Baseline Sensitivity Of Podosphaera Xanthii To Fluopyram And Its Resistance Risk Assessment

Fluopyram is a new, broad-spectrum SDHI fungicide, which was developed by Bayer CropScience. It had been registered in China as a highly active fungicide for cucumber powdery mildew control. In this paper, the antifungal spectrum of fluopyram was determined for some important plant pathogenic fungi, as well as the baseline sensitivity, and the risk of resistance both in the laboratory and in the field were studied for Podosphaera xanthii on cucumber, which will provide a fundamental reference for the application of fluopyram to control cucumber powdery mildew in the field.The sensitivity test conditions for P. xanthii to fluopyram were established by comparison of several different bioassay methods as follows:detached cucumber cotyledon was used as media, treated with fungicide and moisturized on water-agar medium, then the pathogen spores were sprayed on leaves and cultured under conditions of 20±1℃, RH50%,14h-light/10h-dark for efficacy investigation.Fluopyram played a broad-spectrum antimicrobial activity to tested plant pathogenic fungi, such as Alternaria solani, P. xanthii, Alternaria panax, Sclerotinia sclerotiorum, Colletotrichum orbiculare, Cercospora arachidicola, Botrytis cinerea, Alternaria alternate, Alternaria kikuchiana, while had no effect on oomycete pathogen Phytophthora capsici.The sensitivities of 62 P. xanthii strains to fluopyram were determined by the cotyledon assay in vitro. The results indicated that the mean EC50 value is (0.2217±0.0528)μg/mL. The sensitivity frequency of P. xanthii strains to fluopyram distributed as a unimodal curve, which showed that there was no resistant subpopulation among these strains. Thus, it can be used as baseline-sensitivity of P. xanthii to fluopyram for field resistance monitoring.In laboratory, three fluopyram-resistant mutants were obtained by fungicide generatrion selection method. The emergency of resistance is getting faster as the increase of fungicide selection pressure. Their resistance levels ranged from low to moderate, and the resistance of all mutants were stable. As compared to their parental strain, the pathogenicity, fitness and the competition of resistant mutants with no significantly difference.The simulating application of single fluopyram, and mixture or alternation use with some other fungicides with different mode of action were conducted in the greenhouse. The excellent efficacies were obtained for all treatments, and there was no resistant mutants were detected in these treatments.