Abstract:In order to evaluate the resistant risk of the pathogen of powdery mildew on cucumber to hexaconazole, the sensitivity of Sphaerotheca fuliginea of 50 strains collected from Shandong Province, China to hexaconazole was measured on cucumber seedling cotyledons. The results showed that the EC50 values of strains varied from 0.0017μg/mL to 0.16μg/mL with the mean of 0.028μg/mL, which could be regarded as the baseline sensitivity of S.fuliginea to hexaconazole because the 50 strains were collected from the greenhouse where no demethylation inhibitor group of fungicides were applied, and frequencies distribution of the 50 strains was similar with Gaussian distribution. Two hexaconazole-resistant strains were obtained by UV irradiation and hexaconazole screening in the laboratory, and then the fitness parameters and competition ability of resistant and sensitive strains were measured. The results showed that fitness of resistant strains and sensitive strains had no significant difference in absence of fungicide. No significant difference occurred in ratio of resistant strains to sensitive strains when conidia of resistant and sensitive strains were mixed at the ratio of 80∶ 20 and incubated for 7 generations in absence of fungicide; when mixed with 50∶ 50, the ratio of moderately resistant strain decreased significantly, meanwhile the ratio of high resistant strain didnt change significantly; when mixed with 20∶ 80, the ratio of resistant strain decreased.