The demethylation inhibitor (DMI) fungicide prochloraz has been widely used in China to control citrus green mold, which is caused by Penicillium digitatum. The 50% effective concentration (EC50) values of prochloraz for 129 isolates of P. digitatum collected in 2017 from citrus groves of four provinces of China ranged from 0.0032 to 0.4582 mg/liter. Analysis of the distribution of natural logarithms of EC50 values indicated that 111 isolates with EC50 values lower than 0.05 mg/liter could be considered sensitive to prochloraz. Relative baseline sensitivity was established based on the 111 sensitive isolates, and the mean EC50 value was 0.0090 ± 0.0054 mg/liter (SD). Prochloraz at 60, 100, and 140 mg/liter provided preventive efficacies of 67.8, 93.0, and 96.4%, respectively. Prochloraz at 0.005 and 0.01 mg/liter disrupted cell membrane integrity of conidia but reduced cell membrane permeability of mycelia. Prochloraz at 0.01 mg/liter reduced ergosterol content in mycelia by 41.8%. Two prochloraz-resistant isolates with EC50 values of 3.97 and 5.68 mg/liter were attained by consecutive subculturing on prochloraz-amended PDA. Studies on the expression levels of three potential target genes, CYP51A, CYP51B, and CYP51C, demonstrated that whether in the absence or presence of prochloraz, only CYP51B in the resistant isolates was overexpressed at least 10-fold higher than that of the sensitive ones. Sequencing of the three genes showed that only CYP51B in the resistant isolates had a 199-bp insertion in the promoter region. In addition, only CYP51B displayed point mutations of G405S, G389C, and Y390S in the coding regions in the resistant isolates. These results were important for understanding the resistance mechanisms of P. digitatum to prochloraz.
