Roland F. Line

USDA-ARS, WSU

Common Bunt. Common bunt (stinking smut), caused by the fungus Tilletia tritici, is a disease characterized by replacement of the normal wheat kernel with bunt balls containing a black, powdery mass of spores.Slide 1 For the first half of the twentieth century, common bunt was considered to be the most important disease of wheat in the Pacific Northwest. Massive clouds of sooty black spores released during harvest were a common sight in the region. When severe, the disease totally destroyed the wheat crop. When less severe, grain harvested from fields with bunt was contaminated with the spores and had a fishy odor and a darkened appearance. When that grain was milled, it produces off-white flour, which affected its market value. Wheat graded as smutty received a lower price because of the cost of cleaning the grain. During the mid 1940's, more than 40% of the grain brought to the local elevators was graded as smutty.

The large masses of spores released during harvest were easily ignited by sparks and frequently caused explosions and fires in threshing machines, combines, and grain storage facilities. Such fires destroyed machinery and crops and were a danger to people working in the area.

In addition to the destruction of a major food crop and the detrimental effect of the disease on the economy of the region, the air-borne spores released into the atmosphere caused respiratory problems for people who were allergic to the spores. The disease was so important that the Pacific Northwest became known by agriculturist and plant scientists as the smut capital of the world.

In most regions of the world, spores on the seed are the major source of inoculum. Under the right environment, the spores germinate and infect the plant before the wheat seedlings emerge. The fungus grows within the plant and replaces the normal wheat kernel with bunt balls containing a black, powdery mass of spores. The Pacific Northwest, however, has a unique environment that allows the pathogen to survive in the soil as well as on the seed. Thus, many of the methods used to control the disease in other regions were not adequate for control of the disease in the Pacific Northwest. Use of seed protectants did not prevent infection by the fungus in the soil, and adjusting planting dates and other management practices only reduced disease severity. Use of resistant varieties was at best short-lived. Within a few years after the release of new resistant varieties, new races of the pathogen that could circumvent that resistance evolve.

The discovery that polychlorobenzenes, such as hexachlorobenzene (HCB) and pentachloronitrobenzene (PCNB), would control soilborne common bunt was a major break-through in the 1950's. Use of those seed treatments has reduced common bunt from the most important disease of wheat in the region to a disease of minor importance. The treatments not only controlled common bunt but also extended the life of resistant varieties by delaying the appearance of new races. More recently, new fungicides that also control common bunt have effectively controlled the disease.

Flag Smut. Flag smut caused by the fungus Urocystis tritici was first detected on wheat in the Pacific Northwest in 1940. During the following 20 years, the disease spread to additional counties of eastern Washington and Oregon, but little was done to control the disease primarily because other diseases were more important, especially common bunt.

By 1968 when I arrived in the Pacific Northwest, flag smut had become the most important disease in Klickitat County, WA and Wasco County, OR and had spread to several other counties of the two states. Like common bunt, clouds of flag smut spores were often observed during harvest. It was common for flag smut to cause annual losses of 10% or greater. Many of the commercial varieties grown in the region were susceptible and several new varieties being considered for release were also susceptible; the seed treatments that were so highly effective for control of soilborne common bunt did not control flag smut; information on the epidemiology and control of the disease was inconsistent, often contradictory, or unavailable; and the distribution and potential destructiveness of the disease was not clear. To add to the problem, there was concern about the possible effect of the disease on the export market, since some countries have quarantines against importing grain contaminated with flag smut.

Like common bunt, flag smut survives in the soil as well as on the seed and infects the plant before emergence. However, the disease does not appear in the heads. Flag smut appears as gray-black stripes between the veins of leaf blades and sheaths. Infected plants are usually dwarfed with distorted and twisted leaves. When severe, heads never develop.

Research on control of flag smut was initiated in 1968. By 1972, the environmental and managerial factors that contributed to severe flag smut were determined and the disease was controlled. The major break-through was the discovery that two, new systemic seed treatments, oxycarboxin (Plantvax) and carboxin (Vitavax), would provide complete control of flag smut. Carboxin became the registered treatment, because it was also effective against common bunt. Because of the use of carboxin, flag has become an insignificant disease of wheat in North America and is no longer a threat to the marketing of wheat in the Pacific Northwest. More recently, newer seed treatments have been developed that control flag smut at lower rates.

Loose Smut. Loose smut, caused by the fungus Ustilago tritici, is most obvious when new heads emerge. The disease first appears as deformed spikelets filled with a dry, dusty, powdery mass of black spores enclosed by a fine membrane. The membrane quickly disintegrates, and within a few days, only the bare spike with a few black spores remains. The exposed spores are dispersed by wind to newly emerging healthy heads where they germinate and infect the plant when the spikelets open for pollination. The fungus develops in the embryo as the seed develops and becomes dormant as the seeds ripens. The pathogen remains dormant within the seed until the seed germinates. Then it grows along with the plant and ultimately occupies the developing spike. Therefore, infection that occurs in one year does not produce smutted heads until the next year.

Loose smut had been difficult to control for centuries primarily because it is dormant within the seed embryo. Applications of protectants to the seed surface were ineffective, and other treatments that penetrated the seed were difficult to use. The break through in the control of loose smut was also seed treatment with the systemic fungicide, carboxin.

Dwarf Bunt. Dwarf bunt, caused by the fungus Tilletia controversa and also called dwarf smut or TCK smut, is similar to common bunt, except that it infects tillers of wheat plants in the winter under snow at temperatures near freezing, and it can survive in the soil for many years. Because of those characteristics, the disease was not controlled by the seed treatments that were effective in controlling common bunt, flag smut, and loose smut. Planting very early or very late in the fall and combining several genes for resistance into one variety reduces losses in yield caused by dwarf bunt but does not provide complete control and does not prevent contamination of the grain. Furthermore, the People's Republic of China has a quarantine to prevent the import of grain contaminated with dwarf bunt spores. Thus, a few spores in a grain sample can affect international marketing of wheat from a region, even when the disease does not significantly reducing yield in that region or reduce flour quality . Such restrictions have had the effect of eliminating a major world market. Results of our recent research show that a new systemic seed treatment called difenoconazole (Dividend) provided complete control of dwarf bunt under a wide range of environmental and managerial conditions. The new treatment was registered in the summer of 1994 and was used extensively to treat fall planted wheat in 1994 and 1995. The fungicide is now used in many areas of the region. Hopefully, difenoconazole will be as effective in controlling dwarf bunt as the other treatments were in controlling common bunt, flag smut, and loose smut and will alleviate the problems related to marketing wheat in China.

Karnal Bunt. Karnal bunt (partial bunt), caused by Tilletia indica, is a new smut disease of wheat in the United States. Karnal bunt is a disease occurs under the environmental conditions that occur in India, Pakistan Nepal, and Mexico and is seldom severe and seldom affects grain quality even in those countries. The disease was detected in Arizona and California in regions adjacent to Mexico in March 1996. The smut spores survive in the soil and germinate when wet to produce a type of spore that is carried to wheat heads where infection occurs. Because of its environmental requirements for survival, germination, and infection, Karnal bunt is not expected to become a production problem in the Pacific Northwest or in regions of northern United States. It should not affect yield or quality in those regions. However, because of national and international quarantine regulations, the disease has an impact on the marketing of wheat. It is to the southwestern United States what dwarf bunt is to the Pacific Northwest. Some seed treatments reduce the spread of Karnal bunt, but none of the currently registered seed treatments provide 100% control of the bunt. Future control of Karnal bunt on seed will depend upon the ability of the seed treatments to kill all spores in and on the wheat seed or in the soil or to kill all germination spores. Some systemic foliar fungicides prevent infection of the heads or prevent disease development in the field. There is a need for research aimed at elucidating the epidemiological factors affecting the disease in order to understand its importance to the wheat industry and implement control methods.

General Comments Regarding Control of Smuts and Bunts. Quarantine regulations have proven to be ineffective and unnecessary for control of the smuts and bunts. Whereas, use of resistant cultivars, and management, has controlled the diseases. The use of seed treatments for the control of smuts and bunts is the best example of the effectiveness, value, usefulness, and importance of chemicals for control of cereal diseases. Several seed treatments provide outstanding control of common bunt, flag smut, and loose smut with essentially no adverse environmental impact and a minimum cost to the grower. Difenoconazole provides similar control of dwarf bunt. Seed treatments and foliar fungicides control Karnal bunt. Use of chemical seed treatments has prevented world-wide crop losses and saved the economy of the Pacific Northwest while preventing pollution of the environment with bunt and smut spores. The newest systemic fungicides control the diseases at rates lower than 0.5 ounces per acre, rates that have essentially no affect on the environment. If the seed treatments were not available, those diseases would return to their former importance, and the economy of the Pacific Northwest would be drastically affected.