Development of Pythium oligandrum drechsler for biological control of fungal soil-borne diseases
The nutritional and environmental requirements for mycelial growth, oospore production and germination of Pythium oligandrum were examined. Optimum temperatures for growth of several isolates were in the range of 20 - 30 0 , with little growth occurring below 100 or above 350 Oospore germination occurred over the range of 10-35°. Both growth and oospore germination occurred over the range of pH 4.5 - 9.0 and were optimum between pH 6.0 - 7.5. Growth was reduced markedly below -1.0 to -1.5 MPa osmotic potential and ceased at approximately -2.5 to -3.5 MPa; similar results were obtained for oospore germination. Growth and oospore germination were affected more by low matric than by low osmotic potentials. Oospore production required an exogenous supply of sterols; it was also increased by the presence of calcium and affected by the C:N ratio. Semi-solid, static and aerated culture systems were developed for bulk production of P. oligandrum oospores. A liquid cane molasses medium was particularly convenient and efficient. A range of formulations were prepared using oospores produced mainly in this medium. Formulations were evaluated against pathogens causing damping-off in cress and the level of biocontrol in artificially infested sand was not as good as that obtained in naturally infested soil. Alginate pellets and a perlite preparation survived well in laboratory storage at 5-25° for at least 24 wk. Seeds of cress and sugar beet were coated with oospores using commercial seed-pelleting and film-coating procedures. Both types of seed treatment reduced damping-off of cress caused by P. ultimum in artificially infested sand and potting compost, and by Rhizoctonia solani in artificially infested sand. In general, pelleting of P. oligandrum on cress gave better control than film-coating treatments. P. oligandrum also reduced damping-off of sugar beet in soil naturally infested with Aphanomyces cochlioides and Pythium spp.. Control was equivalent to that achieved with hymexazol fungicide seed-coating treatments and was related to the inoculum potential of A. cochlioides in the soil.