Abstract:In order to investigate the effect of the molecular weight less than 8kD secretion of Pythium oligandrum on plant pathogenic fungi, the inhibition effect on 9 species total 16 isolates of plant pathogenic fungi was tested on Petri dishes with cellophane paper and growth rate methods. The secretion of P.oligandrum distinctly depressed the mycelia growth of P.sylvaticum, Botrytis cinerea, and Fusarium culmorum with inhibition rates of 88.9%, 86.7% and 84.6%, respectively. The inhibition rates of four Pythium species and four formae speciales of F.oxysporum were significant difference (P=0.05), respectively. The morphology of B.cinerea treated with the secretion of P.oligandrum was abnormal under scanning electron microscope with mycelia shriveled, small spores appeared in the cell wall and spore output reduced significantly. In the greenhouse trial, the filtrate of P.oligandrum had significantly protective and curative effect against tomato gray mould, and the protective effect of the filtrate was equivalent strong with carbendazim 1000 times dilution.