Baseline sensitivity of Botrytis cinerea strawberry isolates to mefentrifluconazole and cross-resistance with other DMIs

Gray mold, caused by the fungus Botrytis cinerea Pers., is one of the main diseases affecting strawberry, representing a problem during cultivation and post-harvest. The intensive use of chemical fungicides has favored the selection of resistant isolates to the products currently available on the market. This study aimed to establish a baseline sensitivity of Botrytis cinerea isolates from Brazil and the USA to mefentrifluconazole, assess potential cross-resistance with other DMI fungicides, identify mutations in the CYP51 gene associated with resistance, and evaluate the occurrence of practical resistance by comparing the efficacy of mefentrifluconazole with other commercially available fungicides. To build the baseline, the EC 50 of 155 isolates (70 isolates from Parana, Brazil and 85 from Florida, USA) was determined. Isolates from Parana were less sensitive than those from Florida, with two isolates presenting EC 50 values above 1.00 µg/ml. Average EC 50 of isolates from Parana was 0.169 µg/ml (0.046 to 1.303 µg/ml), while isolates from Florida averaged 0.084 µg/ml (0.030 to 0.384 µg/ml). Twenty out of 155 isolates were selected for a cross-resistance study, and their EC 50 values were determined for difenoconazole and triflumizole. Pearson's correlation analysis suggested the presence of cross-resistance between fungicides. DNA was extracted for three isolates with the highest and three with the lowest EC 50 for mefentrifluconazole, followed by PCR and sequencing of the CYP51 gene. All three high-EC 50 isolates carried mutations: two had the G461S mutation, and one had the R464K mutation. The efficacy of mefentrifluconazole was compared with cyprodinil and fludioxonil on detached fruit assays using three isolates (one sensitive, one with the G461S mutation and one with R464K). Mefentrifluconazole was effective against sensitive isolates, performing similarly to fludioxonil and cyprodinil, and represents a valuable option for gray mold management.