Natural spread of Cryphonectria parasitica and persistence of hypovirulence in three Italian coppiced chestnut stands

Three coppiced chestnut stands near Florence (Tuscany, Italy) were monitored for 15 years. Presence of blight, damage, evaluation of different types of cankers and their relationship with stand structure were assessed. Disease incidence ranged from 67% to 99% of infected shoots, and mortality caused by blight was between 6% and 12%. Canker types were defined in the study sites; healing and healed cankers represented 70–88% of the total infections recorded, whereas normal cankers represented 9–12.5%. In general, more than one infection was detected for each shoot, and on some trunks up to nine cankers were observed. During the entire investigation period, a constant trend of hypovirulence spread was observed that was symptomatic of a substantial stability of the phenomenon: most (63.5%) undifferentiated new infections evolved into healing and healed cankers, compared with 6.5% that developed into normal infections. Isolations confirmed that a mixed inoculum (normal, intermediate and white strains) of the pathogen was spreading in the coppices. Inocula produced by pycnidia from healing cankers were examined and tested in an artificial inoculation trial. dsRNA was detected in white strains isolated from healing cankers. In this natural condition and in the complete absence of silvicultural treatments, this inoculum seems capable of ensuring both the spread of hypovirulence and its persistence over the years. Data suggest that hypovirulence and its spread are complex phenomena based on the interaction between fungal inoculum, host resistance and site conditions.