Evaluating the efficacy and molecular mechanisms of the effects of Trichoderma harzianum T15 against Bursaphelenchus xylophilus

BACKGROUND Pine wilt disease, caused by Bursaphelenchus xylophilus , devastates pine ecosystems. As a well‐known biocontrol fungus, Trichoderma harzianum has been shown to exhibit broad‐spectrum antagonistic effects against multiple pathogens and nematodes. In this study, we employed morphological, gas chromatography–mass spectrometry (GC–MS), and RNA‐sequencing analyses to systematically evaluate its efficacy against Bursaphelenchus xylophilus and to elucidate the underlying molecular mechanisms. These findings clarify the biocontrol potential of T. harzianum and support developing new agents. RESULTS We isolated and identified T. harzianum T15 from the rhizosphere soil of Pinus koraiensis . Following nematode death, the body darkened and the cuticle ruptured. T15 mycelium penetrated the nematode cuticle while releasing volatile toxin (phthalates and terpenoids). Trichoderma harzianum killed nematodes through binding to and degrading body walls, proteases and toxins. Ethyl acetate and dichloromethane extracts were used from fermentation; nematode mortality increased with both time and concentration. The 15% ethyl acetate fraction achieved 99% mortality within 72 h. T15 revealed significant enrichment of biological regulation and microbial metabolism in diverse environments, with up‐regulation of peptidase_S8 , chitinase , and detoxification/antioxidant genes in transcriptome. Glycolysis, fructose/mannose, and methane metabolism were activated; elevated pgm and gpi enabled carbohydrate scavenging from nematodes, while increased ribose‐5‐phosphate, cyp450, adh and d ‐gluconolactone‐6‐phosphate supported nucleic‐acid synthesis and antioxidant defense, facilitating mycelial growth and ultimately killing nematodes. CONCLUSION This study provides important insights into the potential of T. harzianum as a viable biological control agent for Bursaphelenchus xylophilus . Additionally, this research increases our understanding of biological control and contributes to the development of sustainable forestry practices.