Diversity ofCryphonectria parasiticain callused chestnut blight cankers on European and American chestnut

Infection of American and European chestnuts with the chestnut blight fungusCryphonectria parasiticaresults in the formation of cankers, lesions caused by the growth of mycelia within bark tissue of the host plant. Infection of the fungus withCryphonectria hypovirus 1(CHV-1) results in conversion of the mycelial phenotype from virulent to hypovirulent, thus allowing production of callus around cankers as a reaction by infected trees, rendering active into inactive cankers. In this study, we sampled one USA and six European chestnut stands and assessed frequency of hypovirulentC. parasiticaand diversity of vegetative compatibility (vc) types present in calluses and randomly sampled cankers. Callused cankers onC. dentataat West Salem in the USA yielded significantly more hypovirulentC. parasiticaisolates compared with four sampled populations onC. sativa, while all six sampled European populations did not show any statistically significant differences among themselves. We observed no correlation between hypovirulence frequencies in randomly sampled cankers and calluses, as well as no correlation ofC. parasiticavc type diversity in calluses and residential populations of the fungus. Furthermore, even though we have observed calluses with more than one vc type, they do not occur regularly. Even when present inC. parasiticapopulations with high vc type diversity, no more than three different vc types were observed in a single callus.