Coinfection of Rosellinia necatrix by a partitivirus and a virga-like virus is associated with hypovirulence

Many Mediteranean isolates of Rosellinia necatrix, causing root rot in avocado, have previously been screened for viruses. Among them is isolate Rn459 that has been shown to be infected by at least three viruses such as Rosellinia necatrix partitivirus 10 (RnPV10), Rosellinia necatrix hypovirus 2 (RnHV2), Rosellinia necatrix fusagravirus 1 (RnFGV1), and Rosellinia necatrix virga-like virus (RnVLV). Here, we attempted to eliminate the viruses by hyphal tip cultures to examine their effect on colony growth in vitro and virulence on avocado plants. The obtained fungal strain termed, Rn459_PV10F/VLVF, which was confirmed to be cured of RnPV10 and RnVLV, but still retaining RnHV2, manifested a phenotype different from the original Rn459. Colony growth comparison showed that Rn459_PV10F/VLVF grew faster than the original Rn459 isolate and the virulence on avocado plants of this Rn459_PV10F/VLVF strain was higher than the original Rn459 strain. These combined results suggest that RnPV10 and RnVLV, alone or together, contribute to confer hypovirulence on the R. necatrix isolates.
This study was supported in part by the Plan Nacional I + D + I Ministerio de Economía y Competitividad (AGL 2014–52,518-C2–2-R) Spain. In addition, this research was co-financed by FEDER funds (EU) and by Yomogi Inc. and Grantsin-Aid for Scientific Research on Innovative Areas from the Japanese Ministry of Education, Culture, Sports, Science and Technology (KAKENHI 25252011 and 16H06436, 16H06429 and 16 K21723 to N.S. and H. K.).