BmSdhB S231L or BmSdhC R76C Mutations Derived from Pydiflumetofen and Fluopyram Domestication Confer Differential SDHI-Resistant Mechanisms in Bipolaris maydis

Bipolaris maydis (B. maydis), the pathogen causing southern corn leaf blight, is spreading rapidly in China. New succinate dehydrogenase inhibitor (SDHI) fungicides pydiflumetofen (Pyd) and fluopyram (Flu) are potential control agents. Baseline sensitivities of B. maydis to Pyd and Flu were 0.0110 ± 0.0059 and 0.1211 ± 0.0377 μg/mL, respectively, by the mycelial growth method. In vitro, one Pyd-resistant mutant and two Flu-resistant mutants were generated via domestication of Pyd and Flu, respectively. Genetic validation confirmed that the BmSdhBS231L and BmSdhCR76C mutations individually confer differentiated resistance: high Flu/low Pyd resistance and low Flu/medium Pyd resistance, respectively. Fitness penalties were observed in some mutants, indicating a moderate to high resistance risk. Using AlphaFold3, the three-dimensional structure of BmSdh was modeled, showing that BmSdhBS231L is adjacent to the Qp site (BmSDHB-Pro230) and BmSdhCR76C is at the Qp site (BmSDHC-Arg76). Molecular docking revealed that these mutations reduce fungicide affinity. This study provides a basis for resistance management of B. maydis and insights into the SDHI's resistance mechanism.