Cryphonectria parasitica isolates of the same vegetative compatibility type display different rates of transfer of CHV1 hypovirus

Abstract

Hypovirulent strains of the chestnut blight fungus Cryphonectria parasitica have not been registered in the autonomous region of Castilla y Leon (Spain), except in the province of Leon. In this laboratory-based study, we analyzed the rates of horizontal transmission of hypovirus CHV1 subtype F1, isolated from chestnut stands in Leon. We tested the conversion capacity of the six vegetative compatibility (vc) types of C. parasitica isolates most commonly distributed in Castilla y Leon (EU1, EU11, EU12, EU66, CL5 and CL6). We investigated conversion rates of virulent isolates into hypovirulent isolates between pairings of isolates of the same vc type (EU1, EU11 and CL6) and also tested cross conversion rates between isolates of different vc types (EU1, EU11 and CL6 as donors and EU12, EU66, CL5 and CL6 as recipients). We carried out the hypovirus transmission assay with 1700 pairings, of which 700 had donor and recipient isolates of the same vc type and 1000 pairings of isolates had different vc types. Our results show that the conversion frequency to hypovirulent isolates was significantly affected by the vc type, the genotype of isolates with the same origin (province) and the interaction between both factors. In the conversion between isolates of the same vc type, the conversion rates were better with EU1 (ranging between 56 and 94 %) than with EU11 (varying from 4 to 58 %). In the cross conversion between donor and recipient isolates of different vc types, only CL5 and CL6 recipients were converted and the conversion of recipient isolates of EU12 and EU66 failed in all cases. For CL5 as recipient isolate, the conversion rates were similar with EU11 and CL6 as the donor isolates. Recipient isolates of CL6 had good conversion with donor isolates from the same vc type (CL6) and in cross conversion the results were better with donor EU1. Fungal isolates from chestnut stands in Leon displayed the best conversion rates, followed by those from Zamora, Salamanca and Avila when the donor and the recipient isolates were of the same vc type. The fungal isolates appear to have a strong influence on the transmission rate of the hypovirus, at least between strains isolated from chestnut stands in Castilla y Leon. EU1 isolates were more susceptible to conversion than the isolates from EU11. The results highlight the differences in hypovirus transmission, regarding vc types and the genotype of isolates from the same province, when donor and recipient isolates are of the same vc type. Further transmission assays would be useful to determine why hypovirus transmissions, when compared to the hypovirus transmission of other European assays, have different conversion rates with fungal isolates from Castilla y Leon.