Occurrence of Resistance in Grapevine Powdery Mildew (Erysiphe necator) to Azoxystrobin + Difenconazole (Ortiva®) and Cross Resistance 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 of Khorasan Razavi province within two time periods i. e. May to June and August to September 2009. Their resistance to azoxystrobin + difenconazole (Ortiva®) fungicide as well as cross resistance to penconazole and hexaconazole fungicides registered in Iran for the control of powdery mildew was investigated. 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. These subcultures were analyzed for resistance to azoxystrobin + difenconazole. The mean EC50 values of azoxystrobin + difenconazole on E. necator subcultures from vineyards was 0. 279 mg L-1. The highest and the lowest EC50 values were found in Mashhad (0. 622 mg L-1) and Khalilabad (0. 143 mg L-1)، respectively. Frequency distributions were skewed most toward higher resistance to azoxystrobin + difenconazole. The present study demonstrated a steady and significant increase in EC50 values for azoxystrobin + difenconazole during the growing season after multiple applications. Also، there was no cross resistance (P> 0. 05) among fungicides.