The Smuts of Wheat, Oats, Barley

C. S. Holton, V. F. Tapke.

Many millions of dollars' worth of grain are destroyed every year by the smuts of wheat, oats, and barley.

For purposes of study and control, we can consider the smuts as being seedling-infecting or floral-infecting.

The seedling-infecting species come in contact with the host plants as follows: The microscopic spores from smutted plants are carried by wind, rain, insects, and other agencies to the heads of healthy plants (as in loose smut of oats). Or, smutted heads are crushed in threshing and spores are distributed to the clean seed or blown to fields, where later they come in contact with the host at seeding time (as in the stinking smuts of wheat). As the seed germinates and the seedling grows through the soil to the surface of the ground, the smut inoculum develops thin threads, which penetrate the seedling and initiate infection. The fungus then grows internally in the plant and eventually forms spores in the young heads. That completes the cycle and sets the stage for a new crop of smut.

STINKING SMUT, or bunt, apparently has been a plague of wheat since wheat was first cultivated. It was prominent among the diseases studied by the earliest plant scientists. It has been widely investigated, and certain control measures have long been known. Nevertheless bunt is still an economic threat to the production of wheat, especially winter wheat, in all important wheat regions of the world.

The average annual loss in the United States is estimated to be 1.3 percent of the crop, or about 25 million dollars. Its severity fluctuates from year to year and from region to region. The heaviest losses occur in the winter wheat regions of the Midwest and Pacific Northwest. Estimated annual losses in Kansas over 30 years range from less than 200,000 dollars to almost 20 million dollars; the average is almost 3 million dollars. In recent years the Pacific Northwest has suffered the most. The loss there was about 10 million dollars in 1950.

The main loss comes from the reduction in yield. The percentage of reduction in yield roughly equals the percentage of smutted heads in the field. Losses in quality result when market grain is discounted in price because of the dark color and the offensive odor of the smut spores that adhere to the kernels. Smut that is removed from the grain by cleaning and washing is assessed as dockage against smutty grain. Stinking smut increases the cost of processing. It also is a fire hazard to threshing equipment and storage bins.

TWO KINDS OF BUNT occur on wheat. The common bunt is caused by Tilletia caries and T. foetida. Dwarf bunt is caused by T. caries.

Common bunt is more prevalent and better known than dwarf bunt.

Dwarf bunt stunts infected plants severely. It has been recognized as a distinct type only since 1925. Its prevalence and severity have increased steadily. Its principal region of distribution is the Pacific Northwest, but it also occurs in Wyoming, Colorado, and New York.

Wheat plants infected with bunt are recognized first by their reduced height. Common bunt shortens the plants a few inches or as much as half the height of healthy plants, depending on the physiologic race of the bunt fungus, the host variety, and the conditions of growth. Plants with dwarf bunt are one-half to one-fourth the height of healthy plants. Because of intergrading types, identification solely on the basis of stunting is sometimes difficult.

The smutted wheat heads are bluish green when they emerge from the boot. The healthy heads are yellowish green. Smutted wheat heads tend to be long and lax and to ripen sooner than healthy ones. The smut balls protrude beyond the glumes as they enlarge. Dwarf bunt infected heads usually are more compact than those infected with common bunt, and the glumes are spread apart so that the smutted heads have a feathery look.

The smut balls vary considerably. The size depends on host variety, location in the spikelet, physiologic race, and climate. Common bunt balls are elongate or round. Dwarf bunt balls are always round. Common bunt balls are more fragile and absorb water less readily than those of dwarf bunt.

Dwarf bunt stimulates excessive tillering of infected plants. The effect is more pronounced in the Turkey variety than in others. Forty tillers on dwarf bunt infected plants are not uncommon.

Bunt-infected flowers have longer pistils and longer and broader ovaries than do healthy flowers. Diseased ovaries are green; healthy ones are white. Stamens in diseased flowers are reduced in length and breadth, and the anthers have a pale-yellow color instead of a pronounced green, as in healthy ones. The stamens in diseased heads fail to extrude at flowering time.

Bunted plants are more susceptible to seedling blights and to yellow stripe rust but more resistant to powdery mildew than are healthy plants. Winter injury is more pronounced in bunted plants. Increased sensitivity of infected plants to gravity has been reported.

The spores of Tilletia caries are uniformly globose and have "netted" spore walls. Spores of T. foetida are globose, elongate, or oval and have smooth walls. The bunt balls of T. foetida are larger and more elongate than those of T. caries on the same variety of wheat. In both, the ball shape tends to conform to the shape of the wheat kernel.

The spore balls are broken in threshing. The grain becomes contaminated with spores. If it is used for seed, the spores germinate and penetrate the young seedling in the course of its subterranean emergence from the seed to the surface of the soil and infection is established in the growing point. The growth of the parasite keeps pace with plant development; at maturity, bunt balls are formed in place of wheat kernels.

Wheat seedlings also may become infected by soil-borne spores. In less humid regions like the Pacific Northwest, airborne spores from the combine harvester settle on summer fallow land and remain dormant until moisture and temperature are favorable for germination in the fall. Usually that occurs at the time winter wheat is seeded, so that smut spores and wheat seeds germinate at the same time, thus exposing the seedlings to infection. Spores of common bunt perish in the soil in less than a year. Those of dwarf bunt may remain viable for 7 years. Even so, dwarf bunt does not attack spring wheat.

The greatest infection occurs at soil temperatures of 40 to 60 F., with moisture content ranging from 15 to 60 percent of field carrying capacity. Other factors affecting the development of bunt are soil fertility, depth of seeding, and length of day. The spore load is important. About 0.5 gram of spores to 100 grams of seed are necessary for maximum infection. The greater the concentration of spore load around the embryo, the higher the degree of infection.

Both species of the bunt fungus are highly specialized into distinct physiologic races. The races differ in their ability to attack different varieties, also in the size and shape of their smut balls and rate and mode of spore. germination. In the dwarf bunt race infection comes primarily from soil-borne inoculum.

Pathogenic specialization in the bunt fungi was discovered in 1925 or so. Different races have been identified from various parts of the world. In the United States about 25 races are recognized by their reaction to a set of eight differential wheat varieties.

New or previously unrecognized races frequently appear, especially in places where varieties resistant to the recognized races have been introduced. The new races attack and spread with the new varieties. Every smut-resistant variety introduced in the Pacific Northwest eventually has become the distributor of one or more new races of bunt.

The two species as well as different races of the bunt fungus have been intercrossed artificially and new pathogenic lines were selected from the hybrid populations. The frequent occurrence of intermediate spore forms in field collections indicates that natural hybridization also occurs. Invariably the intermediate forms are found on highly susceptible varieties. These varieties, however, tend to perpetuate the old, established races, despite their susceptibility to the new races.

EFFECTIVE CONTROL OF BUNT depends on the use of clean seed of a smut-resistant variety properly treated with an appropriate fungicide. Also effective, when it is practical, is the seeding of wheat when soil temperatures are unfavorable to bunt development.

Those control measures seem so simple that they give no hint of the long and costly struggle of plant scientists to develop them. And the struggle continues today in an effort to keep pace with the ever-changing problem, brought on by shifts in wheat varieties and by the adaptive nature of the smut fungi.

The accidental discovery of seed treatment as a means of controlling wheat smut was made long before the parasitic nature of the disease was known.

Some wheat seed salvaged from a grounded sailing ship off the southern coast of England was sown by nearby farmers. The crop they got had less smut than that from local seed. That may have been the origin of the 100-year-old practice of brining the wheat seed.

A scientific approach to the problem of smut control by seed treatment began about 1800 with the discovery of spore germination and the harmful effect of copper sulfate on their germination. Fifty years later the parasitic nature of wheat smut was established, and the quest for more effective control by seed treatment was begun.

Scientists recognized at the outset that a good chemical for treating seed should be highly toxic to the smut spores but harmless to the seed. Copper sulfate did not always give good control and often injured the seed. Seed had to be soaked in the solution and dried afterwards. It left much to be desired as a suitable fungicide for seed treatment. It took almost 50 years of search to find a better treatment, but progress was faster thereafter.

The discovery of the value of formaldehyde about 1895 was hailed as the answer to smut control by seed treatment. After its adoption in the United States, formaldehyde rapidly replaced copper sulfate because of its several advantages. Even so, it still had the disadvantage of being a wet treatment and was likely to injure the seed unless used properly. The impelling need for a dry treatment led to the discovery of the value of copper carbonate dust in Australia. Copper carbonate was first tested in the United States about 1918. It gradually displaced formaldehyde for the control of wheat smut but was ineffective against oat and barley smuts, for which formaldehyde continued to be used.