Abstract: Diplodia tip blight, caused by Diplodia sapinea, is a global pine necrotic disease causing heavy economic losses to the pine industry. Chemical control, its main current management, easily induces pathogen resistance and environmental pollution, which biological control avoids. This study investigated juniper essential oil’s efficacy against the disease on Mongolian Scots pine (Pinus sylvestris var. mongolica) and its induced resistance mechanisms via pot experiments, physiological assays (defense enzyme activities, resistant substances) and metabolomic sequencing (secondary metabolites). Results showed varied efficacy: three foliar sprays of 10 μL·mL^⁻1 oil achieved the best control (82.9%). The 20 μL·mL^⁻1 treatment significantly increased phenylalanine ammonia-lyase (PAL), polyphenol oxidase (PPO) activities, and contents of lignin, flavonoids and total phenolics. Metabolomic analysis showed 326 upregulated and 527 downregulated different metabolites in essential oil-induced and pathogen-inoculated pines, compared to 483 upregulated and 277 downregulated metabolites in non-induced but inoculated pines. The differentially expressed metabolites in treated pines were primarily enriched in pathways related to amino acid metabolism and plant secondary metabolite biosynthesis, with notably increased expression levels of ferulic acid, scopoletin, pipecolic acid, D-proline, and DL-arginine. Therefore, juniper essential oil protects against D. sapinea by inducing systemic acquired resistance in Mongolian Scots pine. In conclusion, juniper essential oil controls D. sapinea by inducing systemic acquired resistance (SAR) in Mongolian Scots pine, clarifying the molecular mechanism and supporting biological control of the disease.

Key words: Diplodia tip blight, Biological control,  , Induced resistance, Metabolomics, Plant defense compounds