Isofetamid Sensitivity and SDHI Cross-Resistance in Botrytis cinerea 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 Botrytis cinerea populations to isofetamid collected from strawberry orchards across various districts of Shanghai (2019 to 2023), examined SdhB 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 SdhB 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 B. cinerea. These results suggest that isofetamid is a highly effective fungicide against B. cinerea 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.