Baseline sensitivity and fungicidal action of propiconazole against Penicillium digitatum.

Green mold, caused by Penicillium digitatum, is the most important citrus postharvest disease worldwide and often causes substantial economic losses to the citrus industry. The demethylation inhibitor (DMI) fungicides are highly effective against a broad range of fungal pathogens, but the DMI fungicide propiconazole has not been registered yet in China for the control of citrus green mold. In this study, baseline sensitivity of P. digitatum to propiconazole was determined. The frequency distribution of logarithms of EC values for 118 isolates collected from five regions in China was bimodal, and among the 118 isolates, 18 isolates were less sensitive or had low resistance to propiconazole. The mean EC value of the sensitive 100 isolates was 0.104 mg/L. Preventive control efficacies on Satsuma mandarin for propiconazole at 200 and 400 mg/L were 63.1 and 84.3%, respectively. The fruit treated with propiconazole at 40 and 100 mg/L produced significantly fewer conidia, and the virulence of the conidia decreased by 12.3 and 14.8%, respectively. Studies with propidium iodide showed that the membrane integrity was damaged for 25.6% of conidia produced on PDA amended with propiconazole at 0.1 mg/L. Fluorescence microscopy observations of P. digitatum conidia stained with 2,7-dichlorofluorescin showed that propiconazole significantly induced the generation of intracellular reactive oxygen species (ROS). Compared with the sensitive isolates, no point mutations were detected in either the coding or promoter region of the target gene CYP51A of the isolates with low resistance to propiconazole. However, the relative expression levels of CYP51A for three resistant isolates were higher than sensitive isolates, and the mean relative expression was 2.08 for resistant isolates versus 0.62 for sensitive isolates in the absence of propiconazole and 3.12 versus 1.44 in the presence of propiconazole. These results indicate increased expression of CYP51A is the molecular mechanism for low resistance of P. digitatum to propiconazole.