INOCULATION OF CUCUMBER ROOTS WITH ZOOSPORES OF MYCOPARASITIC AND PLANT-PATHOGENIC PYTHIUM SPECIES - DIFFERENTIAL ZOOSPORE ACCUMULATION, COLONIZATION ABILITY AND PLANT-GROWTH RESPONSE

Hydroponically grown cucumber (Cucumis sativus) seedlings were inocula ted with zoospores of 1 mycoparasitic (Pythium oligandrum) and 2 patho genic (Pythium aphanidermatum and Pythium 'group F') Pythium spp. Duri ng the first 2 days after inoculation, all the Pythium spp. caused red uction in the root length. However, roots treated with Pythium oligand rum quickly reached the length of the control and on the 8th day, and for the rest of the experimental period, stimulation of root elongatio n was noted. Pythium oligandrum was not pathogenic on cucumber and no differences in the fresh weights of control and Pythium oligandrum ino culated plants were observed in the course of the experiment. Pythium 'group F' and Pythium aphanidermatum were pathogenic on cucumber seedl ings, but their pathogenicities differed. Thus, while Pythium 'group F ' had a constant, negative influence on root length and plant growth, measured as fresh weight, Pythium aphanidermatum caused generalized ne croses of the root system, inhibiting consistently root elongation and plant growth and finally causing plant death. Moreover, the zoospores of 2 mycoparasitic species, Pythium oligandrum and Pythium periplocum , were not attracted to roots of cucumber and accumulated on the roots in very low numbers compared to those of the pathogenic species, Pyth ium aphanidermatum, which were strongly attracted and accumulated in l arge numbers. Finally, it was also found that Pythium oligandrum colon ized the roots very poorly, while Pythium 'group F' and Pythium aphani dermatum were significantly better root colonizers. The significance o f these findings is discussed in relation to the ecology of Pythium sp ecies and biocontrol.