Solution of diethyl ether extracts from culture filtrate of Valsa ceratosperma, a causal agent of apple Valsa canker, grown in a medium containing apple bark components caused similar bark necrosis to the canker on naturally infected apple trees. Three toxic substances were purified from culture extracts by column chromatography and were identified by co-chromatography and by ultraviolet absorption spectra. The toxic substances included phloretic acid (p-hydroxypropionic acid), p-hydroxybenzoic acid and protocatechuic acid which are known to be breakdown products of phloridzin. In cut stem bioassay these compounds showed a toxicity only in high concentrations. The toxicities of the compounds were not host specific.