Isofetamid Sensitivity and SDHI Cross-Resistance in <i>Botrytis cinerea</i> from Strawberry in Shanghai, China

Isofetamid is a succinate dehydrogenase inhibitor (SDHI) fungicide used to combat gray mold. As a single-site fungicide, it inhibits cellular respiration and offers a novel chemical solution for managing strawberry gray mold in Shanghai. This research evaluated the response of 260 <i>Botrytis cinerea</i> populations to isofetamid collected from strawberry orchards across various districts of Shanghai (2019-2023), examined <i>SdhB</i> gene mutations linked to resistance, and studied cross-resistance relationships among four SDHIs: boscalid, isofetamid, fluopyram, and fluxapyroxad. The results demonstrated that the EC₅₀ values of 183 isolates for isofetamid ranged from 0.001 to 0.33 μg/mL, with a mean value of 0.16 ± 0.01 μg/mL. The frequency distribution of these EC₅₀ values exhibited a unimodal curve, suggesting it can be considered as the baseline sensitivity. From 2019 to 2023, a significant upward trend in resistance frequency to isofetamid was observed. Four distinct <i>SdhB</i> mutations were detected: H272Y/R, N230I, and P225F, at frequencies of 5.38%, 26.15%, 20.0%, and 9.62%, respectively. H272Y isolates showed the highest susceptibility to isofetamid and fluopyram, but were either resistant (R) or highly resistant (HR) to fluxapyroxad and boscalid. The N230I and P225F mutations resulted in R or HR to all tested SDHIs. Additionally, the cross-resistance analysis indicated that mutations conferring resistance to fluopyram also reduced isofetamid efficacy, while mutations conferring resistance to boscalid similarly compromised fluxapyroxad activity against <i>B. cinerea</i>. These results suggest that isofetamid is a highly effective fungicide against <i>B. cinerea</i> in Shanghai, but a significant increase in resistance frequencies occurred year by year. Consequently, adopting strategic fungicide rotations will reduce future selection pressure on existing SDHI-resistant isolates.