Amino acid substitutions in succinate dehydrogenase complex conferring resistance to the SDHI fungicide pydiflumetofen in Cochliobolus heterostrophus causing southern corn leaf blight

Southern corn leaf blight (SCLB) caused by Cochliobolus heterostrophus, is a widespread foliar disease that has a substantial impact on maize yield in the Huang-Huai-Hai Region of China. Pydiflumetofen (Pyd), a new succinate dehydrogenase inhibitor (SDHI), has been found as a promising fungicide for the efficient control of SCLB, however, resistance of C. heterostrophus to Pyd has not been studied well. Here, five Pyd-resistant mutants were generated through fungicide adaptation. Sequence alignment analysis revealed that these mutants primarily mutated in ChSdhB and ChSdhD, with three genotypes: ChSdhB^H277Y, ChSdhB^I279T and ChSdhD^H133Y, exhibiting two distinct categories of resistance: high resistance (HR) and moderate resistance (MR), among which the resistance factors were 214.22 and 44.33–53.67, respectively. These mutants were more pathogenic than the wild-type parental strains, but there was a significant reduction in mycelial growth rate and sporulation in the resistant mutants, indicating a significant fitness cost associated with resistance to Pyd. In addition, this study revealed a positive cross-resistance between Pyd and another SDHI fungicide cyclobutrifluram. However, no cross-resistance was found between Pyd and other classes of fungicides, including prochloraz, fludioxonil, iprodione or pyraclostrobin. Homology modeling and molecular docking further confirmed that point mutations of ChSdhB^H277Y, ChSdhB^I279T, and ChSdhD^H133Y could reduce binding affinity between Pyd and its target subunits from –74.07, –74.07, –152.52 kcal mol^–1 to –3.90, –4.95, –9.93 kcal mol^–1, respectively. These findings not only provided valuable insights for managing SCLB caused by C. heterostrophus, but also enhanced our understanding of molecular mechanism underlying plant pathogen resistance to Pyd.