Control of Pythium ultimum by antagonistic fungal metabolites.

Microorganisms which suppressed the mycelial growth of Pythium u11imum in vitro due to the production of extracellular antagonistic metabolites were detected using a cellophane overlay method. An agar plate bioassay was subsequently employed to screen for organisms which produced large quantities of inhibitors in liquid culture. The most potent antagonist was Penicllllum clavlforme. the metabolites of which restricted P. ultimum propagule germination in addition to mycelial development. The yield of anti-Pythium agents in Pen, claviforme culture filtrate (PCF) was optimized by manipulation of the growth medium and cultural conditions, and the principle active component was identified as the polyketide antibiotic patulin.

An extract of PCF was commercially incorporated into sugar beet (Beta vulgaris) seed pellets. The treatment caused a delay in germination and abnormal seedling morphology on filter paper, but had no effect on emergence in peat-based compost. PCF significantly improved both emergence and establishment 7d after planting in compost artificially infested with P, ultimum. The extract was less effective than the synthetic fungicide hymexazal in reducing a high incidence of disease, but performed equally well in combating a milder pre-emergence infection.

PCF was rapidly inactivated in compost (half-life 11.2h) and seedling protection was extended after the concentration of active compounds had declined to levels sub-inhibitory to P. ultimum. The proposition that PCF induced quantitative or qualitative changes in the spermosphere microflora which augmented its known direct Pvthiumantagonistic activity was investigated.

Total microbial activity, measured in terms of fluorescein diacetate hydrolysis, in compost adjacent to PCF-treated seed pellets was reduced at l-3d after planting compared with that in compost surrounding control pellets without additives. This coincided with a decrease in the populations of bacteria and actinomycetes, but numbers of fungi were apparently not affected. A large proportion of fungal isolates recovered from the spermosphere of both PCF-containing and control pellets inhibited the growth of P. ultimum in vitro, and all were less sensitive to PCF than was the pathogen. Various possible interpretations of these results are discussed.

It is concluded that the exploitation of antagonistic fungal metabolites as natural fungicides incorporated into seed pellets is a feasible approach to the suppression of Pythium-induced seed and seedling diseases. Even if short-lived in the plant growth medium such agents may nevertheless confer a useful degree of protection if sufficiently selective to elicit an integrated mode of control mediated by indigenous microbial antagonists.