Overexpression of the CcCYP51A and CcCYP51B genes confer Corynespora cassiicola resistance to prochloraz.

Cucumber Corynespora leaf spot caused by Corynespora cassiicola is the primary disease responsible for reducing cucumber yield, and prochloraz is the main fungicide used to control C. cassiicola. This study investigated the sensitivity and resistance mechanism of C. cassiicola isolates to prochloraz, and found that C. cassiicola has developed resistance to prochloraz. The prochloraz EC values ranged from 0.02 to 2.33 μg/mL, with a mean of 0.436 ± 0.447 μg/mL. In total, 36 of 146 isolates exhibited prochloraz resistance. The resistant isolates had no fitness cost and could not be completely controlled by 50 μg/mL prochloraz on detached leaves. Prochloraz exhibited positive cross-resistance with propiconazole and tebuconazole but not with difenoconazole, carbendazim, trifloxystrobin and pydiflumetofen. The sensitive isolates had significantly lower ergosterol content than the resistant isolates after prochloraz treatment. Compared to sensitive isolates, prochloraz-resistant isolates had no CcCYP51 gene mutation, but the CcCYP51A and CcCYP51B gene expression levels were significantly higher under the treatment of prochloraz. The overexpression of CcCYP51A and CcCYP51B were associated with prochloraz resistance in C. cassiicola.