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

Pydiflumetofen, a succinate dehydrogenase inhibitor (SDHI), has been widely used to control <i>Fusarium</i> head blight (FHB) caused by <i>Fusarium asiaticum</i> since 2020. Resistance monitoring across different provinces in China from 2020 to 2023 detected no pydiflumetofen-tolerant or resistant <i>F. asiaticum</i> isolates. In 2024, 58 out of 539 <i>F. asiaticum</i> 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 <i>SdhB</i>/<i>C1</i>/<i>C2</i>/<i>D</i> of Pyd-TR. However, the expression of <i>SdhC1</i> 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 <i>F. asiaticum</i> to pydiflumetofen in fields, which is associated with <i>SdhC1</i> overexpression.