Antifungal Efficacy and Mechanisms of Methyleugenol from Asarum Essential Oil Against Rhizoctonia solani Causing Ginseng Damping-Off Disease

Abstract

Damping-off disease caused by Rhizoctonia solani is a devastating soilborne disease that poses a serious threat to sustainable ginseng (Panax ginseng) production. In an effort to develop environmentally friendly control alternatives, we investigated the volatile constituents of Asarum heterotropoides essential oil using GC-MS in 2023 and identified methyleugenol as the predominant bioactive component, accounting for 21.32% of the total volatile fraction. Mycelial growth rate assays demonstrated that methyleugenol exerted pronounced antifungal activity against R. solani, with EC50 and EC90 values of 60 mg/L and 800 mg/L, respectively. Treatment at 0.1 mg/mL resulted in 51% growth inhibition after 4 days of incubation. To elucidate the underlying mechanisms of action, R. solani mycelia exposed to 0.1 mg/mL methyleugenol were harvested at 4 and 6 days post-treatment for integrated transcriptomic and untargeted metabolomic profiling. Transcriptome analysis revealed 1444 and 1157 significantly downregulated genes at 4 and 6 days, respectively, with 836 genes consistently repressed at both time points. GO and KEGG enrichment analyses indicated that these persistently suppressed genes were predominantly associated with protein processing in the endoplasmic reticulum, protein export, tyrosine metabolism, and peroxisome biogenesis. Metabolomic profiling identified 95 differentially accumulated metabolites that were commonly reduced across both time points, with KEGG pathway enrichment highlighting significant suppression of starch and sucrose metabolism and galactose metabolism pathways central to carbon source utilization. Integrative transcriptome–metabolome correlation analysis further uncovered three key transcription factors (gene-RDB_LOCUS74145, gene-RDB_LOCUS26344, and gene-RDB_LOCUS52181) implicated in the regulation of carbohydrate metabolism and tricarboxylic acid cycle-related metabolite production. Collectively, our findings indicate that methyleugenol suppresses R. solani growth through a multi-target mechanism involving impairment of carbon source metabolism, disruption of protein processing and secretion, and attenuation of cellular energy supply. These results provide a mechanistic foundation for the development of botanical fungicide-based strategies for the green management of ginseng damping-off disease.