Sensitivity of Erysiphe necator to quinoxyfen in Italian vineyards

Objectives: Grape powdery mildew, caused by Erysiphe necator, is of major economic importance on cultivated grapevines worldwide. Quinoxyfen is a preventive fungicide that specifically controls powdery mildew pathogens by interfering with germination or pre-infection development. The aim of this study was to evaluate quinoxyfen sensitivity of Erysiphe necator collected from Italian vineyards characterized by different type of usage of this fungicide. Methods : The study was carried out on populations of E. necator sampled in 23 vineyards localised in different viticultural regions of Italy during the 2009 and 2010 growing seasons. Sensitivity tests were performed by application of different concentrations of quinoxyfen on “Trebbiano” grape seedlings followed by experimental inoculation of conidia suspension. A preliminary comparison among data obtained from plant assays, leaf disc sporulation tests in a settling tower and conidial germination tests was also carried out. In some of vineyards where samples were collected field trials were also conducted. Results : The distribution of EC50 values was found to range from 0.0008 to 6.28 mg/L. These wide variability may be related to the peculiar mode of action of quinoxyfen as also observed in previous baseline studies. Overestimation of leaf disc test was also preliminary pointed out and the results of field trials showed a good control of the disease. Conclusions : The EC50 values detected in 2009-2010 in Italy did not show any changes in sensitivity of E. necator to quinoxyfen both in bioassays and in field trials. Evaluation of sensitivity has been in progress through qualitative plant assays together the quantitative conidial germination tests on monoconidial isolates.