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

<i>Bipolaris maydis</i> (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 <i>B. maydis</i> to Pyd and Flu were 0.0110 ± 0.0059 and 0.1211 ± 0.0377 μg/mL, respectively, by the mycelial growth method. <i>In vitro</i>, one Pyd-resistant mutant and two Flu-resistant mutants were generated via domestication of Pyd and Flu, respectively. Genetic validation confirmed that the BmSdhB^S231L and BmSdhC^R76C 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 BmSdhB^S231L is adjacent to the Qp site (BmSDHB-Pro230) and BmSdhC^R76C 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 <i>B. maydis</i> and insights into the SDHI's resistance mechanism.