Biocontrol via mycoviruses: a neglected option for bioprotection?

‘Mycoviruses’ is the term used for viruses infecting fungi and fungus-like organisms. Mycoviruses have been reported in Ascomycota, Basidiomycota, Chytridiomycota, Deuteromycota and Zygomycota (Ghabrial et al., 2015). Most reported mycoviruses consist of linear double-stranded RNA (dsRNA; ~70%), while a small number are composed of negative single-stranded RNA (−ssRNA) or positive single-stranded RNA (+ssRNA; ~30%) (Ghabrial et al, 2015; King et al, 2011). More rarely, mycoviruses with circular single-stranded DNA (ssDNA) have been found (Jiang et al., 2013; Li et al., 2020; Yu et al., 2010). Like animal and plant viruses, mycoviruses require the living cells of their host to replicate. The main differences between animal and plant viruses on the one hand, and mycoviruses on the other, lie in the fact that most mycoviruses lack an extracellular route for infection and are transmitted between cells through cell division, sporulation and occasionally cell fusion, as well as lacking movement proteins (Son et al., 2015)Many of these mycoviruses are considered to be cryptic, and supposedly have little to no effect on their host. However, some notable exceptions have been described, in which the viruses cause deleterious effects on their host, and where the host is a plant pathogen, may cause hypovirulence, reducing the infection, growth and reproduction of the fungus. Virocontrol, the biocontrol of pathogens via hypovirulence-causing mycoviruses, is a small field, but one which is now receiving more attention. This chapter discusses the characteristics of mycoviruses and the conditions that need to be met to make a successful virocontrol agent. The chapter describes one of the success stories so far, that of the virocontrol of Cryphonectria parasitica, the chestnut blight pathogen. The chapter ends by reviewing future trends and information on sources of more information on mycoviruses and virocontrol