Summary1 In an ecological classification of the soil fungus flora, a number of groups can be distinguished, such as obligate saprophytes, root infecting fungi and fungal parasites of animals. This article has been concerned with the ecological relationships between root infecting fungi and obligate saprophytes. The specialized plant parasites, together with mycorrhizal fungi (symbionts), have been grouped together as root inhabiting fungi. The remainder of the root infecting fungi, together with the obligate saprophytes, have been designated as soil inhabiting fungi.2. The root inhabiting fungi are characterized by an expanding parasitic phase on the living host plant, and by a declining saprophytic phase in its absence. This restriction of the saprophytic phase of such root inhabitants is enforced by the competition of other soil micro‐organisms, and they can therefore be considered as ecologically obligate parasites, inasmuch as their continued survival is dependent upon that of their host plants. With few exceptions, such fungi can readily be grown as saprophytes in pure culture and they are not, therefore, obligate parasites in the strict nutritional sense.3. The soil inhabiting fungi are characterized by ability to survive indefinitely as soil saprophytes. Some characteristics suggested as contributing to competitive saprophytic ability are a high growth rate, excretion and tolerance of antibiotic toxins, and a capacity for rapid and complete decomposition of dead plant tissues.4. Some distinguishing characteristics for the separation of these two groups of organisms have been listed. Root diseases caused by soil inhabiting fungi are most important in seedlings, and in older plants in which normal host resistance has been impaired by an adverse environment. Diseases of annual crops caused by such fungi are less amenable to control by crop rotation than are those caused by root inhabiting fungi.5. Evolution of the host‐parasite relationship is suggested as the determinant in the segregation of root inhabiting fungi. Competitive saprophytic ability is incompatible with such evolution, and is gradually lost, but the sheltered host‐borne phase of the root inhabitant is prolonged. Evolution appears to have progressed furthest in the mycorrhizal fungi, as evidenced by their loss of competitive saprophytic ability, which has culminated in the nutritionally obligate parasitism of the vesicular‐arbuscular endophytes (Rhizophagus spp.).6. Interference by other soil micro‐organisms with the parasitic activity of a root inhabiting fungus is greatest at the root surface, where the soil micro‐flora is greatly intensified, and is also changed in composition, by root excretion and by other activities of the living root. The rhizosphere and root surface microfloras thus appear to constitute the root's outermost barrier against invasion by pathogenic fungi.7. In its invasion of the host root, a root inhabiting fungus is likely to be accompanied or followed by weak secondary parasites and obligate saprophytes. With progressive improvement in the relationship between host and primary parasite, gross disorganization of host root physiology and defence mechanisms is likely to diminish, and so the infected root may still be able to repel secondary invaders. Thus the ectotrophic mycorrhizal symbiosis is typically a closed association, and functions as a symbiosis for defence as well as for nutrition.8. The saprophytic survival of a root inhabiting fungus in dead infected host tissues is curtailed by the competition and antagonism of associated saprophytes, and is likely to be shortest under soil conditions optimum for microbiological activity.Grateful acknowledgement is made to the Agricultural Research Council for their provision of a research grant, during the tenure of which this article was prepared. I am especially indebted to Dr J. Rishbeth, both for permission to quote from his data on Fomes annosus before publication and for much helpful discussion on root disease problems, and to Prof. F. T. Brooks, F.R.S., for his valuable comments on the manuscript.
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Jan 1, 1965
R W Smith 
Open Access