Activity of fungicide cyclobutrifluram against Fusarium fujikuroi and mechanism of the pathogen resistance associated with point mutations in FfSdhB, FfSdhC₂ and FfSdhD

Rice bakanae disease (RBD) is a devastating plant disease caused by Fusarium fujikuroi. This study aimed to evaluate the potential of cyclobutrifluram, a novel succinate dehydrogenase inhibitor (SDHI), to control RBD, and determine the risk and mechanism of resistance to cyclobutrifluram in F. fujikuroi. In vitro experiments showed that cyclobutrifluram significantly inhibited mycelial growth and spore germination, and altered the morphology of mycelia and conidia. Treatment with cyclobutrifluram significantly decreased mycotoxin production and increased cell membrane permeability in F. fujikuroi. The baseline sensitivity of 72 F. fujikuroi isolates to cyclobutrifluram was determined using mycelial growth and spore germination inhibition assays, which revealed EC₅₀ values of 0.0114–0.1304 and 0.0012–0.016 μg mL^–1, with mean EC₅₀ values of (0.0410±0.0470) and (0.0038±0.0015) μg mL^–1, respectively. Pot experiments demonstrated that the protective effect of cyclobutrifluram against F. fujikuroi was more significant than that of phenamacril and azoxystrobin, indicating that cyclobutrifluram is a promising antifungal agent for the control of RBD. Six cyclobutrifluram-resistant mutants of F. fujikuroi were obtained via fungicide adaptation. Moreover, these mutants exhibited weaker fitness than their parental isolate and positive cross-resistance with other SDHI fungicides, including pydiflumetofen and penflufen; however, no cross-resistance was detected with other classes of fungicides, including phenamacril, fludioxonil, prochloraz, or azoxystrobin. These results indicated that the resistance risk of F. fujikuroi to cyclobutrifluram might be moderate. Sequencing analysis revealed that mutations, including H248D in FfSdhB, A83V in FfSdhC₂, and S106F and E166K in FfSdhD, contributed to resistance, which was confirmed by molecular docking and homologous replacement experiments. The results suggest a high potential for cyclobutrifluram to control RBD and a moderate resistance risk of F. fujikuroi to cyclobutrifluram, which are meaningful findings for the scientific application of cyclobutrifluram.