Download Fungi: Living Organisms and Causers of Plant Diseases and more Schemes and Mind Maps Physiology in PDF only on Docsity!
THE FUNGI ARE LIVING ORGANISMS
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 pro- duce 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 valu- able drugs and chemicals and give the desired flavor to our choicest cheeses. The most important detail to re- member about fungi is that they are living organisms. They require, there- fore, 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 ex- ternal 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 environ- ment in which they exist. If the en- vironment is another plant, particu- larly if it is one of our cultivated agri- cultural plants, the resulting situation is regarded as a plant disease. Thus the apple scab fungus, familiar to any- one 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 na- ture they can grow successfully no- where else. A pathogenic fungus is but one of countless individuals in the organic w^orld. To maintain its position it must live and grow, reproduce, be scattered about, and, under certain circum- stances, survive periods during which the environment is relatively unfavor- able 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 fila- ments, which may be colorless or vari- ously colored but which never contain chlorophyll, the green pigment re- sponsible for binding the sun's energy in the formation of sugar. Considered individually, the filaments are called hyphae (singular: hypha). In the aggre- gate they are spoken of as a mycelium. The filaments are essentially like the cells of which other plants are com- posed, 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 appear- ance 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 afifectecl host. Others develop aggregates of fila- ments with a recognizable size and shape, of distinctive color, and often decidedly firm to the touch. Such aggregates are commonly associated w4th 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 sug-
28 YEARBOOK OF AGRICULTURE 1953
gest that the physical and chemical processes that take place in the proto- plasm of the fungi arc 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 diges- tive enzymes as do the cells of the diges- tive 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 l)een made of the terms parasite and saprophyte^ supposedly to describe these tv/o alter- native conditions. Attempts to apply the terms soon demonstrate their inad- equacy, at least when used in unmiod- ified form. It is much more usefvil to speak of obligate parasites^ which can grow only on living host plants, and facultative parasites^ w^hich can grovv^ 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 im.portance in a discussion of plant diseases. Besides an ability to utilize food and grow, a fungus must reproduce. Be- cause it is the most intriguing phase of the life history and because it fur- nishes most of the information neces- sary for identification, the reproduc- tive aspects of fungi have received much attention. By reproduction we ought always to m.ean an increase in the total number of individuals of a given species, a process which usually represents suc- cessive 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 sur- vive; 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, meas- uring several hundreds or even thous- ands to the inch. Some spores arc one- celled. Some are tv/o-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 independ- ent 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 form.ation. No one, be he beginner or experi- enced professional, can truly appreci- ate 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 constric- tion 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 sur- face of the relatively undifferentiatcd mass of hyphae, or they may be more or less enclosed in a fruiting body, which in turn is made up of nonspore- bearing filaments.
30 YEARBOOK^ OF^ AGRICULTURE^1953
which there are no available host plants. Survival may depend upon the presence of thick-walled spores, of sclerotia, or upon the ability to live for some time as a saprophyte upon the organic matter in the soil.
MUCH EMPHASIS is placed, in teach- ing 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 where- by the pathogen and host come inti- mately into contact, as by the falling of an airborne spore onto a leaf sur- face. Inoculation might also be ac- complished 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 de- velopment of the pathogen. Incubation —development of patho- gen, in association with the host, prior to the appearance of conspicuous, recognizable symptoms of disease. Characteristics of host (age, abun- dance, presence of wounds, degree of resistance) as well as of the pathogen enter into the above-mentioned events, as do considerations of the environ- ment. Not infrequently a third organ- ism—man, insect, bird, nematode, or other animal—may have a decisive role. Whatever the factors involved, and at whatever stage, the final de- velopment of well-defined disease is the result of a vast complex of ele- ments. Sporulation, important as it is from
other viewpoints, is of little significance with regard to the particular host plant involved. Damage is done dur- ing 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 dis- ease 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 compe- tition, toxin secretion, enzyme dis- turbance, or the like. Some specific considerations are nonetheless useful. Parasitism may be either obligate, in the sense that the pathogen will riot 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 macroscopic outgrowths of the hyphac 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 indis- criminate, 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.
IDENTIFYING A PATHOGENIC FUNGUS 31
in general, leaf, stem, and fruit dis- eases are caused by airborne or insect- carried fungi, root diseases by soil- inhabiting species. Some vascular wilts arc caused by soil fungi, some by fungi possessing insect vectors. While there is probably no really simple host-pathogen relation, the complexity achieved in certain in- stances 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 be- tween 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 de- tails 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 character- istics. 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 spo- radic diseases of this nature that bring about the greatest hardships on the in- dividual farmer. Small wonder then that our most publicized maladies are wheat rust, apple scab, potato blight,, and the like.
RUSSELL B. STEVENS is associate pro- fessor of botany in the University of Ten- nessee. He obtained his doctorate at the University of Wisconsin in ig40 and served in the Army Medical Department during the Second World War. He is author with his latefather of a textbook. Disease in Plants.
Identifying a
Pathogenic
Fungus
William W. Diehl
From the beginning to the end of its life the health of every seed plant, wild or cultivated, is afi'ected by fungi. Even though a seed within a fruit or capsule may be sterile, it comes into contact with fungal spores and hyphae as soon as it is exposed to the air or is in contact with the ground. Spores are microscopic, seedlike, reproductive bodies, and hyphae are the micro- scopic vegative growths of fungi. The air is literally charged with spores, and the soils of the whole earth are full of living spores and hyphae of different kinds of fungi. Most of the fungi are innocuous. Many are bene- ficial. But some thousands of recog- nizably different kinds of fungi are now known to be pathogens, or agents of disease, in plants. Practical measures for the prevention and control of plant diseases depends in large part upon scientific knowledge of each pathogen and its role in nature. Since there arc more than 100, recorded names of supposedly different kinds or species of fungi, the specific identification of a single specimen or culture of a fungus involves the exclu- sion of some 99,999 names. That is a technical problem akin in complexity and difficulty to the isolation and iden- tification of any one out of 100, chemical compounds. But the problem is not insuperable. There is a general procedure that leads the way out of the apparent chaos of more than 100,000 names.
