DIFFERENTIAL SENSITIVITY TO TRIADIMENOL OF ERYSIPHE NECATOR ISOLATES BELONGING TO DIFFERENT GENETIC GROUPS

In populations of Erysiphe necator, the causal agent of grapevine powdery mildew, two genetic groups have been described, based on several molecular markers. In single vineyards, the two groups seem to occupy different temporal niches, with a temporal alternation that is clear-cut in vineyards intensively treated with chemical fungicides. To shed light on this behaviour, 80 isolates of E. necator from different vineyards and belonging to the two genetic groups were characterized for sensitivity to triadimenol, as representative of sterol demethylation inhibitor fungicides (DMIs). A leaf bioassay and an allele-specific PCR assay were used, based on the point mutation A495T in the CYP51 gene that causes the amino acid change Y136F, associated with high level of resistance to DMIs in E. necator. The CYP51 gene from isolates with different resistance levels was cloned and sequenced. The sensitivity to triadimenol (EC₅₀) of isolates of genetic group A was significantly higher than in group B isolates, to which all the moderately and highly resistant isolates belonged. The point mutation A495T was found in the CYP51 gene of isolates showing different levels of resistance, but additional point mutations were found in the same gene. The higher sensitivity to DMIs in group A isolates can be at the basis of their precocious disappearance in vineyards, and can have important implications for powdery mildew control strategies. The molecular basis of E. necator resistance to DMIs was confirmed to be complex and not always associated with the point mutation A495T.