Allelopathic control of pine needle litter on Ginseng rust rot under forest: Mechanisms uncovered by metabolomics and targeted validation

Panax ginseng C. A. Meyer is a high-value medicinal crop increasingly cultivated in understory systems for sustainable production. However, its yield and economic return are severely constrained by ginseng rust rot, caused by Ilyonectria robusta . To address this constraint, we elucidated how canopy tree species regulate disease incidence via distinct allelopathic compounds released from decomposing needle litter. Using UHPLC-Orbitrap-HRMS-based untargeted metabolomics, we identified vanillic acid as the phenolic acid displaying the most significant differential abundance between decomposing red pine (RP) and Japanese larch (JL) needles. Targeted quantification confirmed its presence in RP forest soil at an ecologically relevant concentration of 0.18 μmol·L ⁻¹ . Bioassays revealed a concentration-dependent biphasic effect of vanillic acid on Ilyonectria robusta . Specifically, a low concentration (0.1 μmol·L ⁻¹ ) stimulated mycelial growth and increased spore germination rate to 92.5%. In contrast, concentrations ≥ 1.0 μmol·L ⁻¹ were inhibitory, with mycelial growth suppressed by 34.7% at 5.0 μmol·L ⁻¹ . Concurrently, vanillic acid at 0.1 μmol·L ⁻¹ suppressed radicle elongation of ginseng. These findings elucidate a biochemical mechanism for the elevated disease incidence under RP canopies and provide a direct scientific rationale for selecting JL over RP as canopy trees in understory ginseng systems. This supports the development of ecology-based management strategies to enhance sustainable production.