Baseline sensitivity and resistance monitoring of Botrytis cinerea isolates from Florida strawberry to pydiflumetofen

Botrytis fruit rot (BFR), caused by Botrytis cinerea, is a major fruit rot disease in Florida strawberry fields. The intensive use of fungicides to control BFR has resulted in resistance to several fungicide groups over the years, consequently requiring continuous monitoring of resistant populations for appropriate management recommendations. Pydiflumetofen is the newest single-site fungicide within the SDHI’s (FRAC group 7) for use on strawberry, and the development of resistance in B. cinerea populations from Florida fields is unknown. In this study, we determined the baseline sensitivity of B. cinerea isolates collected from Florida strawberry fields prior registration of pydiflumetofen, characterized Sdh-B and Sdh-C mutations conferring resistance to pydiflumetofen, and determined the frequency of pydiflumetofen resistance of B. cinerea isolates collected during four strawberry seasons in Florida. Baseline sensitivity of seventy isolates with no prior exposure to pydiflumetofen ranged from 0.020 to 0.365 µg/ml. During the 2019–20, 2020–21, 2021–22, and 2022–23 seasons, 271, 195, 156, and 116 B. cinerea isolates were collected from commercial fields and the resistance frequency to pydiflumetofen was 0.4% (one isolate), 8.7% (17 isolates), 15.4% (24 isolates), and 1.7% (two isolates), respectively. Sixteen isolates resistant to pydiflumetofen had a substitution of asparagine by isoleucine at codon 230 (N230I) and one isolate had a substitution of proline by phenylalanine at codon 225 (P225F) in the Sdh-B, conferring moderate or high resistance to pydiflumetofen. One highly resistant isolate also had a substitution of asparagine to serine at codon 87 of Sdh-C. Isolates were confirmed to be resistant to pydiflumetofen in fruit detached assays, with disease incidence of 72 to 88.1% for isolates harboring N230I mutation, 91.7% for the isolate harboring the P225F mutation, and 93.8% for the isolate harboring mutations in both Sdh-B and Sdh-C. Given the current low-level of resistant B. cinerea to pydiflumetofen, Miravis Prime®, a commercial mixture of pydiflumetofen and fludioxonil, might be a good alternative for managing BFR in Florida. Nevertheless, resistance management strategies should be taken to reduce selection pressure to this and other SDHI fungicides.