Abstract: The inhibitory activity of Citri grandis oil on Stagonosporopsis cucurbitacearum of Siraitia grosvenorii was determined by the plate bacteriostatic method combined with pot experiment. The antifungal mechanism of the volatile oil of Citri grandis was investigated by observing the morphology of pathogenic mycelia and measuring the change of conductivity and soluble protein content in the treatment solution of pathogenic mycelia treated by volatile oil. The results of the plate antibacterial test indicated that the inhibition rate of bacterial filamentum growth was 100.0% when the volatile oil was diluted less than 1 000 times, which was significantly higher than the inhibition rate of 80% carbendazim wettable powder diluted 1 000 times （88.7%）. The results of pot experiment showed that when the volatile oil was diluted 1 000 times, the inhibition rates of 45% pentazole-imimidazole emulsion and 10% difenoconazole emulsion were both above 80.0%, and significantly higher than that of 80% carbendazim wettable powder （70.68%）. Under the microscope, it was found that the pathogenic mycelia treated with Citri grandis volatile oil for 7 days showed a moniliform expansion and fracture phenomenon. The results of co-culture of mycelium with volatile oil showed that the conductivity and soluble protein content of the mixture increased with the increase of volatile oil concentration and the prolongation of interaction time. The conductivity of the culture medium was 279.208 μS/cm, which was the maximum value of each treatment, when volatile oil with the concentration of 2.0 g/L was co-cultured with pathogenic mycelia for 48 h. When the final mass fraction of volatile oil was 0.2%, the soluble protein content in culture medium reached 90.6 μg/mL after 60 h of treatment, which was the maximum value of each treatment.

Key words: Siraitia grosvenorii, Stagonosporopsis cucurbitacearum, Citri grandis Exocarpium, essential oil, inhibition and mechanism