INTERACTION OF SOIL MICRO‐ORGANISMS WITH OPHIOBOLUS GRAM IN IS SACC, THE FUNGUS CAUSING THE TAKE‐ALL DISEASE OF WHEAT

Abstract

Summary Isolations of Ophiobolus from infected wheat roots gave the maximum percentage of recovery 2–4 weeks after inoculation. Later the percentage of recovery declined. Disappearance of Ophiobolus mycelium was most rapid in acid soils, and slower in sand and in alkaline soil. In soils of more normal type an intermediate behaviour was shown. From such infected roots a large number of soil fungi and bacteria were isolated and, in part, identified. It is suggested that disappearance of Ophiobolus mycelium from invaded roots is conditioned by the activity of these contaminants, and more particularly of such species as Trichoderma lignorum. Long persistence of the parasitic mycelium in roots growing in sand suggests that this is a poor medium for the growth of micro‐organisms. A systematic study in plate cultures of the interaction between Ophiobolus and the various soil contaminants isolated showed various degrees of interference with the growth of the former. These effects are described and tabulated. Similar interference effects were seen when Ophiobolus was grown in sterilized soil which had been inoculated with each of the contaminants. These effects of the living contaminants can, to some extent, be reproduced by their metabolic products, viz. by filtered cultural solutions in which these organisms have grown. The inhibiting or retarding effects so produced are not due to the abstraction of food substances but to the presence of deleterious metabolites. In certain cases it was shown that Ophiobolus mycelium when exposed to such metabolic products was killed. This result was established both by cultural and pathogenic tests. Tests with wheat seedlings planted over Ophiobolus mycelium, with or without the simultaneous presence of the various contaminating organisms, showed that the antibiotic effect varied from no appreciable effect to complete inhibition of attack. The various organisms are divided into three groups on this basis.