The Ecology of Soil-Borne Fungal Disease

Abstract

The behaviour in soil of root-infecting fungi is sometimes considered to be a complex subject, and the inherent principles and implications are indeed commonly misunderstood by plant pathologists not specialist in this field. There has, of course, been no shortage of reviews recently on various aspects of the ecology of fungi in soil. Yet despite, or perhaps even because of the extent of this literature, confusion is still common. From the large field avail­ able two works can best be recommended. Firstly Garrett's 1956 book (1), which provides a considered and balanced account of the relative importance of work up to that date and presents the theoretical concepts important in fully understanding the subject. Secondly the account of the 1958 Liverpool International Symposium (2) since it includes much recent work that indi­ cates the trend of current work and thought, some of it more controversial than others. The apparent complexity of the biology in soil of plant pathogens is partly the result of the soil's opacity, which makes observations difficult and necessitates the use of indirect methods, the interpretation of which is some­ times ambiguous. But another potent source of difficulty is the presence in the soil of an established population of microorganisms with their own biology without any necessary connection with a host plant. This situation contrasts markedly with the simple one existing for most air-borne diseases, in which not only is observation of the host-pathogen interaction relatively easy but also the system is a simple two-component one with two pathways of interac­ tion, and can be represented: H;:=P. This biological system is affected by the inorganic environment acting directly on both components which can then adjust the equilibrium as indicated by the arrows. The variations within this system are limited, and, using the symbolic signs for neutral, positive and negative effects (3), the pathogen may show 0 or towards the suscept, and the suscept +,0, or towards the pathogen. Little difficulty attends under­ standing the possibilities in such a system. Occasionally air-borne diseases may involve more than two components as in the synergistic production of disease in the white blister rust of cabbage, and there is also the possibility that the phyllosphere (4) might affect the outcome as has been recorded by Bamberg (5) and Koch (6), but there is no indication that such phenomena are either general or generally considered in air-borne disease. In soil, by contrast, it is the simple condition, H;:=P, that is uncommon, even rare. It occurs occasionally, owing to bad technique in hydroponics or in plant culture in sterilized composts. Where such a situation is allowed to develop disease is often exceptionally severe, and even organisms not