Relationship between pre-formed inhibitors in oats and infection by Gaeumannomyces graminis and Phialophora radicicola

Abstract

Growth by varieties of Gaeumannomyces graminis and Phialophora radicicola, and Leptosphaeria narmari (LN), was studied on oat and wheat seedlings and on water-extracts of oats and wheat. All fungi grew less well on oat than on wheat roots, though the degree of growthreduction varied: P. radicicola var. graminicola (PRG) was generally least affected by hosttype and P. radicicola var. radicicola (PRR, lobed hyphopodia) was most affected, though isolates within these taxa differed, one from another. For all fungi, the most pronounced difference on oat and wheat roots was in density rather than extent of runner-hyphae from the inoculum. On coleoptiles, G. graminis var. graminis (GGG) grew equally well on oats and wheat, whereas growth by PRR was markedly reduced on oats. The differences in behaviour on seedlings were paralleled by differences on oat and wheat extracts. G. graminis var. tritici (GGT) and PRR were strongly inhibited by oat root or shoot extracts; G. graminis var. avenae (GGA) and some isolates of PRG were inhibited by oat shoot but not oat root extract, and GGG, LN and most isolates of PRG grew well on root and shoot extracts of oats. Within this last group, PRG and LN detoxified oat shoot extract and enabled growth by GGT, whereas GGG did not do so. The results support and extend Turner's (1961) hypothesis, that pre-formed inhibitors in oat sap are implicated in host-resistance; they also suggest that oats might be used to alter the population balance between GGT and similar fungi, which act as biological control agents.