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In Vitro and in Vivo Effects of Pseudomonas spp. on Pythium aphanidermatum: Zoospore Behavior in Exudates and on the Rhizoplane of Bacteria-Treated Cucumber Roots. T. Zhou, Postdoctoral research associate, Department of Plant Science, Macdonald Campus of McGill University, Ste. Anne de Bellevue, Québec, Canada H9X 3V9, Present address: Department of Environmental Biology, University of Guelph, Guelph, Ontario, Canada N1G 2W1; T. C. Paulitz, assistant professor, Department of Plant Science, Macdonald Campus of McGill University, Ste. Anne de Bellevue, Québec, Canada H9X 3V9. Phytopathology 83:872-876. Accepted for publication 18 May 1993. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/Phyto-83-872.

Roots of 4-day-old cucumber (Cucumis sativus cv. Corona) seedlings grown in hydroponic solution in test tubes were either not treated or treated with one of five strains of Pseudomonas corrugata and P. fluorescens. After 24 h, one-half of the plants were transferred to tubes with fresh nutrient solution (changed treatment) and one-half were left in the original solution with the bacteria (no-change treatment). After 48 h, the nutrient solution containing root exudates was filter-sterilized and tested for ability to attract zoospores of Pythium aphanidermatum, using a capillary tube bioassay. Fewer zoospores were attracted to root exudates from seedlings treated with any bacterial strain (no-change treatment) than to root exudates from seedlings not treated with bacteria; the distance of zoospore travel in the capillary tubes was also reduced. Germination of zoospores in root exudates from bacteria-treated roots was also significantly reduced by four of the five strains, in both the changed and the no-change treatments. Three-day-old seedlings treated or not treated with bacteria were inoculated with a zoospore suspension of P. aphanidermatum. Two hours after inoculation, the roots were fixed, stained with malachite green, counterstained with acridine orange, and observed with epifluorescence microscopy. Observations of 0.3-mm sections from the first 20 mm of root tip from each plant were recorded with video microscopy, and the number of encysted zoospores and percent zoospore germination were determined. Frequency of sections with no zoospores was higher in roots treated with bacterial strains than in those not treated, and the frequency of sections with >30 zoospores was reduced. One of the strains also increased the frequency of sections with no zoospore germination, whereas another strain decreased the frequency of sections with 51–75% germination. These bacteria reduced the attraction, encystment, and germination of zoospores of P. aphanidermatum both in situ and in root exudates from treated cucumber seedlings, possibly by utilizing carbon and nitrogen compounds in the root exudates that are required by P. aphanidermatum prior to infection.