Evaluation of Rhizosphere Bacteria for Biological Control of Pythium Root Rot of Greenhouse Cucumbers in Hydroponic Culture. L. RANKIN, Former Graduate Research Assistant, Department of Plant Science, Macdonald Campus of McGill University, 21,111 Lakeshore Rd., Ste. Anne de Bellevue, Qu6bec, Canada H9X 3V9. T. C. PAULITZ, Assistant Professor, Department of Plant Science, Macdonald Campus of McGill University, 21,111 Lakeshore Rd., Ste. Anne de Bellevue, Qu6bec, Canada H9X 3V9. Plant Dis. 78:447-451. Accepted for publication 20 January 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-0447.

Five bacterial isolates were evaluated for their ability to reduce root rot caused by Pythium aphanidermatum (Pa) on cucumber (Cucumis sativus L. cv. Corona) grown under simulated commercial conditions in a rock wool hydroponic system. Two isolates of Pseudomonas corrugala (Pcl3 and Pc35) and three of Pseudomonas fluorescens (Pf 15, Pf 16, and Pf27) were evaluated in both spring and early fall 1991 crops. Isolates Pcl3, Pc35, and Pf 15 were also evaluated in a 1992 spring crop. In all crops, 5-wk-old plants were set onto rock wool slabs and treated with water or 200 ml of a bacterial suspension (106 cells per milliliter). One-half of the plants were inoculated with 105 zoospores of Pa strain 186. In 1991, Pa inoculum was applied 6 days after treatment with biocontrol agents. In 1992, it was applied three times: 1 wk before, at the same time, and I wk after treatment with the biocontrol agents. In the 1991 spring crop, inoculated plants treated with Pcl3 or Pfl5 produced 88% more marketable fruit than the inoculated control (0.10 > P> 0.05, orthogonal contrasts). In the absence of the pathogen, these two isolates significantly increased the number of fruit (32 and 41 %, respectively), compared to the noninoculated control (P < 0.05). In the fall crop, treatment of Pa-inoculated plants with Pc 13 or Pf 15 significantly increased marketable fruit production (almost 600% over that of the inoculated control). Pc 13 and Pf 15 also increased total fruit weight produced by inoculated plants. These isolates significantly increased the shoot dry weight in plants not inoculated with the pathogen. In both crops, treatment with any of the bacterial isolates resulted in significantly reduced cull rates compared to the Pa-inoculated control. Disease development in the 1992 crop was lower than in the previous year, and the effect of treatments with biocontrol agents was not significant. However, treatment with Pfl5 alone resulted in significantly greater fruit production compared to plants not inoculated with Pa or treated with biocontrol agents.