce soybean [Glycine max (L.) Merr.] yield and seed quality, but the magnitude of these reductions depends on several factors including the soybean developmental stage at infection and the incidence of infection. While most research has involved 100% infection at a defined growth stage, natural SMV infectious are usually not 100% and occur at different times in different years. The objectives were to better define the relationship between incidence level and timing of infection and to determine the best approach for evaluating potential yield losses due to SMV infection. The effects of SMV on soybean seed yield and quality in relation to (i) the incidence level of SMV infection, (ii) the soybean growth stage at infection time, and (iii) the interaction between these were investigated using blends with different proportions of one pair of isolines, 'Williams' (susceptible to SMV) and L78-379 (resistant to SMV). The effects of incidence level of SMV infection on yield, percentage of seed transmission and seed mottling were all linear in both early and late planting environments. A greater reduction in yield, and higher percentages of seed transmission and seed coat mottling resulted from infection of soybean by SMV at or before flowering compared to infection after flowering. When soybean plants were infected by SMV after flowering, negative effects were small or negligible. The interaction between incidence level of infection and inoculation stage was highly significant. When data from inoculation after flowering were dropped, the interaction between incidence of infection and inoculation stage was nonsignificant. 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 R₁.