Parasitic relationships between Pythium oligandrum Drechsler and some other species of the Oomycetes class.

Parasitic relationships between Pythium oligandrum Drechsler and some phytopathogenic species of the Oomycetes class were investigated on agar plates. A high parasitic ability of Pythium oligandrum on Pythium ultimum Trow, P. debaryanum Hesse, and Aphanomyces laevis De By species was proved in this study of relationships among these fungi, commonly populating the rhizosphere of emerging sugar beet. P. oligandrum produces numerous thin haustorial threads, searching the hyphae of host species and enwrapping them during the parasitation. In a later parasitation stage the host organs were enwrapped by thicker hyphae of P. oligandrum as well. P. oligandrum grew through the colony of parasitized species mostly within 24 hours after inoculation and was able to destroy it entirely within 5--6 days. P. oligandrum produced numerous fructification organs in contradistinction to parasitized species. P. oligandrum oospores germinated abundantly when host species were present. Hyperparasitism of P. oligandrum was found both in cultivation with a single host and in common cultivation with all three host species simultaneously.