Activity and Resistance Assessment of Three Acetohydroxyacid Synthase Inhibitors on Phytophthora sojae 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 Phytophthora species. In this study, we evaluated the activity of nine herbicides targeting acetohydroxyacid synthase (AHAS) in this pathway against Phytophthora sojae. Three herbicides, iodosulfuron-methyl, chlorimuron-ethyl, and florasulam, exhibited inhibitory effects on P. sojae 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 P. sojae. Furthermore, P. sojae 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, P. sojae 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 P. sojae AHAS represents a potential target for the development of novel control agents.