Sensitivity to the Demethylation Inhibitor Difenoconazole Among Baseline Populations of Various spp. Causing Blue Mold of Apples and Pears.

Difenoconazole (DIF), a demethylation inhibitor fungicide, was registered in 2016 for the control of postharvest diseases of pome fruits. In this study, 162 isolates from ( = 31) and 13 other "non-" spp., i.e., ( = 52), ( = 32), ( = 15), ( = 9), ( = 8), ( = 5), ( = 3), ( = 2), along with one isolate each of , , , , and , were collected from multiple packinghouses in the U.S. Pacific Northwest. In vitro sensitivity assays showed similar sensitivities of spores and mycelia across species with the mean EC values ranging from 0.01 for ( = 8) to 1.33 μg mL for ( = 2), whereas the mean EC were 0.03, 0.12, 0.19, and 0.51 μg mL for ( = 31), ( = 9), ( = 52), and ( = 5), respectively. The recommended rate of DIF controlled and isolates but not all isolates of four other spp. on Fuji apples after five months at 1.5 °C. The mixture of DIF + fludioxonil (FDL) (Academy) controlled all the dual-sensitive isolates (DIFFDL) and DIF single-resistant (DIF) isolates among the six species tested but not the FDL and dual DIFFDL isolates. Notable polymorphism was detected in the gene of the "non-" species with four mutations located at four residues. Although the isolates analyzed in this study had not previously been exposed to DIF, the findings indicate variable sensitivity levels among the spp.