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Artificial Infection of Rosellinia necatrix with Purified Viral Particles of a Member of the Genus Mycoreovirus Reveals Its Uneven Distribution in Single Colonies

First, fifth, and sixth authors: Department of Plant Protection, National Institute of Fruit Tree Science, National Agricultural Research Organization (NARO), Independent Administrative Institution, 2 Fujimoto, Tsukuba, Ibaraki 305-8605, Japan; and second, third, and fourth authors: Department of Apple Research, National Institute of Fruit Tree Science, National Agricultural Research Organization (NARO), Independent Administrative Institution, 92 Shimokuriyagawa, Morioka, Iwate 020-0123, Japan

Rosellinia necatrix mycoreovirus 3 (W370) (RnMYRV-3/W370, described as RnMYRV-3 in this paper), a member of the newly established genus Mycoreovirus within the family Reoviridae, is the hypovirulence factor of the white root rot fungus, Rosellinia necatrix. Two virus-free fungal isolates (W37 and W97) that were somatically incompatible with the virus-harboring field isolate (W370) were transfected with purified RnMYRV-3 particles. Virus infection was confirmed by electrophoresis and northern hybridization of viral double-stranded RNA. RnMYRV-3 was transmissible from transfected strains to their respective, virus-free counterparts via hyphal anastomosis. Virus-transfected strains produced smaller lesions on apple fruits than did their virus-free counterparts. Virus-cured strains were indistinguishable from wild-type strains in culture morphology and displayed approximately the same virulence level on apples. Virus-transfected strains had “mosaic” colony portions consisting of thin, fast-growing and dense, slow-growing mycelia, and grew more slowly as a whole than their virus-free, parental strains. The level of virus accumulation varied among virus-transfected subcultures and within its single colonies. Virus-transfected strains were occasionally cured, as was W370. Such a phenomenon may be ascribed to uneven viral distribution in single colonies and the difficulty in viral transmission to virus-free hyphae.