Abstract:In order to clarify the main antifungal compound of Inula japonica, the chloroform extraction of crude extract from I.japonica flowers was separated and compound 1-O-acetylbritannilactone (ABL) was obtained. The content of the compound ABL in flowers crude extract was tested with high performance liquid chromatography. The inhibitory effects of compound ABL against Sphaerotheca fuliginea, Pseudoperonospora cubensis, Botrytis cinerea, Alternaria solani and Phytophthora infestans were tested. The chemical structure of ABL reference substance was elucidated by using ultraviolet spectrometry (UV), infrared spectroscopy (IR), nuclear magnetic resonance (NMR). The purity of ABL reference substance was 99.5%, which satisfies the need of reference substances of traditional Chinese medicines. A method of high performance liquid chromatography was used for the determination of the content of ABL in crude extract from I.japonica flowers (it’s 2.45%), then the content of ABL in I.japonica was calculated (it’s about 0.25%). Inhibitory activities of compound ABL against the five phytopathogenic fungi were different, which against the cystospore germination of P.infestans was significant, the inhibition rate was 100% at the concentration 0.1mg/mL.