Antifungal Activity and Resistance Risk of Cyclobutrifluram against <i>Fusarium solani</i> , the Causal Agent of Peanut Root Rot

Peanut root rot (PRR), caused by <i>Fusarium solani</i>, is a common and destructive disease. Cyclobutrifluram exhibits antifungal activity against <i>Fusarium</i> spp., yet its bioactivity and resistance risk against <i>F. solani</i> remain unclear. In this study, the EC₅₀ values of cyclobutrifluram against 155 <i>F. solani</i> strains ranged from 0.0279 to 0.1102 μg/mL, with a mean of 0.0513 ± 0.0170 μg/mL. Treatment with cyclobutrifluram significantly inhibited conidial production and germination, reduced succinate dehydrogenase (SDH) activity, increased hyphal branching, and shortened the distance between septa or cell nuclei. Cyclobutrifluram provided excellent control over PRR. Seven laboratory-generated cyclobutrifluram-resistant mutants showed no or slight fitness penalties and exhibited positive cross-resistance only with pydiflumetofen, indicating a moderate-to-high resistance risk. Resistance to cyclobutrifluram in <i>F. solani</i> was conferred by four SDH substitutions (<i>FsSdhB</i>^H248Y, <i>FsSdhC1</i>^A83V, <i>FsSdhC2</i>^A85V, and <i>FsSdhD</i>^D184G), which was confirmed by molecular docking and gene replacement. This study provides crucial insights for developing effective control strategies to manage PRR.