| Osthol, a natural coumarin compound (7-methoxy-8-[3-methylpent-2-enyl] coumarin) from Cnidium monnieri, showed a wide spectrum of antifungal and antibacterial activities against phytopathogens. In this study, antifungal activity of osthol against pumpkin powdery mildew was investigated by comparing the difference of morphology and biochemical parameters between the osthol-treated samples and the controls. Both Sphaerotheca fuliginea and pumpkin seedlings were studied after the treatment with osthol. Chitinase gene from pumkin leaves was isolaled and its differential expression in transcript level was studied by reverse transcription-polymerase chain reaction. Results showed that 100μg·mL-1 osthol had remarkable protection effects on pumpkin powdery mildew by inhibiting spore germination, hyphal growth and conidia formation of Sphaerotheca. fuliginea. Further research indicated that osthol could impair the adhesion of powdery mildew fungi to pumpkin cuticle by inhibiting the activity of nonspecific esterase secreted from the fungi. In addition, scanning and transmission electron microscope photos revealed that mycelia of Sphaerotheca fuliginea became abnormal (wizened appearance, melting cell wall and homogeneous cell contents) after treated with osthol. Besides, osthol could induce the increase of chitinase peroxidase and total soluble proteins, which conduced to the defence reactions of host plants. The increasing expression of pumpkin chitinase gene in transcript level explained the enhancement of the enzyme activity suppressing the growth of the fungi. In conclusion, osthol had a multiple mode of action against pumpkin, powdery mildew fungi not only for its direct antifungal activity but also for its resistance induction to plants.
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