Observation on Baseline Sensitivity of Erysiphe necator Genetic Groups to Azoxystrobin

Abstract

Powdery mildew, caused by Erysiph necator, is a common and severe fungal disease of grapevine all over the world. The disease costs millions of dollars to vine growers, due to intensive use of fungicides and yield losses. Recently in population of E. necator two genetic groups have been described, the two groups seem to occupy different temporal niches, with a temporal alternation that is clear-cut in vineyards intensively treated with chemical fungicides. QoI-STAR (Quinol Outside Inhibitors-Strobilurin Type of Action and Resistance) fungicides are widely used to control the disease, and generally carry a high risk of pathogen resistance development. To clarify the behaviors of the biotrophic fungus when treated with azoxystrobin as a representative of QoI-STAR, baseline sensitivity of laboratory isolates were determined. A leaf bioassay and the primers RSCBF1 and RSCBR2 designed on the highly conserved regions of cytb gene in fungi were used. Partial sequence of E. necator cytb gene were obtained. Attempts to obtain a laboratory mutant were not totally successful. The sensitivity to azoxystrobin (EC50) in isolates of genetic group B was significantly higher than in isolates of group A, to which all the isolates collected later in the season belonged. The higher sensitivity to azoxystrobin fungicides observed in group B isolates can be at the basis of their precocious disappearance in vineyards, and can have important implications for powdery mildew control strategies.