Activity and Resistance Assessment of Three Acetohydroxyacid Synthase Inhibitors on <i>Phytophthora sojae</i> Development, Pathogenicity, and Inhibitor–Target Interaction Analysis

The branched-chain amino acid biosynthesis pathway was predicted to be one of the essential pathways in the plant pathogen <i>Phytophthora</i> species. In this study, we evaluated the activity of nine herbicides targeting acetohydroxyacid synthase (AHAS) in this pathway against <i>Phytophthora sojae</i>. Three herbicides, iodosulfuron-methyl, chlorimuron-ethyl, and florasulam, exhibited inhibitory effects on <i>P. sojae</i> mycelial growth, with EC₅₀ values of 1.04, 1.16, and 1.86 μg/mL, respectively. These herbicides significantly reduced spore production and pathogenicity of <i>P. sojae</i>. Furthermore, <i>P. sojae</i> herbicide-adapted strains displayed low-to-moderate resistance levels and reduced fitness, indicating a low resistance risk. No cross-resistance among the three herbicides was observed. Notably, <i>P. sojae</i> AHAS expression was upregulated following herbicide treatment, and molecular docking revealed that herbicides bind to a conserved binding pocket via hydrophobic interactions, hydrogen bonds, or π-stacking. Collectively, these findings demonstrated that <i>P. sojae</i> AHAS represents a potential target for the development of novel control agents.