Pine-Extracted Volatile Oils Suppress Root Rot in Psammosilene tunicoides Through Direct Antifungal Activity and Rhizosphere Microbiome Modulation

Frequent outbreaks of root rot in Psammosilene tunicoides W. C. Wu & C. Y. Wu severely compromise the quality of its medicinal materials and hinder its large-scale cultivation. Interestingly, wild P. tunicoides growing under pine trees rarely experience this disease. To explore the potential basis of root rot suppression, we evaluated the direct antifungal activity of pine-derived volatile oils and the associated changes in the rhizosphere microbiome. GC-MS showed that pine turpentine was dominated by α-pinene (45.50%) and longifolene (28.20%). In vitro assays confirmed its highly efficient inhibition (81.65–94.71%) against major root rot pathogens in P. tunicoides. Beyond direct antifungal effects, metagenomic analysis indicated that volatile oil (SYR) treatment was associated with shifts in the rhizosphere microbiome, including increased relative abundances of potentially beneficial taxa, such as Paenibacillus, Trichoderma, and Geosiphon. Pine volatiles might be associated with shifts in the rhizosphere microbial community of P. tunicoides, potentially involving plant-mediated changes in root exudation and the enrichment of certain beneficial microbes. However, it remains to be further elucidated regarding the specific mechanisms underlying these community changes. Functional prediction of the microbial community suggested a predominance of metabolic pathways, secondary metabolite biosynthesis, and flagellar assembly in the SYR group. Conclusively, pine volatiles may contribute to root rot suppression through two potential processes: direct pathogen inhibition and beneficial microbiome enrichment. This study provides a theoretical basis for establishing sustainable agroforestry co-planting systems for P. tunicoides.