MUCH EMPHASIS is placed, in teaching plant pathology, for example, on the life history, or life cycle, of each fungus. On that basis, one may think of diseases as developing during that part of the life of a fungus when it is actively growing in association with a host plant.

Pathogenic disease, after all, is not a thing but a condition a state of unstable equilibrium that results from the interaction of at least two living organisms.

Numerous technical terms have come into wide usage. A few at least are instructive:

Inoculation that circumstance whereby the pathogen and host come intimately into contact, as by the falling of an airborne spore onto a leaf surface. Inoculation might also be accomplished through the intervention of man or animal.

Invasion entry, passive or active, of the pathogen into the host.

Establishment development by the fungus of a successful association with the host tissues, enabling further development of the pathogen.

Incubation development of pathogen, in association with the host, prior to the appearance of conspicuous, recognizable symptoms of disease.

Characteristics of host (age, abundance, presence of wounds, degree of resistance) as well as of the pathogen enter into the above-mentioned events, as do considerations of the environment. Not infrequently a third organism man, insect, bird, nematode, or other animal may have a decisive role. Whatever the factors involved, and at whatever stage, the final development of well-defined disease is the result of a vast complex of elements.

Sporulation, important as it is from other viewpoints, is of little significance with regard to the particular host plant involved. Damage is done during the essentially vegetative phase of pathogen development and is largely realized by the time spore formation commences.

The very core of the whole problem of fungi as causal agents of plant disease lies in the phrase "host-parasite relation." It is disappointing, but not surprising, to realize that of all phases this is presently the least well known. We do not even know, in the vast majority of cases, in just what way a given pathogen is harmful to its host, whether it is a matter of food competition, toxin secretion, enzyme disturbance, or the like. Some specific considerations are nonetheless useful.

Parasitism may be either obligate, in the sense that the pathogen will not grow actively save on living tissue; or facultative, indicating that it may grow well at certain stages of its life cycle on nonliving organic matter. The obligate parasites usually have a very specific association with their hosts, the cells of the latter being invaded by microscopic outgrowths of the hyphae called haustoria. These are thought, but not fully proved, to be absorptive in function. Facultative parasites, on the other hand, often injure by the secretion of extracellular enzymes.

CERTAIN FUNGI are strictly local in their effect, producing lesions on leaf, stem, or root system, though their localism may at times be an expression of host resistance. Other fungi are selective with respect to particular tissues, as exemplified by the vascular wilt organisms, which are confined to the water-conducting tissues, or the chestnut blight fungus, injuring the cambium layer. Still others are indiscriminate, establishing themselves at various points, and at times destroying the entire plant.

There is the anticipated correlation between the mode of dissemination of the fungus and its relation to the host.

In general, leaf, stem, and fruit diseases are caused by airborne or insect-carried fungi, root diseases by soil-inhabiting species. Some vascular wilts are caused by soil fungi, some by fungi possessing insect vectors.

While there s probably no really simple host-pathogen relation, the complexity achieved in certain instances is truly impressive. Possibly the best understood instances of a complex interrelationship are to be found in the so-called heteroecious rusts. Here one is confronted with a pathogen that is an obligate parasite, having as many as five distinct spore types, and compelled to alternate from season to season between two botanically very different host species. How such a situation evolved over the past ages remains a complete mystery.

It goes almost without saying that critically accurate knowledge of the details of pathogen life cycles is essential to the development and application of effective control measures.

The attention given the fungi as causes of plant disease seems in large measure due to two further characteristics. In the first place, it is the fungus diseases of plants (by contrast with those of bacterial and virus origin) that are most easily controlled by chemical applications in the form of sprays and dusts. Added to this is the fact that fungi are responsible for a much larger number of the rapidly spreading, hence epidemic, diseases than are viruses or bacteria.

Whatever the reason, it is the sporadic diseases of this nature that bring about the greatest hardships on the individual farmer. Small wonder then that our most publicized maladies are wheat rust, apple scab, potato blight, and the like.

RUSSELL B. STEVENS is associate professor of botany in the University of Tennessee. He obtained his doctorate at the University of Wisconsin in 1940 and served in the Army Medical Department during the Second World War. He is author with his late father of a textbook, Disease in Plants.