Synergistic and Fungicide-Reducing Effects of Biological Agents with Chemical Fungicides against Ginseng Rusty Root Rot.

Ginseng rusty root rot caused by Ilyonectria robusta threatens ginseng production. This study screened synergistic combinations of biocontrol agents and chemical fungicides. Combined treatments inhibited mycelial growth, yield, and conidial production and germination. Among nine combinations, five achieved over 50% control efficacy; NJF 5:1 reached 81.48% field efficacy and reduced I. robusta in roots and soil. Biocontrol strain colonization increased versus singlestrain treatments. Ginseng defense enzyme genes and soil enzyme activities were enhanced. Three-stage soil microbiomics showed improved diversity, increased beneficial Granulicella and Pseudogymnoascus, and decreased Ilyonectria pathogen, which positively correlated with disease index, while beneficial genera correlated with soil enzyme activity. Seven lipopeptide biosynthesis genes were upregulated by the combination versus single strain. Metabolomics and transcriptomics confirmed that fludioxonil promoted the accumulation of antifungal p-hydroxybenzaldehyde and 4-methylphenol in NJ13. This study presents a novel synergistic interaction model reducing chemical usage and clarifies multiple synergistic effects against ginseng rusty root rot.