360 YEARBOOK OF AGRICULTURE 1953 consist of chlor otic spots and streaks. The virus is transmitted by at least two species of leafhoppcrs, Nephoiettix apicalis (hipunctatus) var. cincticeps and Deltocephalus dor salis. Experiments with N. apicalis have shown that the virus The Smuts of passes through part of the eggs to the next generation, for as many as seven generations. Wheat, Oats, H. H. McKiNNEY holds degrees from Michigan State College and the University Barley of Wisconsin. In igig he joined the staff of the division of cereal crops and diseases of the Bureau of Plant Industry^ Soils, and C. S. Holton, V. F. Tapke Agricultural Engineering, where he has devoted most of his lim.e in research on Many millions of dollars' worth of viruses and virus diseases. grain are destroyed every year by the smuts of wheat, oats, and barley. For further reading: For purposes of study and control, //. H. McKinney: Evidence of Virus Muta- we can consider the smuts as being tion in the Common Mosaic of Tobcicco, seedling-infecting or floral-infecting. Journal of Agricultural Research, volume 5/, The seedling-infecting species come pages g^i-gSi, 1^33; Mosaic Diseases of Wheat and Related Cereals, U. S. D. A. Circular ^42, in contact with the host plants as ig37; Mosaic of Bromus inermis, Knih H. follows: The microscopic spores from Fellows and C. 0. Johnston, Phytopaihology, smutted plants are carried by wind, volume j2, page 331, ig42; Genera of the Plant rain, insects, and other agencies to Viruses, Journal oj the Washington Academy oj Sciences, volume 34, pages I3g-i54, 1944; De- the heads of healthy plants (as in scriptions and Revision of Several Species of loose smut of oats). Or, smutted Viruses in the Genera Marmor, Fractilinea, heads are crushed in threshing and and Galla, Journal of the Washington Academy spores are distributed to the clean of Sciences, volume 34, pages 322-329, 1944; Soil Factors in Relation to Incidence and Symp- seed or blown to fields, where later tom Expression of Virus Diseases, Soil Science, they come in contact with the host volume 6r, pages g3-ioo, 1946; Mosaics of at seeding time (as in the stinking Winter Oats Induced by Soil-borne Viruses, smuts of wheat). As the seed germi- Phytopathology, volume 36, pages 359-3^9, 1946; Stability of Labile Viruses in Desiccated nates and the seedling grows through Tissue, Phytopathology, volume 37, pages 139- the soil to the surface of the ground, 142, 1947; Wheats Immune From Soil-borne the smut inoculum develops thin Mosaic Viruses in the Field, Susceptible threads, which penetrate the seedling When Inoculated Manually, Phytopathology, volume 38, pages 1003-1013, 1948; Tests of and initiate infection. The fungus Varieties of Wheat, Barley, Oats, and Corn then grows internally in the plant for Reaction to Wheat Streak-Mosaic Viruses, and eventually forms spores in the Plant Disease Reporter, volume 33, pages 359—369, young heads. That completes the 1949; Mosaics of Winter Oats "and Their Control in the Southeastern States, with T. R. cycle and sets the stage for a new crap Staîiton, J, L. Seal, T. H. Rogers, W. R. Padcn, of smut. G. K. Middleton, and U. R. Gore, U. S. D. A. Circular 809, 1949', ^ Seed-borne Virus Caus- STINKING SMUT, or bunt, apparently ing False-Stripe Symptoms in Barley, Plant Disease Reporter, volume 35, page 48, ig5i; A has been a plague of wheat since wheat Method for Inoculating Varietal Test Nur- was first cultivated. It was prominent series With the Wheat Streak-Mosaic Virus, among the diseases studied by the with H. Fellows, Plant Disease Reporter, volume earliest plant scientists. It has been 35> P^g^^ 264-266, 1951; Wild and Forage Grasses Found To Be Susceptible to the Wheat widely investigated, and certain con- Streak-Mosaic Virus, with H. Fellows, Plant trol measures have long been known. Disease Reporter, volwne 35, pages 441-442, igji. Nevertheless bunt is still an economic THE SMUTS OF WHEAT, OATS, BARLEY 361 threat to the production of wheat, of the bunt fungus, the host variety, especially winter wheat, in all im- and the conditions of growth. Plants portant wheat regions of the world. with dwarf bunt are one-half to one- The average annual loss in the fourth the height of healthy plants. United States is estimated to be 1.3 Because of intergrading types, identifi- percent of the crop, or about 25 cation solely on the basis of stunting is million dollars. Its severity fluctuates sometimes difiiicult. from year to year and from region to The smutted wheat heads are bluish region. The heaviest losses occur in green when they emerge from the the winter wheat regions of the Mid- boot. The healthy heads are yellowish west and Pacific Northwest. Estimated green. Smutted wheat heads tend to annual losses in Kansas over 30 years be long and lax and to ripen sooner range from less than 200,000 dollars than healthy ones. The smut balls to almost 20 million dollars; the protrude beyond the glumes as they average is almost 3 million dollars. enlarge. Dwarf bunt infected heads In recent years the Pacific Northwest usually are more compact than those has sufiTered the most. The loss there infected with common bunt, and the was about 10 million dollars in 1950. glumes are spread apart so that the The main loss comes from the smutted heads have a feathery look. reduction in yield. The percentage The smut balls vary considerably. of reduction in yield roughly equals The size depends on host variety, the percentage of smutted heads in location in the spikelet, physiologic the field. Losses in quality result when race, and climate. Common bunt balls market grain is discounted in price are elongate or round. Dwarf bunt because of the dark color and the balls are always round. Common bunt offensive odor of the smut spores that balls are more fragile and absorb water adhere to the kernels. Smut that is less readily than those of dwarf bunt. removed from the grain by cleaning Dwarf bunt stimulates excessive and washing is assessed as dockage tillering of infected plants. The effect against smutty grain. Stinking smut is more pronounced in the Turkey increases the cost of processing. It also variety than in others. Forty tillers on is a fire hazard to threshing equipment dwarf bunt infected plants are not and storage bins. uncommon. Bunt-infected flowers have longer Two KINDS OF BUNT occur on wheat., pistils and longer and broader ovaries The common bunt is caused by than do healthy flowers. Diseased Tilletia caries and T. Joetida, Dwarf ovaries are green; healthy ones are bunt is caused by T. caries. white. Stamens in diseased flowers are Common bunt is more prevalent reduced in length and breadth, and and better known than dwarf bunt. the anthers have a pale-yellow color Dwarf bunt stunts infected plants instead of a pronounced green, as in severely. It has been recognized as a healthy ones. The stamens in diseased distinct type only since 1925. Its heads fail to extrude at flowering time. prevalence and severity have increased Bunted plants are more susceptible steadily. Its principal region of distri- to seedling blights and to yellow stripe bution is the Pacific Northwest, but rust but more resistant to powdery it also occurs in Wyoming, Colorado, mildew than are healthy plants. and New York. Winter injury is more pronounced in Wheat plants infected with bunt are bunted plants. Increased sensitivity of recognized first by their reduced infected plants to gravity has been height. Common bunt shortens the reported. plants a few inches or as much as The spores of Tilleiia caries are uni- half the height of healthy plants, formly globose and have '^netted" depending on the physiologic race spore walls. Spores of T, Joetida are 362 YEARBOOK OF AGRICULTURE 1953 globose, elongate, or oval and have balls and rate and mode of spore smooth walls. The bunt balls of T, germination. In the dwarf bunt race foetida are larger and more elongate infection comes primarily from soil- than those of T, caries on the same borne inoculum. variety of wheat. In both, the ball Pathogenic specialization in the bunt shape tends to conform to the shape fungi was discovered in 1925 or so. of the wheat kernel. Different races have been identified The spore balls are broken in from various parts of the world. In the threshing. The grain becomes con- United States about 25 races are taminated with spores. If it is used for recognized by their reaction to a set of seed, the spores germinate and pene- eight differential wheat varieties. trate the young seedling in the course New or previously unrecognized of its subterranean emergence from the races frequently, appear, especially in seed to the surface of the soil and infec- places where varieties resistant to the tion is established in the growing point. recognized races have been introduced. The growth of the parasite keeps pace The new races attack and spread with with plant development; at maturity, the new varieties. Every smut-resistant bunt balls are formed in place of wheat variety introduced in the Pacific kernels. Northwest eventually has become the Wheat seedlings also may become distributor of one or more new races of infected by soil-borne spores. In less bunt. humid regions like the Pacific North- The two species as well as different west, airborne spores from the combine races of the bunt fungus have been harvester settle on summer fallow land intercrossed artificially and new patho- and remain dormant until moisture genic lines were selected from the and temperature are favorable for hybrid populations.