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.