Abstract: A biologically pure culture of Pythium rostratum is disclosed which produces zoosporangia which produce zoospores capable of killing knapweed. Also disclosed is a method for killing knapweed, comprising: (a) growing Pythium rostratum which selectively kills knapweed on a carrier saturated with a liquid nutrient capable of supporting Pythium rostratum growth and the production of zoosporangia; and (b) applying the Pythium rostratum carrier to knapweed-infested land under moisture and temperature conditions sufficient to support Pythium rostratum growth and zoospore release by the zoosporangia, such that the zoospores are produced which are capable of killing knapweed.

Abstract: Disclosed is an agricultural chemical composition in the form of an aqueous suspension, which comprises fine particles of a water-insoluble physiologically active substance having a melting point of 15.degree. to 70.degree. C., which are stably dispersed in water by a water-soluble or water-dispersible polymer comprising at least one monomer selected from the group consisting of unsaturated carboxylic acids and derivatives thereof as the indispensable component, as a dispersant, and a phthalic acid ester as an agent for preventing precipitation of crystals.In this agricultural chemical composition, the phthalic acid ester bleeds on the surface of the physiologically active substance to modify the surface, and the storage stability of the composition in the form of a suspension is highly improved.