Infection of Two Heterologous Mycoviruses Reduces the Virulence of , a Fungal Agent of Apple Valsa Canker Disease.

Mycovirus infection has been widely shown to attenuate the virulence of phytopathogenic fungi. is an agriculturally important fungus that causes Valsa canker disease in apple trees. In this study, two unrelated mycoviruses [ 1 (CHV1, genus , and single-stranded RNA) and Mycoreovirus 1 (MyRV1, genus , double-stranded RNA)] that originated from (chestnut blight fungus) were singly or doubly introduced into via protoplast fusion. CHV1 and MyRV1 stably infected and caused a reduction in fungal vegetative growth and virulence. Co-infection of both viruses further reduced the virulence of but compromised the stability of CHV1 infection and horizontal transmission through hyphal anastomosis. Infections of MyRV1 and, to a lesser extent, CHV1 up-regulated the transcript expression of RNA silencing-related genes in . The accumulation of CHV1 (but not MyRV1) was elevated by the knockdown of , a key gene of the RNA silencing pathway. Similarly, the accumulation of CHV1 and the efficiency of the horizontal transmission of CHV1 during co-infection was restored by the knockdown of . Thus, CHV1 and MyRV1 are potential biological control agents for apple Valsa canker disease, but co-infection of both viruses has a negative effect on CHV1 infection in due to the activation of antiviral RNA silencing by MyRV1 infection.