Problems of Variability in Fungi

E. C. Stakman, J. J. Christensen.

Many plant disease fungi are known to be complex and variable, but the degree of complexity is imperfectly known for most of them and is not well enough known for any of them.

This complexity is not surprising because many of the crop plants in which the fungi live are equally complex. There are more than 14,000 varieties of wheat; there are unnumbered varieties of apples, beans, corn, roses, iris, and other cultivated and wild plants. Moreover, man and Nature are continually making new varieties by mutation and by hybridization and selection.

Plant disease fungi also are plants. They are parasitic on higher plants. Most are microscopic in size and relatively simple in structure, but they are plants nevertheless. They germinate, grow, and fructify; they multiply by the countless billions; they Mutate and hybridize; consequently Nature has made countless kinds in the past and is continually making More kinds through mutation and hybridization. As plant breeders produce better kinds of useful plants, therefore, Nature may produce more destructive fungi. It is the job of plant pathologists to learn what the parasitic fungi are now doing and what they may do in the future to jeopardize or nullify man's success in plant improvement. To understand present Plant disease situations and to predict Possible future developments, it is essential to learn as much as possible about the variability in fungi pathogenic to plants. Other kinds of causal agents of plant diseases, such as viruses and bacteria, must be studied similarly, but the present discussion is restricted to fungi.

The principles of growth and reproduction in plant pathogenic fungi are similar to those for higher plants, except that fungi have no chlorophyll, which enables green plants to make their food from simple materials in the soil and air. The fungi therefore must live on living plants or animals or their products. The fungus parasites of plants live in or on living plants and get their nourishment from them by means of minute tubes, or hyphae, that grow, branch, and form a network known as the mycelium, which finally produces many tiny spores.

Spores serve the same propagative and reproductive function as the seeds of higher plants, but spores are much smaller and simpler in structure than seeds. Spores usually have one or at most a few cells and do not contain an embryo as do seeds. Nevertheless they germinate and produce new fungus plants with the parental characters. The conditions required for germination are similar to those for seeds, moisture and suitable temperature being the most important. The time needed for germination varies with the kind of spore and the temperature; it varies from about an hour to several days. Many fungi produce millions or even billions of spores in a few days to a few weeks and can therefore produce countless numbers in a few short generations. That is one reason why they are so dangerous.

Some spores are produced asexually and some only as a result of sexual fusions. The same fungus may produce several kinds of spores asexually, but usually produces only one kind sexually. Successive crops of asexual spores are generally produced abundantly and quickly when conditions favor growth of the fungus, and the sexual spores are usually formed when the vegetative period is terminating. Each kind of spore has a special name; thus some of the rusts produce five kinds of spores or sporelike bodies teliospores, sporidia, pycniospores, aeciospores, and uredospores, each with special form and function.

The principles and procedures in classifying and naming fungi are similar to those for higher plants. Species of higher plants are determined by morphologic characters of the vegetative and fruiting structures, including seeds. Similarly species of fungi are determined by the characters of the mycelium, of the spore-producing organs, and of the spores themselves. The unit of measurement for fungus spores is the micron, which is one one-thousandth of a millimeter; 25 millimeters make an inch. The range in size is from about 4 microns to more than 100 microns and, together with shape, color, and certain other characters, is characteristic for each species.

Visible characters are used for grouping higher plants and plant pathogenic fungi into classes, orders, families, genera, species, and varieties. Thus it is easy to distinguish larger groups of agricultural plants, such as grasses and legumes. It is equally easy to distinguish larger groups of fungi, such as smuts and rusts. But there are many kinds of wild and cultivated grasses, such as timothy, bromegrasses, oats, barley, rye, rice, corn, and wheat; and there are many kinds of legumes, such as beans, peas, soybeans, alfalfa, and clover. Likewise, there are many kinds of smuts, such as the loose smut of wheat, the stinking smuts of wheat, common corn smut, and rye smut, and there are more than 3,000 kinds of plant rusts. It is relatively easy to recognize these major groups of higher plants and of fungi, but it becomes increasingly harder to classify the smaller groups, such as species and varieties.

The agronomist must know not only wheat, a genus scientifically designated by its Latin name Triticum, but he must know the species of wheats, such as the common bread wheat, Triticum vulgare; the durum wheats, T. durum; and several other species or subspecies. He must know also thousands of varieties of bread wheats and hundreds of varieties of durum. And he must learn about the new lines and potential varieties that continually are being produced. Likewise the plant pathologist must know the genera, species, varieties, and "lines" of the fungi that parasitize different kinds of wheat, and he must learn about the new ones that continually appear.

Moreover, the agronomist must know not only what crop plants look like, but also how they behave and what they are good for. He must know whether a variety of wheat is a winter wheat or a spring wheat, whether it is winter hardy, where it grows well, whether it makes good flour or poor flour, and whether it is susceptible or resistant to disease; and the plant pathologist must know not only what species of plant disease fungi look like but must also know their pathogenicity for the many different kinds and varieties of plants.

Most species of plant pathogenic fungi are equally as complex in composition as species of crop plants, and it is therefore necessary to find out what is within the species. As an example, the species Puccinia graminis, the stem rust of small grains and grasses, is recognized easily by its general appearance and by certain characters of its spores. But there are several varieties within the species that are alike in some characters and different in others, including the kinds of crop plants that they can attack. Thus the variety tritici of Puccinia graminis parasitizes wheat, barley, and many wild grasses; the variety avenae parasitizes oats and certain wild grasses but not wheat and barley; within the tritici variety there are races that differ in their ability to attack certain varieties of wheat; within the variety avenae there are races that differ in ability to attack varieties of oats. Kanred, as an example, is immune to some races of the variety tritici and susceptible to others. These rust races in turn can be subdivided into the ultimate subdivisions, the biotypes. A biotype is a population of individuals that are identical genetically.

The descendants of a single non- Sexually produced stem rust spore constitute a biotype, unless mutation occurs to cause genetic diversity. The test of genetic purity of a population of a pathogenic fungus is the consistency of its behavior. Most species of plant pathogenic fungi comprise many biotypes, and attempts are made to group the most closely related ones into races and then to group the most closely related races into varieties, which in turn are grouped into species. Actually the procedure usually is in the reverse direction: The larger groups are recognized first, and the successively smaller groups are discovered by successively more refined methods. This can be illustrated by results of investigations of Puccinia graminis, the fungus that causes stem rust of small grains and grasses. (The discussion of stem rust is based on results of cooperative investigations between the United States Department of Agriculture and the University of Minnesota, which were begun in 1914 and continued by many investigators under the general supervision of the senior author, who regrets that the contributions of the many administrators and investigators cannot be specifically mentioned without interrupting the continuity of the discussion.)

Puccinia graminis is a good species Within which there are clearly recognizable varieties, races, and biotypes. There are at least six rather distinct varieties that differ in size of spores and in the kinds of plants that they can attack: (1) tritici (wheat), whose host plants are wheat, barley, and many wild grasses; (2) secalis (rye), rye, barley, and many wild grasses like those attacked by tritici; (3) avenae (oats), oats and wild grasses different from those attacked by tritici; (4) Aleifiratensis (timothy), timothy and certain wild grasses; (5) agrostidis (redtop), redtop and other species of Agrostis; (6) poae (bluegrass), Kentucky bluegrass (Poa pratensis) and related species.

There probably are more varieties than the six listed, and new ones may appear in future as a result of mutation or hybridization. Races are known within the varieties tritici, avenae, and secalis. Although there sometimes are at least slight morphologic differences between races within a variety, the most important and most easily recognizable differences are in the degree of infection (pathogenicity) on certain varieties within the genera Triticum (wheat), Avena (oats), and Secale (rye), respectively. The crop varieties, designated as differential varieties, now used to distinguish races within the tritici and avenae varieties, were selected from hundreds that were tested, and, at the time of selection, appeared to be representative and adequate for distinguishing races.

The differential varieties of wheat are:

Triticum compactum (club wheats): Little Club, C. I. 4066 (Cereal Investigations accession number, Department of Agriculture). (Certain lines of Jenkin, C. I. 5177, notably Hood, C. I. 11456, may be substituted for Little Club.)

Triticum vulgare (bread wheats): Marquis, C. I. 3641; Reliance, C. I. 7370 (certain lines of Kanred, C. 1. 5146, can generally be substituted for Reliance); Kota, C. I. 5878.

Triticum monococcum (Einkorn): Einkorn, C. I. 2433.

Triticum durum (durum or macaroni wheats): Arnautka, C. I. 1493; Mindum, C. I. 5296; Spelmar, C. I. 6236; Kubanka, C. I. 2094; Acme, C. I. 5284.