Interactions between the mycoparasite Pythium oligandrum and sclerotia of the plant pathogen Sclerotinia sclerotiorum

Pythium oligandrum Drechsler is a mycoparasite which parasitizes hyphae of many fungal species. A detailed study of the interactions between P. oligandrum and the sclerotia of the plant pathogen Sclerotinia sclerotiorum (Lib.) de Bary is presented. Pythium oligandrum was present in Danish soils at concentrations between 4 and 26 cfu g(-1) soil. An increase in the natural population of P. oligandrum by addition of P. oligandrum zoospores to a soil reduced the ability of sclerotia of S. sclerotiorum to germinate myceliogenically and the sclerotia were colonized internally by P. oligandrum. This colonization and reduction of germination of sclerotia were also seen when sclerotia and P. oligandrum were incubated together in water. Small sclerotia were significantly more susceptible to parasitism by P. oligandrum than large sclerotia, and increasing the incubation time caused a further reduction in the germination ability of the sclerotia. P. oligandrum was able to pass through its entire life-cycle from zoospores to oogonia both with sclerotia as sole nutrient-source and in water containing exudates from the sclerotia. The cell wall degrading enzymes N-acetyl-beta-D-glucosaminidase (NAGase), endo-chitinase, protease, beta-glucanase, beta-glucosidase and cellobiohydrolase were detected in culture filtrates of P. oligandrum cultivated with S. sclerotiorum. These findings suggest that P. oligandrum has a potential to reduce the survival of S. sclerotiorum sclerotia present naturally in soils, through mycoparasitic activity.