The efficacy of osthol, a natural coumarin compound, in controlling powdery mildew was evaluated in 2004-2005 in Anhui and Hebei Provinces of China. In both years, the treatments (osthol 15.0 and 18.0 g ai ha(- 1)) showed a stable control efficiency of 75.42, 81.24% and 76.36, 84.84%, respectively, at the Institutes of Plant Protection of Hebei Academy of Agricultural Sciences. In field experiments, osthol was as effective as difenoconazole in controlling powdery mildew and was more effective than triadimefon against Sphaerotheca fuliginea. Protection was expressed as a significant reduction (up to 87% compared with the control) in the mildewed leaf area in young pumpkin plants. Osthol strongly inhibited spore germination and mycelial growth of S. fuliginea in vitro, damaged the cell wall and the organelles of the pathogen. At 48 h after incubation, 50 microg ml(- 1) osthol could completely inhibit spore germination. These findings suggested that the effect of osthol on powdery mildew may be associated with the direct fungitoxic property against the pathogen. We conclude that osthol would be an attractive natural compound for practical agronomic use against powdery mildew.