Cryphonectria is a fungal genus associated with economically significant disease of trees. Herein we characterized a novel double-stranded RNA virus from the fungal species Cryphonectria naterciae, a species unexplored as a virus host. De novo assembly of RNA-seq data and Sanger sequencing of RACE (rapid amplification of cDNA ends) clones gave the complete, non-segmented genome (10,164 bp) of the virus termed Cryphonectria naterciae fusagravirus (CnFGV1) that was phylogenetically placed within the previously proposed viral family Fusagraviridae. Of 31 field-collected strains of C. naterciae, 40% tested CnFGV1-positive. Co-cultivation resulted in within-species transmission of CnFGV1 to virus-free strains of C. naterciae. Comparison of the mycelium phenotype and the growth rate of CnFGV1-infected and virus-free isogenic strains revealed frequent sectoring and growth reduction in C. naterciae. Co-culturing also led to cross-species transmission of CnFGV1 to Cryphonectria carpinicola and Cryphonectria radicalis, but not to Cryphonectria parasitica. The virus-infected C. naterciae and the experimentally infected Cryphonectria spp. readily transmitted CnFGV1 through asexual spores to the next generation. CnFGV1 strongly reduced conidiation and in some cases vegetative growth of C. carpinicola, which is involved in the European hornbeam disease. This study is the first report of a fusagravirus in the family Cryphonectriaceae and lays the groundwork for assessing a hypovirulence effect of CnFGV1 against the hornbeam decline in Europe.

A double-stranded RNA (dsRNA) virus was isolated and characterized from a strain EW220 of the phytopathogenic fungus Botryosphaeria dothidea. The full-length cDNAs of dsRNAs were 6,434 bp and 5,986 bp in size, respectively. The largest dsRNA of BdBRV1 encodes a cap-pol fusion protein that contains part coat protein gene and a RNA-dependent RNA polymerase (RdRp) domain, and the second dsRNA encodes a hypothetical protein. The dsRNA virus was named Botryosphaeria dothidea botybirnavirus 1 (BdBRV1). BdBRV1 contains spherical virions that are 37 nm in diameter consisting of two segments. The structural proteins of BdBRV1 virus particles were 110 kDa, 90 kDa, and 80 kDa, which encoded by dsRNA1 and 2-ORFs. The analysis of genome sequences revealed that the sequences of BdBRV1 shared 99% identity with Bipolaris maydis botybirnavirus 1(BmBRV1). Phylogenetic reconstruction indicated that BdBRV1 and BmBRV1 are phylogenetically related to the genus Botybirnavirus. Importantly, BdBRV1 influences the growth of B. dothidea and confers hypovirulence to the fungal host. BdBRV1 and BmBRV1 were two strains of the same virus. To our knowledge, this is the first report of a botybirnavirus in B. dothidea and our result might be the first funding in the different fungi infected by the same virus.