Occurrence of Resistance in Grapevine Powdery Mildew (Erysiphe necator) to Penconazole and Hexaconazole in Khorasan Razavi Province

Thirty seven isolates of Erysiphe necator were collected from different vineyards from three regions and nine cities Khorasan Razavi province within two time periods i.e. May to June and August to September 2009. A leaf disk bioassay was carried out to determine the resistance of powdery mildew isolates on the basis of EC50 values derived from log-logistic dose-response curves. Treatments were consisted of penconazole at doses of 0, 0.125, 1.25, 12.5 and 125 ppm and hexaconazole at doses of 0, 0.3, 3, 30 and 300 ppm. The results showed that maximum values of EC50 (maximum resistance) for penconazole and hexaconazole on E. necator subcultures were 0.932 mg/l in Dargaz and 3.15 mg/l in Qoochan, respectively. For hexaconazole EC50 values for early-formed ascospores were higher than that of conidia. In other words early-formed ascospores were more resistant than conidia to both fungicides. However, penconazole EC50 values for conidia were higher than that of early-formed ascospores. Results revealed the occurrence of resistance of Erysiphe necator to penconazole and hexaconazole in different levels and cities