Abstract. Pythium spp. that cause damping-off of seedlings also can cause root rot of older plants and lead to yield reductions. This can be especially severe in soilless cultures where the fungus can spread easily with the nutrient solution. 39 Pythium isolates obtained from discolored roots were assayed for their ability to cause damping-off on cucumber seedlings in sand-peat and for their pathogenicity in soilless culture of cucumber in rockwool or hydroponic solution. Isolates of Pythium aphanidermatum, P. irregulare, P. sylvaticum and P. ultimum were highly pathogenic in sand-peat, but only P. aphanidermatum strains were pathogenic in soilless conditions and led to root decay, plant death in rockwool culture and growth reduction in hydroponic culture. One strain of P. aphanidermatum significantly reduced the yield of cucumber grown in rockwool under conditions similar to those of commercial cultures.

Root colonization of cucumber with P aphanidermatum OP4 was examined in the absence and in the presence of either fluorescent pseudomonad strain CH31 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 of the previously developed biotest enabled us to assess the ability of the two strains of fluorescent pseudomonad to suppress root rot caused by P. aphanidermaium 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 fluorescent pseudomonad CHI had no effect on either root colonization oe disease sevenity.