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

Roots of 4-day-old cucumber (Cucumis sativus cv. Corona) seedlings gro wn in hydroponic solution in test tubes were either not treated or tre ated with one of five strains of Pseudomonas corrugata and P. fluoresc ens. 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-steril ized and tested for ability to attract zoospores of Pythium aphaniderm atum, 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 wit h 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 stra ins, in both the changed and the no-change treatments. Three-day-old s eedlings treated or not treated with bacteria were inoculated with a z oospore suspension of P. aphanidermatum. Two hours after inoculation, the roots were fixed, stained with malachite green, counterstained wit h acridine orange, and observed with epifluorescence microscopy. Obser vations 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 bacteri al strains than in those not treated, and the frequency of sections wi th >30 zoospores was reduced. One of the strains also increased the fr equency of sections with no zoospore germination, whereas another stra in decreased the frequency of sections with 51-75% germination. These bacteria reduced the attraction, encystment, and germination of zoospo res of P. aphanidermatum both in situ and in root exudates from treate d cucumber seedlings, possibly by utilizing carbon and nitrogen compou nds in the root exudates that are required by P. aphanidermatum prior to infection.