Keywords: benomyl, thaibendazole, grey mildew, roses.
To identify and quantify strains of Botrytis cinerea that are resistant to benomyl and thiabendazol, one susceptible isolate was collected in Santa Catarina, Texcoco, and 12 in Villa Guerrero, State of Mexico. Bioassays were conducted and the data were processed with PROBIT-SAS to obtain EC50 and EC95. With benomyl, the results indicate that the susceptible isolate had a EC50 of 0.055 mg·ml-1 and a EC95 of 0.49 mg·ml-1. The EC50 of the isolates from Villa Guerrero fluctuated between 250.59 and 831.68 mg·ml-1, and the factor of resistance (FR), relative to the susceptible control, was between 4,555.18 and 15,120.45. In contrast, EC95 was found to be between 12,241.09 and 18,279.08 mg·ml-1, and FR fluctuated between 24,980.63 and 37.303.24. With thiabendazole, the EC50 and EC95 of the susceptible isolate reached 0.028 and 0.327 mg·ml-1, respectively. The EC50 of the isolates of Villa Guerrero was 88.51 to 129.96 mg·ml-1, and FR was 3,160.00 to 4,440.42, while EC95 was between 2,830.00 and 10,839.00 mg·ml-1. These results confirm that the fungus is resistant to benomyl and thiabendazole in the study region.