Characterization of multiple amino acid substitutions in succinate dehydrogenase subunits conferring low level resistance to bixafen in Magnaporthe oryzae.

Rice blast caused by Magnaporthe oryzae is one of the most destructive diseases of rice. Due to issues of resistance and low control efficacies of traditional fungicides, it is essential to find a novel fungicide to solve these problems. Bixafen is a succinate dehydrogenase inhibitor, while its inhibitory activity against M. oryzae is not well studied. In this study, we established the baseline sensitivity of M. oryzae to bixafen and the average EC value was 1.6730 μg/mL for mycelial growth. Besides, the sensitivity curve was unimodal over the sensitive range. Fifteen resistant mutants were induced with 10 mutation types, including MoSdhB, MoSdhB, MoSdhB, MoSdhC, MoSdhD, which all conferred low level resistance to bixafen. Molecular docking suggested that all the mutations decreased the binding-force score for the affinity between bixafen and MoSdhB/C/D, which further confirmed that these point mutations should be responsible for the resistance to bixafen in M. oryzae. All bixafen-resistant mutants showed decreased mycelial growth, mycelial dry weight, and sporulation, indicating fitness penalty for these mutations. Moreover, positive cross-resistance was detected between bixafen and carboxin/fluxapyroxad, but was not found with other classes of fungicides, such as difenoconazole or kresoxim-methyl. Taken together, bixafen could be treated as a candidate for the control of rice blast with a moderate to low risk of fungal resistance. Also, combined or rotated use of bixafen with other classes of fungicides is recommended and the field resistance should be continuously monitored.