Verticillium albo-atrum partitivirus 1 causes latent infection in Verticillium nonalfalfae isolated from hop (Humulus lupulus L.)
The soilborne plant-pathogenic fungus Verticillium nonalfalfae (formerly V. albo-atrum sensu lato) causes Verticillium wilt in hop, and infections caused by its highly virulent (lethal) pathotype lead to severe disease development and pose the greatest threat to hop production. In this study, the presence of mycoviruses was investigated in isolates representing different geographical regions and virulence groups using double-stranded RNA isolation and high-throughput sequencing. Notably, Verticillium albo-atrum partitivirus 1 (VaaPV1) was detected in 46 of the 91 isolates examined. VaaPV1 is a bipartite member of the genus Partitivirus (family Partitiviridae) that encodes an RNA-dependent RNA polymerase and a coat protein. Thirty isolates were sequenced, and they showed a low genetic divergence. Comparisons between infected and isogenic virus-free isolates revealed no significant effects on fungal growth or virulence (on tobacco and hop), indicating that VaaPV1 causes a cryptic infection. Hyphal tipping failed to eliminate the virus, and only a small proportion of regenerated protoplasts were virus-free, indicating efficient virus maintenance during fungal growth. This study provides the first evidence of VaaPV1 infecting V. nonalfalfae from hop and contributes to a broader understanding of partitivirus prevalence and biology in this vascular wilt pathogen.