| Summary: | <i>Magnaporthe oryzae</i>, the fungus that causes rice blast, is the most destructive pathogen of rice worldwide. A number of <i>M. oryzae</i> mycoviruses have been identified. These include <i>Magnaporthe oryzae.</i> viruses 1, 2, and 3 (MoV1, MoV2, and MoV3) belonging to the genus, <i>Victorivirus</i>, in the family, <i>Totiviridae</i>; <i>Magnaporthe oryzae.</i> partitivirus 1 (MoPV1) in the family, <i>Partitiviridae</i>; <i>Magnaporthe oryzae.</i> chrysovirus 1 strains A and B (MoCV1-A and MoCV1-B) belonging to cluster II of the family, <i>Chrysoviridae</i>; a mycovirus related to plant viruses of the family, <i>Tombusviridae</i> (<i>Magnaporthe oryzae.</i> virus A); and a (+)ssRNA mycovirus closely related to the ourmia-like viruses (<i>Magnaporthe oryzae.</i> ourmia-like virus 1). Among these, MoCV1-A and MoCV1-B were the first reported mycoviruses that cause hypovirulence traits in their host fungus, such as impaired growth, altered colony morphology, and reduced pigmentation. Recently we reported that, although MoCV1-A infection generally confers hypovirulence to fungi, it is also a driving force behind the development of physiological diversity, including pathogenic races. Another example of modulated pathogenicity caused by mycovirus infection is that of Alternaria alternata chrysovirus 1 (AaCV1), which is closely related to MoCV1-A. AaCV1 exhibits two contrasting effects: Impaired growth of the host fungus while rendering the host hypervirulent to the plant, through increased production of the host-specific AK-toxin. It is inferred that these mycoviruses might be epigenetic factors that cause changes in the pathogenicity of phytopathogenic fungi.
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