RHIZOSPHERE COMPETENCE OF PYTHIUM-OLIGANDRUM

The associations of Pythium oligandrum with the root cortex, rhizoplan e, and rhizosphere were measured with 11 crop species. This work was e xpedited by the use of a semiselective technique for isolation of P. o ligandrum from soil and plant material. Cortical colonization of roots by P. oligandrum was not detected, and the rhizoplanes of the roots o f most crops were free of the fungus. However, P. oligandrum was detec ted in large quantities with every crop tested when roots with adherin g soil (rhizosphere soil) were assayed. Different crop species and cul tivars of cantaloupe, cauliflower, and tomato Varied in rhizosphere de nsities of P. oligandrum, but rhizosphere population densities of the fungus were consistently higher than in nonrhizosphere soils with plan ts gown in P. oligandrum-infested sterilized potting mix or an unsteri lized mineral soil. After transplanting tomatoes into potting mix infe sted with fi oligandrum, increases in CFU occurred over time in the rh izosphere but not in the nonrhizosphere soil. In trials on delivery me thods of inoculum of P. oligandrum, the rhizosphere populations of tom ato plants grown in potting mix were about sixfold higher compared to seed-coat treatments when ground alginate pelleted oospores were appli ed to seedlings growing in plug containers prior to transplanting or t o pots containing potting mix before direct seeding.