Study on Mechanisms of Resistance to SDHI Fungicide Pydiflumetofen in Fusarium fujikuroi

Rice bakaenii disease (RBD) is a widespread and devastating disease mainly caused by Fusarium fujikuroi. Pydiflumetofen (Pyd) is a novel succinate dehydrogenase inhibitor (SDHI) with strong inhibitory activity against F. fujikuroi, but the mechanism of resistance to Pyd has not been well studied for this pathogen. Through fungicide adaption, a total of 12 Pyd-resistant mutants were obtained and the resistance level could be divided into three categories of high resistance (HR), moderate resistance (MR), and low resistance (LR) with resistance factors (RF) of 184.04-672.90, 12.63-42.49, and <10, respectively. Seven genotypes of point mutations in FfSdh genes (FfSdhB^H248L, FfSdhB^H248D, FfSdhB^H248Y, FfSdhC₂^A83V, FfSdhC₂^H144Y, FfSdhD^S106F, and FfSdhD^E166K) were found in these mutants, among which genotype FfSdhB^H248L and FfSdhC₂^A83V mutants showed HR, genotype FfSdhB^H248D, FfSdhB^H248Y, FfSdhC₂^H144Y, and FfSdhD^E166K mutants showed MR, and genotype FfSdhD^S106F mutants showed LR. Moreover, all the substitutions of amino acid point mutations including FfSdhB^H248L/D/Y, FfSdhC₂^A83V,H144Y, and FfSdhD^S106F,E166K conferring resistance to Pyd in F. fujikuroi were verified by protoplast transformation. Additionally, a positive cross-resistance was detected between Pyd and another SDHI fungicide penflufen, while no cross-resistance was detected between Pyd and phenamacril, prochloraz, azoxystrobin, carbendazim, or fludioxonil. Although pathogenicity of the mutants was increased compared with that of the wild-type parental strains, the mycelial growth rate and spore production levels of the resistant mutants were significantly decreased (P < 0.05), indicating significant fitness cost of resistance to Pyd in F. fujikuroi. Taken together, the risk of resistance to Pyd in F. fujikuroi might be moderate, and appropriate precautions against resistance development in natural populations should be taken into account when Pyd is used for the control of RBD.