Mycovirome associated with divergent pathogenicity and fungicide sensitivity of Rhizoctonia causing rice sheath blight in Northeast China

Rice sheath blight, caused by Rhizoctonia , is a major threat to global rice production. Hypovirulence-associated mycoviruses offer potential for biocontrol of plant diseases. However, little is known about the diversity of mycoviruses and their effects on the host fungus in Rhizoctonia populations responsible for rice sheath blight in Northeast China. Here, we analyzed 42 strains (39 R. solani AG-1 IA and three R. oryzae-sativae AG-Bb) from this region using metatranscriptomic sequencing, pathogenicity assays, and fungicide sensitivity tests. A total of 67 viral contigs, classified into four families, were identified. From these contigs, eight near‑complete viral genomes were assembled and characterized, including three novel mitoviruses and one novel partitivirus. Pathogenicity assays showed that 21 strains exhibited weak virulence, of which 20 harbored at least one mitovirus, suggesting that mitoviruses are associated with virulence attenuation. In a pot experiment, pre‑inoculation with the hypovirulent strain JMS10, followed by challenge with a highly virulent strain 3 d later, suppressed disease by 53.21%, suggesting a potential protective effect. Fungicide sensitivity analysis showed that virus-infected strains exhibited significantly lower EC 50 values for flutolanil compared to virus-free strains, suggesting a possible link between viral infection and enhanced flutolanil sensitivity. This study expands the known mycoviral diversity in Rhizoctonia causing rice sheath blight in Northeast China, establishing a foundation for developing mycovirus-based biocontrol strategies against rice sheath blight.