Investigating the sensitivity of Venturia inaequalis isolates to difenoconazole and pyraclostrobin in apple orchards in China

The resistance level of 90 single lesion conidial isolates of Venturia inaequalis collected from multiple commercial orchards in Shaanxi and Gansu Provinces and Xinjiang Autonomous Region of China to the demethylation inhibitor (DMI) fungicide difenoconazole and quinone outside inhibitor (QoI) fungicide pyraclostrobin was examined. The EC 50 values of the 90 isolates to difenoconazole and pyraclostrobin ranged from 0.143 to 6.735 μg/mL and 0.084 to 2.026 μg/mL, respectively. Among the isolates, 19 had resistance, 66 had reduced sensitivity, and five had sensitivity to difenoconazole; eight had resistance, 81 had reduced sensitivity, and one had sensitivity to pyraclostrobin. Although a weak correlation between difenoconazole and pyraclostrobin was detected, four isolates were identified as resistant to difenoconazole and pyraclostrobin. However, isolates with practical resistance were not found widely in our study and were only sporadic in a few places, indicating that at present, difenoconazole and pyraclostrobin are still safe for disease management in the apple-growing areas of Shaanxi, Gansu and Xinjiang. However, the risk of fungicide resistance should be managed with caution, and yearly monitoring of resistance development is necessary.