| Summary: | Soilless cultivation has increased in China. Like vegetables or ornamental plants, tomatoes can be cultivated in soilless culture systems. Fusarium wilt (FW) is an economic tomato disease. The nature and amount of volatile organic compounds in <i>Trichoderma asperellum</i> PT-15 were investigated, and 6-pentyl-α-pyrone (6-PP) was detected. Furthermore, the effect of 6-PP on <i>Fusarium oxysporum</i> HF-26 was evaluated. Results revealed that 25 mg/L 6-PP was the optimal concentration inhibiting <i>F. oxysporum</i> HF-26 and that the content of fusaric acid decreased considerably compared to that of the control. <i>FUB1</i>, <i>FUB4</i>, and <i>FUB10</i> toxin synthesis and transport genes were downregulated. Additionally, <i>VelA, velB</i>, and <i>LaeA</i> genes were downregulated, reducing <i>F. oxysporum</i> mycelial growth and hyphae formation. 6-PP was added to the soilless culture solution in a greenhouse experiment. The results showed that an antifungal–nutrient solution containing 25 mg/L 6-PP significantly suppressed FW with 70.71% efficacy and a 27.23% disease index (DI), which were higher efficacy and lower DI than that of the control. Furthermore, treatment with an antifungal–nutrient solution containing 6-PP increased the levels of defence enzymes 24 h post-inoculation (hpi) compared with those at other time periods. The relative expression levels of the <i>PR1, NPR1, PR2</i>, and <i>PR5</i> genes were considerably upregulated at 24 hpi.
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