The range of variation in types of smut is well illustrated in crosses between the loose and covered smuts of oats. When varieties of oats are inoculated with different combinations of monosporidial lines of Ustilago avenae, loose smut, and U. kolleri, covered smut , many kinds of smutted heads (panicles) may be produced, ranging from loose to covered, through many intermediate types. One combination may produce loose smut on one variety and covered smut on another, and another combination may produce the same type of smut on both varieties, which indicates that the type of smutted panicles is determined by both the fungus and the variety of oats.

In interspecific crosses between the barley smuts Ustilago hordei (covered type) and U. nigra (false loose type), made by C. C. Allison at Minnesota, the F₁ head types were intermediate, but tended toward the loose type. The progeny from the F₁ chlamydospores produced not only the parental types of sori, compact heads with smooth spores and loose heads with echinulate spores, but several new types, including intermediates with smooth or echinulate spores, compact heads with echinulate spores, and loose heads with smooth spores.

L. J. Tyler and C. P. Shumway, at Minnesota, made crosses between the sorghum smuts Sphacelotheca sorghi (covered kernel smut) and Sorosporium reilianum (head smut). The characters of the F, sori and of the chlamydospores tended to be intermediate between those of the parents. Syed Vaheeduddin crossed Sorosporium reilianum (head smut of sorghum) with Sphacelotheca cruenta (loose kernel smut of sorghum). Different combinations Of monosporidial lines produced sori differing in shape and size. Some of the F₁ sori resembled one or the other parent, while others were intermediate. Similar types were obtained again in F₂ from inoculating with different combinations of f₁ (gametic) segregates. Naturally, there was segregation of factors for many other characters also such as size and echinulation of chlamydospores, cultural characters, sex, and pathogenicity. Some of the hybrids between the two smuts had pronounced hybrid vigor. They caused extreme elongation of the ovaries of sorghum, thus producing an effect similar to that caused by the long smut of sorghum, Tolyposporium filiferum. Moreover, the chlamydospores, although intermediate in size between the sizes of the parental spores, germinated over an exceptionally wide range of temperature and produced promycelia two and a half to three times as long as those of either parent, and the sporidia and hyphal branches were correspondingly large. In addition, some haploid segregates had extraordinary tolerance for certain chemicals. Since sporidia and promycelia) cells are haploid, not diploid as in higher plants, a study of hybrid vigor in smut fungi may aid in interpreting that in higher plants.

INTERSPECIFIC HYBRIDIZATION apparently produces new parasitic races of some smuts in nature. New virulent races of buff smuts of oats have been produced artificially by crossing Ustilago avenae with U. kolleri. Some of the hybrids had a wider host range than either parent. Some of the hybrid races of the buff smut attacked both the susceptible variety Monarch, and the variety Gothland, which was immune to the parental buff race. The inter-specific hybrids combined some of the factors for pathogenicity of both parents. It appears that new races of buff smut can be produced readily by crossing any buff race with normal black races of U. avenae or of U. kolleri.

Many new parasitic races of Tilletia that cause bunt or stinking smut of wheat have been produced by hybridization between T. caries and T. foetida. Although some hybrid races are less virulent than either parent, others combine factors for pathogenicity of both parents. Crosses between T. foetida and T. caries can result in new morphologic types also. As an example, the hybrid sori and chlamydospores were smaller in one cross than those of either parent. Hybrids also may have varying degrees of spore-wall reticulations, and size of chlamydospores can vary considerably. Such hybrids occur in nature also, and in some cases have been given taxonomic rank, such as T. caries intermedia.

That there are innumerable species, parasitic races, and biotypes of plant disease fungi is a banal truism. That innumerable new biotypes and races can be produced by mutation, hybridization, and heterocaryosis has been shown by extensive experimentation. That new biotypes and parasitic races often appear in nature is known from long-continued observation. The new biotypes and races, even though they were produced infrequently, could still become widely prevalent in a short time because many fungi are prodigiously prolific and can be disseminated widely and quickly by the wind. A single kernel of smutted wheat contains between 2 million and 12 million spores; a single pustule of wheat stem rust may contain a quarter of a million spores; stem rust may produce 70 billion spores on a single barberry bush; there are about 50,000 billion rust spores on one acre of fairly heavily rusted wheat. Countless billions of spores are literally carried on the wings of the wind. Many of the new biotypes that are produced in these enormous populations are not dangerous, but some are. The potentially rapid multiplication of new biotypes and parasitic races of the fungi and the potentially rapid spread make the more virulent ones potentially dangerous, and too often potentiality has become reality.

The practical implications of the vast variation in plant pathogenic fungi are manifold, but the most important is the menace to food supplies. Some of the most devastating epidemic diseases of basic food and feed crops can be controlled economically only by the development of disease-resistant varieties. New parasitic races have repeatedly appeared to attack these varieties in the past and the menace still exists for the future. The genetic diversity and phenotypic variability of many of the most destructive pathogens are so great as to create extremely complex problems of disease control in the present and to raise the question as to how complex they can become in the future. How much virulence can nature put into plant pathogens and how much resistance can man put into crop plants? This is one of the most important questions for present and future agriculture. It can be answered only by basic studies to determine the limits of genic combinations for virulence in pathogens and for productivity and disease resistance in crop plants.

E. C. STAKMAN is head of the department of plant pathology and botany, University of Minnesota, and an agent in the United States Department of Agriculture. He has devoted his professional life principally to basic and practical studies of diseases of crop plants and the fungi that cause them. He is widely known for investigations on the epidemiology and physiologic specialization in stem rust of wheat and the genetics of the smut fungi. At Minnesota and elsewhere he has participated in breeding programs for disease-resistant varieties, particularly of wheat and oats. He was president in 1949 of the American Association for the Advancement of Science. His current activities include membership in the executive committee of the National Science Board, in the Advisory Committee for Biology and Medicine of the United States Atomic Energy Commission, and chairmanship of the Committee on International Cooperation of the American Phytopathological Society.

J. J. CHRISTENSEN is professor of plant pathology at the University of Minnesota, where he obtained the doctor of philosophy degree in 1925. He has made extensive studies of cereal diseases and the genetics of plant pathogens in relation to breeding for resistance and has participated in cereal breeding programs. In addition to work at Minnesota, Dr. Christensen has studied genetics of Plant pathogens in Europe, has been adviser to SCAP on plant diseases in Japan, and has traveled extensively in South America on a study of cereal diseases. He is past president of the American Phytopathological Society.