A mycovirus from entomogenous fungi Beauveria acts as a mobile effector of phytopathogen antagonism through cross-taxonomic transmission.

Fungal antagonism shapes microbial community structure and provides an ecological basis for biological control. Although mycoviruses have been implicated in fungal antagonism, their effects are generally attributed to indirect modulation of resident host physiology. Here, we show that a mycovirus can function as a mobile effector of antagonism through cross-taxonomic transmission, representing the first documentation of viral transfer from an entomopathogenic/endophytic fungus (Beauveria) to a phytopathogenic fungus (Sclerotinia) via a plant‑mediated route. We characterized Beauveria brongniartii victorivirus 1 (BbrVV1), a totivirus infecting the plant-colonizing entomopathogenic fungus Beauveria brongniartii. BbrVV1 enhanced the in planta suppression of Sclerotinia sclerotiorum by B. brongniartii, achieving a biocontrol efficacy of up to 66.7% at 3 days post‑inoculation, compared to 49.1% for the virus‑free strain, an effect associated with transmission of BbrVV1 from B. brongniartii to the invading pathogen within colonized plants. In its native host, BbrVV1 caused only moderate fitness effects, whereas host shift to S. sclerotiorum induced pronounced hypovirulence, including restricted vegetative growth, impaired sclerotial development and attenuated pathogenicity. Furthermore, based on in vitro dual‑culture and in planta co‑inoculation assays, BbrVV1 was found to spread horizontally within S. sclerotiorum populations via hyphal contact, but was not transmitted through mono-sclerotial progeny. These findings expand the ecological significance of mycoviruses as mobile agents of fungal interaction and provide a conceptual foundation for plant-associated viral delivery in biological control.