Characterization of baseline sensitivity and resistance risk of Colletotrichum gloeosporioides complex isolates from strawberry and grape to two demethylation-inhibitor fungicides, prochloraz and tebuconazole

Anthracnose, caused by Colletotrichum gloeosporioides complex, is an important disease of strawberry and grape plants. Demethylation-inhibitor fungicides (DMIs) are potential alternatives to benzimidazole fungicides for the management of anthracnose in strawberry and grape plants in China. However, no information regarding the baseline sensitivity of C. gloeosporioides complex to DMIs is available. In this study, a total of 216 isolates of C. gloeosporioides including 113 from strawberries and 103 from grapes, were tested for their baseline sensitivities to prochloraz and tebuconazole. The sensitivities of the total combined population (n = 216) of the strawberry and grape isolates collected from six different locations showed no geographical variation. The frequency distribution of the EC50 values for the 216 isolates was a unimodal curve. The mean EC50 value of 0.053 ± 0.01 mg/l (ranging from 0.016 to 0.19 mg/l), and 0.62 ± 0.11 mg/l (0.27 to 3.75 mg/l), could be utilized as the baseline for monitoring the shift of sensitivity to prochloraz and tebuconazole, respectively. Some geographical variation was detected among the grape subpopulation (n = 103), which showed significantly lower sensitivity to both fungicides than the strawberry subpopulation (n = 113). Prochloraz and tebuconazole had a significantly lower risk of resistance development than that of kresoxim-methyl, which is a Qo inhibitor. This was indicated by the fewer mutants obtained through UV mutagenesis, the lower resistant factor and the decreased sporulation and pathogenicity ability of the obtained mutants. All of the kresoxim-methyl resistant mutants became resistant to pyraclostrobin, but not to the boscalid or DMIs. Interestingly, however, a positive cross-resistance was observed between tebuconazole and difenoconazole, but not between tebuconazole and prochloraz.