Suppression of Pythium Root Rot of Cucumber by a Fluorescent Pseudomonad s Related to Reduced Root Colonization by Pythium Apbanidermatum

Root rot caused by Fviivum spp. is one ofthe most important diseases of vegetables and flowers grown in soilless culture. The negative effect of the strain OP4 of Fythium apitanidermatum on cucumber yield has previously been demonstrated. Introduction of the antagonistic strain fluorescent pseudomonad CH31 was shown partly to counteract the yield loss caused by F. aphanidermatum OP4. Root colonization of cucumber with F. aphanidermatum OP4 was examined m the absence and in the presence of either fluorescent pseudomonad strain CH3! or strain CHI. Quantification was performed by antigen-coated plate enzyme-linked immunosorbent assay (ACP-ELISA) using an antiserum obtained from a rabbit immunized with zoospore cysts: this antiserum did not cross-react with non-infested roots of cucumber, or with the fluorescent pseudomonads CH31 and CHI. Application ofthe previously developed biotest enabled us to assess the ability of the two strains of fluorescent pseudomonad to suppress root rot caused by F. aphanidermatum OP4. At all bacterial densities tested, the fluorescent pseudomonad CH31 led to a significant reduction of both the root colonization of cucumber with P aphanidermatum OP4 and the severity of Pythium root rot. In contrast, the fiuorescent pseudomonad