Studies on Pythium oligandrum, an aggressive parasite of other fungi

Abstract

Parasitism by Pythium oligandrum Drechsler on Gaeumannomyces graminis (Sacc.) Arx & Olivier, Phialophora radicicola Cain and a range of other, unrelated fungi was studied. It is an aggressive parasite with a wide host range amongst fungi. Host-parasite interactions were studied on filter-paper, cellophane and agar plates. Host fungi were classed as highly susceptible (e.g. Botryotrichum piluliferum Saccardo & Marchai, Phialophora radicicola Cain var. radicicola ), moderately susceptible (e.g. Fusarium roseumf.sp. cerealis (Cooke) Snyder & Hansen, G. graminis var. tritici Walker, P. radicicola var. graminicola Deacon), resistant (e.g. G. graminis var. graminis ) or antagonistic (e.g. most basidio-mycetes); but this depended on the age of both the individual hyphae and the whole colony when parasitized. Thus, young hyphae of most fungi were susceptible to parasitism; resistance increased as hyphae matured but apparently decreased thereafter. Resistance of mature hyphae was possibly related to dark pigmentation in some fungi but not in others. P. oligandrum grew across agar plates pre-colonized by some susceptible fungi as rapidly as on fresh agar plates, but growth on colonies of those fungi that were resistant during cellulolysis was slower and ceased altogether as these matured. Spiny oogonia of the parasite formed only on susceptible hosts or parts of host colonies, presumably in response to sterols released from parasitized hyphae. However, hyphal coiling, which is sometimes considered as evidence of parasitism, occurred only on resistant hyphae; susceptible hyphae were parasitized by apparently unmodified hyphae of P. oligandrum and a parasitic association was therefore not always evident. The possible significance of this to fungal parasitism in general is considered; gross morphological changes of a parasite possibly indicate at least temporary host resistance. Behaviour of P. oligandrum on fungal hosts is compared with that of unspecialized pathogens on higher plants. The biological status of Pythium spp. as unspecialized parasites is thereby confirmed, but hyphal coiling is suggested as equivalent to the ectotrophic infection habit of specialized plant pathogens, like G. graminis .