Damage Assessment of Major Soybean Viruses in Korea: Infection Timing Influences Yield Loss and Seed-Borne Transmission Dynamics

Soybean mosaic virus (SMV; species Potyvirus glycitessellati ) and soybean yellow mottle mosaic virus (SYMMV; species Gammacarmovirus glycinis ) are predominant viral pathogens threatening soybean production. While their high prevalence during early vegetative stages has been increasingly reported, the quantitative epidemiological impact of infection timing on ultimate crop yield and seed-borne transmission dynamics remains poorly understood. In this study, we evaluated the effect of SMV and SYMMV infections at three distinct growth stages on yield components and seed-borne transmission in soybean genotypes (Danbaek, OB13087, and Cheongja5ho) under field conditions. All seeds used in this study were produced under insect-exclusion conditions during the previous year to minimize the risk of natural vector-mediated viral infection. Results indicated that early vegetative infection caused the most severe crop loss, primarily through decreases in pod and seed numbers. Virus was frequently detected in whole seeds regardless of their external appearance, whereas seed-to-seedling transmission varied among the tested host-virus combinations. The numerically highest transmission rates occurred following late vegetative inoculation, although the effect of inoculation timing was statistically significant only in OB13087. No transmission was detected in Danbaek despite frequent virus detection in whole seeds. This study provides quantitative evidence that early-season virus management is critical to prevent irreversible yield losses, while demonstrating that visual seed sorting alone is insufficient for reliable seed health assessment, highlighting the need for molecular-based evaluation approaches.