AUTHOR=Yang Shian , Dai Ruoyin , Salaipeth Lakha , Huang Lili , Liu Jie , Andika Ida Bagus , Sun Liying 

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

JOURNAL=Frontiers in Microbiology

VOLUME=12

YEAR=2021

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2021.659210

DOI=10.3389/fmicb.2021.659210

ISSN=1664-302X

ABSTRACT=<p>Mycovirus infection has been widely shown to attenuate the virulence of phytopathogenic fungi. <italic>Valsa mali</italic> is an agriculturally important fungus that causes Valsa canker disease in apple trees. In this study, two unrelated mycoviruses [<italic>Cryphonectria hypovirus</italic> 1 (CHV1, genus <italic>Hypovirus</italic>, and single-stranded RNA) and Mycoreovirus 1 (MyRV1, genus <italic>Mycoreovirus</italic>, double-stranded RNA)] that originated from <italic>Cryphonectria parasitica</italic> (chestnut blight fungus) were singly or doubly introduced into <italic>V. mali</italic> via protoplast fusion. CHV1 and MyRV1 stably infected <italic>V. mali</italic> and caused a reduction in fungal vegetative growth and virulence. Co-infection of both viruses further reduced the virulence of <italic>V. mali</italic> 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 <italic>V. mali</italic>. The accumulation of CHV1 (but not MyRV1) was elevated by the knockdown of <italic>dcl2</italic>, 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 <italic>dcl2</italic>. 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 <italic>V. mali</italic> due to the activation of antiviral RNA silencing by MyRV1 infection.</p>