Multiple Mutations in SDHB and SDHC₂ Subunits Confer Resistance to the Succinate Dehydrogenase Inhibitor Cyclobutrifluram in <i>Fusarium fujikuroi</i>

Abstract

<i>Fusarium fujikuroi</i> is one of the dominant phytopathogenic fungi causing rice bakanae disease worldwide. Cyclobutrifluram is a novel succinate dehydrogenase inhibitor (SDHI), which shows strong inhibitory activity against <i>F. fujikuroi</i>. The baseline sensitivity of 112 <i>F. fujikuroi</i> to cyclobutrifluram was determinated with a mean EC₅₀ value of 0.025 μg/mL. A total of 17 resistant mutants were obtained by fungicide adaptation and displayed equal or slightly weaker fitness than parental isolates, which suggests that the resistance risk of <i>F. fujikuroi</i> to cyclobutrifluram is medium. A positive cross-resistance was detected between cyclobutrifluram and fluopyram. The amino acid substitutions H248L/Y of FfSdhB and G80R or A83V of FfSdhC₂ conferred cyclobutrifluram resistance in <i>F. fujikuroi</i>, which was validated by molecular docking and protoplast transformation. The results indicate that the affinity between cyclobutrifluram and FfSdhs obviously decreased after point mutations, causing the resistance of <i>F. fujikuroi</i>.