Vegetative compatibility and conversion to hypovirulence among Italian isolates of Cryphonectria parasitica

Summary A total of 850 virulent isolates of Cryphonectria parasitica were isolated from natural cankers on European chestnut trees growing in various Italian regions. Vegetative‐compatibility (v‐c) group membership was tested using the merge‐barrage method. In all, 19 vegetative‐compatibility groups were found, and, for each of these, a representative isolate (v‐c tester) was identified; 49% of the isolates were compatible with the tester of only one v‐c group, whereas 51% were compatible with the testers of more than one v‐c group; 3% of the isolates showed compatibility with seven v‐c groups. The coincidence between vegetative incompatibility and failure of conversion occurred in only a few cases; 42% of the converts were stable with respect to their morphological characteristics and presence of cytoplasmic ds‐RNA. The results obtained in Italy suggest that, for an efficient biocontrol programme, it is preferable to evaluate the tendency of a population to be converted rather than evaluating the v‐c group membership.