Parasitism and Biological Control of Verticillium dahliae by Pythium oligandrum.

Pythium oligandrum was a parasite of Verticillium dahliae in dual culture, impeding the latter's ability to grow and form microsclerotia. Variation occurred among isolates of P. oligandrum in their parasitism toward isolates of V. dahliae, and isolates of V. dahliae expressed a range of susceptibilities to parasitism by P. oligandrum. Temperature and matric potential significantly affected interactions between these fungi. Populations of P. oligandrum in the rhizosphere of pepper (Capsicum annuum) were similar in a potting soil infested and noninfested with V. dahliae, but the populations of V. dahliae in combined samples of rhizosphere and nonrhizosphere soil of pepper were significantly lower in the presence of P. oligandrum than in its absence. In greenhouse experiments with pepper grown in soil infested with V. dahliae, shoot and fruit weights were significantly higher in the presence of P. oligandrum than in its absence. However, when soil was infested only with P. oligandrum, fresh weights of shoots and fruits were 40 to 50% higher than when plants were grown in its absence, evidence that this fungus significantly promoted growth of this crop. Therefore, processes responsible for improved plant health associated with soil treatment with P. oligandrum could be the result of complex interactions between pathogen, host, and mycoparasite.

Rhizosphere Competence of Pythium oligandrum.

ABSTRACT The associations of Pythium oligandrum with the root cortex, rhizoplane, and rhizosphere were measured with 11 crop species. This work was expedited by the use of a semiselective technique for isolation of P. oligandrum from soil and plant material. Cortical colonization of roots by P. oligandrum was not detected, and the rhizoplanes of the roots of most crops were free of the fungus. However, P. oligandrum was detected in large quantities with every crop tested when roots with adhering soil (rhizosphere soil) were assayed. Different crop species and cultivars of cantaloupe, cauliflower, and tomato varied in rhizosphere densities of P. oligandrum, but rhizosphere population densities of the fungus were consistently higher than in nonrhizosphere soils with plants grown in P. oligandrum-infested sterilized potting mix or an unsterilized mineral soil. After transplanting tomatoes into potting mix infested with P. oligandrum, increases in CFU occurred over time in the rhizosphere but not in the nonrhizosphere soil. In trials on delivery methods of inoculum of P. oligandrum, the rhizosphere populations of tomato plants grown in potting mix were about sixfold higher compared to seed-coat treatments when ground, alginate pelleted oospores were applied to seedlings growing in plug containers prior to transplanting or to pots containing potting mix before direct seeding.