EVALUATION OF RHIZOSPHERE BACTERIA FOR BIOLOGICAL-CONTROL OF PYTHIUM ROOT-ROT OF GREENHOUSE CUCUMBERS IN HYDROPONIC CULTURE

Five bacterial isolates were evaluated for their ability to reduce roo t rot caused by Pythium aphanidermatum (Pa) on cucumber (Cucumis sativ us L. cv. Corona) grown under simulated commercial conditions in a roc k wool hydroponic system. Two isolates of Pseudomonas corrugata (Pc13 and Pc35) and three of Pseudomonas fluorescens (Pf15, Pf16, and Pf27) were evaluated in both spring and early fall 1991 crops. Isolates Pc13 , Pc35, and Pf15 were also evaluated in a 1992 spring crop. In all cro ps, 5-wk-old plants were set onto rock wool slabs and treated with wat er or 200 ml of a bacterial suspension (10(6) cells per milliliter). O ne-half of the plants were inoculated with 10(5) zoospores of Pa strai n 186. In 1991, Pa inoculum was applied 6 days after treatment with bi ocontrol agents. In 1992, it was applied three times: 1 wk before, at the same time, and 1 wk after treatment with the biocontrol agents. In the 1991 spring crop, inoculated plants treated with Pc13 or Pf15 pro duced 88% more marketable fruit than the inoculated control (0.10 > P > 0.05, orthogonal contrasts). In the absence of the pathogen, these t wo isolates significantly increased the number of fruit (32 and 41 %, respectively), compared to the noninoculated control (P < 0.05). In th e fall crop, treatment of Pa-inoculated plants with Pc13 or Pf15 signi ficantly increased marketable fruit production (almost 600% over that of the inoculated control). Pc13 and Pf15 also increased total fruit w eight produced by inoculated plants. These isolates significantly incr eased 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 cont rol. Disease development in the 1992 crop was lower than in the previo us year, and the effect of treatments with biocontrol agents was not s ignificant. However, treatment with Pf15 alone resulted in significant ly greater fruit production compared to plants not inoculated with Pa or treated with biocontrol agents.