Baseline sensitivity to the SDHI fungicide pydiflumetofen and resistance mechanisms in Fusarium oxysporum, one of the causal agents of soybean root rot in China.

BACKGROUND

Soybean root rot (SRR) is a devastating disease that severely reduces yield and quality in China, with Fusarium oxysporum recognized as one of the most prevalent causal agents. Pydiflumetofen, a new-generation succinate dehydrogenase inhibitor (SDHI) fungicide, exhibits potent efficacy against Fusarium spp. However, the resistance risk and underlying mechanisms of the SRR pathogen to pydiflumetofen remain largely unexplored.

RESULTS

Baseline sensitivity was determined using 104 isolates collected from five major soybean-producing provinces. The half-maximal effective concentration (EC50) values ranged from 0.0106 to 0.1068 μg mL-1, with a mean of 0.0437 ± 0.0187 μg mL-1. The unimodal frequency distribution indicated no naturally occurring resistant subpopulations in the field. Through fungicide-directed laboratory selection, 12 resistant mutants were obtained, showing resistance factors (RFs) between 82.70 and 7136.98. Phenotypic characterization revealed divergent fitness costs among mutants. Sequence analysis identified two point mutations: A83V in FoSdhC₁ and D133N mutation in FoSdhD. Site-directed mutagenesis via homologous recombination confirmed the causal relationship between these mutations and pydiflumetofen resistance. The A83V mutation conferred high-level resistance (RF > 100), while the D133N mutation conferred moderate-level resistance (RF between 10 and 100). Cross-resistance assays showed positive cross-resistance with cyclobutrifluram, but no cross-resistance with difenoconazole, fludioxonil, carbendazim, or pyraclostrobin.

CONCLUSION

The resistance risk of F. oxysporum to pydiflumetofen is categorized as moderate overall. The FoSdhC₁A83V mutation confers high-level resistance, and the FoSdhDD133N mutation confers moderate-level resistance. The established baseline provides a critical reference for field resistance monitoring, and these findings support the rational use of pydiflumetofen and the development of resistance management strategies. © 2026 Society of Chemical Industry.