SUPPRESSION OF PYTHIUM ROOT-ROT OF CUCUMBER BY A FLUORESCENT PSEUDOMONAD IS RELATED TO REDUCED ROOT COLONIZATION BY PYTHIUM-APHANIDERMATUM

Root rot caused by Pythium spp. is one of the most important diseases of vegetables and flowers grown in soilless culture. The negative effe ct of the strain OP4 of Pythium aphanidermatum on cucumber yield has p reviously been demonstrated. Introduction of the antagonistic strain f luorescent pseudomonad CH31 was shown partly to counteract the yield l oss caused by P. aphanidermatum OP4. Root colonization of cucumber wit h P. aphanidermatum OP4 was examined in the absence and in the presenc e of either fluorescent pseudomonad strain CH31 or strain CH1. Quantif ication was performed by antigen-coated plate enzyme-linked immunosorb ent assay (ACP-ELISA) using an antiserum obtained from a rabbit immuni zed with zoospore cysts; this antiserum did not cross-react with non-i nfested roots of cucumber, or with the fluorescent pseudomonads CH31 a nd CH1. Application of the previously developed biotest enabled us to assess the ability of the two strains of fluorescent pseudomonad to su ppress root rot caused by P. aphanidermatum OP4. At all bacterial dens ities tested, the fluorescent pseudomonad CH31 led to a significant re duction of both the root colonization of cucumber with P. aphanidermat um OP4 and the severity of Pythium root rot. In contrast, the fluoresc ent pseudomonad CH1 had no effect on either root colonization or disea se severity.