Vol.:(0123456789) 1 3 Journal of General Plant Pathology https://doi.org/10.1007/s10327-018-0766-7 FUNGAL DISEASES Infection by Magnaporthe oryzae chrysovirus 1 strain A triggers reduced virulence and pathogenic race conversion of its host fungus, Magnaporthe oryzae Mitsuhiro Aihara 1  · Syun‑ichi Urayama 1,2  · Minh Tuong Le 2,3  · Yu Katoh 1  · Tomoya Higashiura 1  · Toshiyuki Fukuhara 1  · Tsutomu Arie 2  · Tohru Teraoka 2  · Ken Komatsu 2  · Hiromitsu Moriyama 1 Received: 30 June 2017 / Accepted: 30 October 2017 © The Phytopathological Society of Japan and Springer Japan KK, part of Springer Nature 2018 Abstract Magnaporthe oryzae chrysovirus 1 strain A (MoCV1-A) is associated with an impaired growth phenotype of its host fungus, Magnaporthe oryzae. In this report, we assayed the virulence and pathogenicity of MoCV1-A-infected and MoCV1-A-free M. oryzae on rice plants. MoCV1-A infection did not afect virulence-associated fungal traits, such as conidial germination and appressorium formation. However, after punch inoculation of leaves on rice plants, MoCV1-A-infected strain formed smaller lesions than the MoCV1-A-free strain did on all rice varieties tested, showing that MoCV1-A infection resulted in reduced virulence of host fungi in rice plants. In contrast, after spray inoculation of rice seedlings, in some cases, MoCV1- A-infected and MoCV1-A-free strains caused diferent lesion types (resistance to susceptible, or vice versa) on individual international diferential rice varieties. However, we did not fnd any gain/loss of the fungal avirulence genes by PCR, sug- gesting that MoCV1-A infection can convert the pathogenicity of the host M. oryzae from avirulence to virulence, or from virulence to avirulence, depending on the rice variety. We also confrmed the correlation of these race conversion events and invasive hyphae growth of the fungi in a leaf sheath inoculation assay. These data suggested that MoCV1-A infection generally confers hypovirulence to the fungal host and could be a driving force to generate physiological diversity, includ- ing pathogenic races. Keywords Mycovirus · Magnaporthe oryzae chrysovirus 1 · Magnaporthe oryzae · Race · Virulence · Pathogenicity Introduction Magnaporthe oryzae is an ascomycete fungus and the causal agent of rice blast, the most destructive disease of rice worldwide (Dean et al. 2012; Pennisi 2010). To con- trol rice blast, many kinds of fungicides, biological control agents and systemic acquired resistance inducers have been developed and used (Midoh and Iwata 1996). In addition to the application of these products, breeding and use of blast- resistant varieties is considered one of the most important ways to control the disease as it is economical, labor-saving and environmentally safe. Resistance conditioned by a single major resistance (R) gene is typically efective in preventing infection by pathogenic races of M. oryzae containing the correspond- ing avirulence (AVR) gene in a gene-for-gene manner (Liu and Wang 2016; Silué et al. 1992). R genes have evolved to detect the products of the corresponding AVR genes and to trigger efective resistance against plant pathogens. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10327-018-0766-7) contains supplementary material, which is available to authorized users. * Ken Komatsu akomatsu@cc.tuat.ac.jp 1 Laboratory of Molecular and Cellular Biology, Graduate School of Agriculture, Tokyo University of Agriculture and Technology, 3-5-8 Saiwaicho, Fuchu, Tokyo 183-8509, Japan 2 Laboratory of Plant Pathology, Graduate School of Agriculture, Tokyo University of Agriculture and Technology, 3-5-8 Saiwaicho, Fuchu, Tokyo 183-8509, Japan 3 Department of Plant Protection, College of Agriculture and Applied Biology, Can Tho University, Can Tho City, Vietnam