Selection of Rhizosphere Bacteria for Biological Control of Pythium aphanidermatum on Hydroponically Grown Cucumber

Six-hundred and four isolates of bacteria were cultured from the rhizosphere of cucumber (Cucumis sativus cv. Corona) grown in 34 agricultural and forest soils from Quebec. The isolates were tested for inhibition of mycelial growth and zoospore germination of Pythium aphanidermatum in vitro. Ninety-three isolates inhibited mycelial growth and 35 isolates, which reduced zoospore germination after 4.5 h to <50% of the control treatment, were selected for further testing. Thirty-four of the 35 isolates were effective as washed bacterial cells in reducing zoospore germination, but the cell-free culture filtrates of only 3 isolates were effective. All 35 isolates significantly reduced zoospore attraction to capillary tubes filled with 3 mM aspartic acid + 0.05% agar in an attraction assay. These bacterial isolates were also tested in plant bioassays on cucumber seedlings of cv. Straight Eight grown in test tubes with nutrient solution and inoculated with zoospores of P. aphanidermatum and/or bacterial isolates. Inoculation with P. aphanidermatum alone reduced the root length to 67% of the bacterium-free, pathogen-free control. The root lengths of plants inoculated with P. aphanidermatum plus 7 of the isolates ranged from 82 to 99% of the bacterium-free, pathogen-free control and were not significantly different from that control. One isolate significantly increased root length, compared to the treatment with P. aphanidermatum alone. Three isolates, when applied to plants not inoculated with P. aphanidermatum, stimulated root growth to 134% of the bacterium-free, pathogen-free control. Five of the most effective isolates were tested on cucumber plants grown in aerated hydroponic culture for 3 weeks; all were effective root colonizers and were detected at population densities of 10(4)-10(5)/cm(3) on the root surface after 3 weeks. The best isolate in the test tube bioassay also significantly increased the shoot dry weight and root volume of cucumber plants cv. Straight Eight or cv. Corona inoculated with P. aphanidermatum and grown in aerated hydroponic solution. This same isolate also decreased zoospore encystment and germination on the roots of cv. Straight Eight, 4 and 8 h after addition of zoospores to the hydroponic solution. The top 5 isolates in the plant bioassays, in terms of biocontrol ability, were also ranked among the top 5 isolates in the zoospore germination and/or the zoospore motility bioassays. Selection of isolates for in planta testing based on their performance in zoospore bioassays may be a useful tool for prescreening large number of isolates, to increase the likelihood of identifying potential antagonists of zoosporic fungi. In general, in vitro assays were not useful predictors of the relative performance of isolates in planta. The combination of in vitro and in planta bioassays, however, was useful for selecting effective isolates and eliminating ineffective ones. (C) 1992 Academic Press, Inc.