The Fungi Are Living Organisms

Russell B. Stevens.

Most fungi can be easily seen. Some are large and conspicuous, like the fleshy mushrooms of field and forest.

Some are stout and woody, like the bracket fungi on rotting timber. The pathogenic fungi, the ones that produce disease, more closely resemble the molds that overrun our shoes and stored leather goods in damp weather, damage foods, rot fabrics, and, in happier circumstances, produce valuable drugs and chemicals and give the desired flavor to our choicest cheeses.

The most important detail to remember about fungi is that they are living organisms. They require, therefore, a source of energy. Like animals, but unlike green plants, fungi cannot convert the energy of the sun and thus synthesize the food they need. They must therefore get food from some external source. In so doing, the slender, infinitely tiny, often colorless threads that make up the fungus itself live and grow at the expense of the environment in which they exist. If the environment is another plant, particularly if it is one of our cultivated agricultural plants, the resulting situation Is regarded as a plant disease. Thus the apple scab fungus, familiar to anyone who has tried to raise apples in practically any part of the United States, forms olive-green patches on the leaves and fruits of its host and Profits at its expense. The same is true Of the grain rust fungi, the corn smut fungus, and a myriad of others. They grow thus because by their very nature they can grow successfully nowhere else.

A pathogenic fungus is but one of countless individuals in the organic world. To maintain its position it must live and grow, reproduce, be scattered about, and, under certain circumstances, survive periods during which the environment is relatively unfavorable to it. When one knows the essentials of how fungi accomplish this assignment, one knows the essentials of how fungi cause disease in plants.

Structurally, fungi are mostly very tiny but very complex. Most of them are composed of microscopic filaments, which may be colorless or variously colored but which never contain chlorophyll, the green pigment responsible for binding the sun's energy in the formation of sugar. Considered individually, the filaments are called hyphae (singular: hypha). In the aggregate they are spoken of as a mycelium. The filaments are essentially like the cells of which other plants are composed, each being surrounded by a thin membranous wall and composed of a jellylike living protoplasm.

Although the filamentous growth form is common to practically all plant pathogenic fungi, the actual appearance of the fungi is enormously varied.

Some, like the rhizopus soft rot fungi of sweetpotatoes and strawberries or the scab fungi of wheat, have no particularly characteristic form and appear as a downy growth over the surface of the affected host.

Others develop aggregates of filaments with a recognizable size and shape, of distinctive color, and often decidedly firm to the touch. Such aggregates are commonly associated with the actively reproducing phase of development, and the "fruiting body" so formed serves as one of the most useful means of distinguishing one plant pathogenic fungus from another.

The physiology of fungi (like the physiology of all plants and animals) is by no means completely understood. We have enough information to suggest that the physical and chemical processes that take place in the protoplasm of the fungi are essentially the same as those of any other living organism. Indeed, those processes, summed up in the term metabolism, are strikingly similar throughout the organic world. Because the fungi lack chlorophyll, their nutrition is most like that to be found in the colorless, or rather nongreen, parts of higher plants, except that many fungi secrete digestive enzymes as do the cells of the digestive tract of man and other animals.

Because fungi depend for their food on outside sources, and because some establish themselves on other living plants and some on nonliving food sources, a great deal has been made of the terms parasite and saprophyte, supposedly to describe these two alternative conditions. Attempts to apply the terms soon demonstrate their inadequacy, at least when used in unmodified form. It is much more useful to speak of obligate parasites, which can grow only on living host plants, and facultative parasites, which can grow on nonliving food supplies but which at times do establish themselves on a living host. True saprophytes, which live exclusively on nonliving substrata, can hardly be of importance in a discussion of plant diseases.

Besides an ability to utilize food and grow, a fungus must reproduce. Because it is the most intriguing phase of the life history and because it furnishes most of the information necessary for identification, the reproductive aspects of fungi have received much attention.

By reproduction we ought always to mean an increase in the total number of individuals of a given species, a process which usually represents successive generations and thus assures the perpetuation of the species in time as well as its maintenance in space. It is an error in logic, however, to conclude that fungi reproduce in order to survive; one can say simply that they survive because they reproduce.

Characteristically, fungi reproduce by the formation of microscopic bodies collectively called spores.

Such spores, depending on the type and the particular pathogen by which formed, assume an impressive array of different characteristics. They vary greatly in size, although always tiny in terms of more familiar objects, measuring several hundreds or even thousands to the inch. Some spores are one-celled. Some are two-celled. Some have many cells. Some are completely colorless. Some are only lightly tinted, others dark. Still others are coal black. Some are extremely fragile and are killed by exposure to dry air in less than a minute. Some can withstand boiling temperatures for brief periods. A few types are capable of independent movement, but most of them drift passively on wind or water, although often they are forcibly ejected when first mature. Finally, they are formed in myriad different ways by the parent organism. Detailed consideration of specific diseases is by all odds the best way to comprehend the complexity and variability of spore formation.

No one, be he beginner or experienced professional, can truly appreciate the astonishing abundance in which spores are formed by fungi, even though they are produced on fruiting bodies so small as to be scarcely visible to the unaided eye. Total numbers from a single diseased plant must be estimated in millions, billions, even trillions, yet so little is the likelihood of survival that only the tiniest fraction of them are destined to give rise to a subsequent generation.

Fungus spores are often formed vegetatively; that is, they develop at the ends of the filaments by constriction of the walls, or their appearance may follow as the direct result, more or less, of some form of sexual activity. Those of the former type may occur pretty much at random over the surface of the relatively undifferentiated mass of hyphae, or they may be more or less enclosed in a fruiting body, which in turn is made up of non-sporebearing filaments.

The manner in which the spores are produced, and the appearance of the fruiting body is the primary type of information upon which virtually all classification schemes are based. This is particularly true of the sexual spores. We thus have four groups of fungi:

The Phycomycetes the filaments have no cross walls, and sexual activity consists of the fusion of many-nucleated male and female cells. In this group alone the nonsexual spores are frequently capable of swimming about, and these fungi are primarily found in very moist situations.

The Ascomycetes sexual spores, typically in groups of eight, are to be found within a tiny sac, or ascus.

The Basidiomycetes sexual spores, two or four, are borne upon the end of a tiny club, or basidium. The rust and smut fungi, which make up an important segment of this group, have a modified type of basidium clearly homologous to the true basidium.

The Fungi Imperfecti only the nonsexual type of spore is known for this group, which otherwise greatly resembles the Ascomycetes.

Within the group of the Ascomycetes, particularly, further subdivisions are made on the basis of the type of fruiting body in association with which the sexual spores are produced.

THE NONSEXUAL type of spore formation generally is typical of the early stages of disease, often during the active growing season.

Sexual reproduction, on the other hand, characteristically occurs in the later phases of disease development, preliminary to a period of dormancy during the winter. Of the common diseases, the brown rot of stone fruits illustrates the point as well as any.

Here, the fruit as it approaches maturity will be literally enshrouded With a soft, velvety mass of nonsexual Spores, millions of them, which are responsible for the rapid spread of the Pathogen during the summer. Only after the decayed fruit has shriveled, dropped, and lain on the ground for a full winter do the fragile trumpets of the fruiting bodies appear, bearing their countless sacs of ascospores. These, in turn, carry the disease back to the budding and blossoming trees in springtime.

NOT ONLY must an organism survive in point of time; it must also be distributed in space. With the exception of fragile motile cells in certain Phycomycetes (and then only for very limited absolute distances) fungi are not self-propelled. They depend, therefore, on chance distribution by exterior forces wind, water movement, activity of birds and animals, insect carriers, dissemination of host plants and plant parts, and so on. To a large extent the dissemination of fungi is through the medium of their spores, tiny objects often ideally suited for the purpose. Fragments of hyphae, hard masses of mycelium known as sclerotic (singular: sclerotium), and possibly other entities account for the remainder.

Spore movement in the bulk of fungus species is passive, although flagellated forms occur in a few species and in many others the initial discharge is forcible. The distance to which fungus spores may be carried by air currents, for example, is very great. Because of this, and because of the astonishing productivity of some pathogens, we are witness to occasional dramatic epidemics of disease. No better illustrations of this exist in the United States than the northward sweep of the cereal stem rusts in the Great Plains area, and the march of potato late blight, tobacco blue mold, and cucurbit downy mildew up the eastern seaboard.