Protection of cucumber against Pythium root rot by fluorescent pseudomonads: predominant role of induced resistance over siderophores and antibiosis

Four Pseudomonas strains were evaluated for their intrinsic properties conf erring their ability to protect long English cucumber against Pythium aphan idermatum in hydroponic culture. Two of the strains, BTP1 and its sideropho re-negative mutant M3, increased plant yield as compared with the nan-inocu lated control plants. Strain BTP7 was intermediate in its biocontrol activi ty while strain ATCC 17400 failed to reduce disease development. The role o f pyoverdines could not be confirmed since treatment with either BTP1 or it s siderophore-negative mutant M3 provided similar suppression of Pythium di sease. In addition, no siderophores were detected in the nutrient solution. BTP1 did not inhibit pathogen growth in vitro On several media, suggesting that antibiosis was not a mechanism of suppression. Quantification of root bacterial populations did not indicate differences among the strains. On t he other hand, roots treated with either BTP1 or its sid(-) mutant M3 conta ined more antifungal phenolics than roots from any other treatments includi ng controls. These results suggest that antifungal compounds induced by ino culation of cucumber roots with the fluorescent Pseudomomas strains BTP1 an d M3 participate actively in the protection of cucumber plants against P. a phanidermatum.