Molecular and biological characterisation of Magnaporthe oryzae botourmiavirus 15

In this study, we have identified and characterized a novel fungal virus, designated Magnaporthe oryzae botourmiavirus 15 (MoBV15), which was isolated from the rice blast fungus. The genome of MoBV15 is a positive-sense, single-stranded RNA (+)ssRNA 3,228 nucleotides in length with a GC content of 51.4%. It features a single open reading frame (ORF) predicted to encode a putative RNA-dependent RNA polymerase (RdRp). The 5' and 3' ends of the MoBV15 genomic RNA exhibit reverse complementarity and may form a panhandle structure-a feature rarely observed. On tomato-oatmeal medium, the virus‑carrying strain C16‑MoBV15 produced conidia at a concentration of 4.3 × 10³/mL, whereas the virus‑free parental strain C16 produced 4.7 × 10³/mL, representing a reduction of approximately 7.9% in sporulation; however, the difference was not statistically significant (p > 0.05), indicating that the virus does not significantly affect the sporulation capacity of C16. In terms of mycelial growth rate, a clear difference was observed between the virus‑infected and virus‑free strains: the lesion lengths of the virus‑infected strain were reduced by 42.27% (using spore suspension inoculation) and 46.28% (using mycelial plug inoculation) compared with the virus‑free strain, suggesting that MoBV15 can induce hypovirulence in the rice blast fungus host. Based on sequence comparisons and phylogenetic reconstruction, we propose that MoBV15 constitutes a novel species within the genus Scleroulivirus of the family Botourmiaviridae, thereby expanding the known diversity of fungal-associated Botourmiaviridae.