Resistance Evolution and Fitness of Fusarium asiaticum from Fields to Succinate Dehydrogenase Inhibitor Pydiflumetofen in China

Pydiflumetofen, a succinate dehydrogenase inhibitor (SDHI), has been widely used to control Fusarium head blight (FHB) caused by Fusarium asiaticum since 2020. Resistance monitoring across different provinces in China from 2020 to 2023 detected no pydiflumetofen-tolerant or resistant F. asiaticum isolates. In 2024, 58 out of 539 F. asiaticum isolates (10.8%) collected from fields in China were identified as pydiflumetofen-tolerant (Pyd-TR), with their sensitivity distribution displaying a continuous rightward shift as EC50 values increased from 0.0168 to 0.0225 μg/mL and MIC values increased from 0.500 to 5.00 μg/mL. Pyd-TR exhibited growth, reproduction, and pathogenicity comparable to those of sensitive isolates. Cross-resistance was observed between pydiflumetofen and fluopyram but not with non-SDHI. No mutations were detected in SdhB/C1/C2/D of Pyd-TR. However, the expression of SdhC1 in Pyd-TR was significantly higher than in sensitive isolates after pydiflumetofen treatment. This study provides the first evidence of the early resistance evolution of F. asiaticum to pydiflumetofen in fields, which is associated with SdhC1 overexpression.