Evaluation of the risk of development of resistance to QoI fungicide ZJ0712 in Podosphaera xanthii under greenhouse conditions

Greenhouse-grown cucumbers were monitored over two successive growing seasons to investigate the effects of successive application of ZJ0712, a new quinone outside inhibitor (QoI) fungicide, on the development of resistance in cucumber powdery mildew (Podosphaera xanthii). Resistant P. xanthii isolates were detected after nine successive applications of ZJ0712, although the control efficacy of this fungicide against cucumber powdery mildew at that time was still higher than 80%. Seven ZJ0712-resistant P. xanthii isolates with resistance factor values greater than 180 were obtained, which exhibited a stable resistance to ZJ0712. These resistant P. xanthii isolates had similar pathogenicity to the wild-type isolate on cucumber plants. The ZJ0712 showed significant cross-resistance with azoxystrobin, enostrobilurin, or chlorothalonil, but not with the azole fungicide triadimefon. Furthermore, the most commonly reported G143A mutation in Cyt b associated with QoI resistance was found in five of the seven resistant isolates. These findings suggest that there is a high risk of resistance development associated with using ZJ0712 for controlling cucumber powdery mildew under greenhouse conditions, and the underlying resistance mechanisms in different P. xanthii isolates are not consistent and need to be further unraveled.