Ministry of Agriculture

Dwarf Bunt (Stinking Smut) of Winter Wheat

What is dwarf bunt?

Dwarf bunt is a winter wheat disease that is caused by the fungus Tilletia controversa. The kernels of diseased plants are replaced by "bunt balls", which contain masses of black spores with a foul odour. The bunt balls rupture at harvest, contaminating the grain. Yields are also reduced. Severely infected crops may have infection levels as high as 50%.

dwarf bunt - bunt balls Bunt balls containing black spores

Hosts:

In British Columbia, dwarf bunt has been observed on winter wheat and spelt. It has also been reported on some grasses, rye and winter barley in other areas, but this is much less common than on winter wheat. Spring wheat is not affected.

Regulated Areas in B.C.

Dwarf bunt is a regulated disease in Canada. As a result, there are restrictions on movement of grain and straw from infested areas to non-infested areas in Canada to prevent disease spread. Regulated (infested) areas in B.C. include the Regional Districts of Central Kootenay, North Okanagan, and Columbia Shuswap west of the Monashee Mountain Range. Grain, seed, straw, hay, chaff or screenings of wheat, barley, rye, oats or triticale may not be transported out of the regulated areas without a movement certificate issued by the Canadian Food Inspection Agency (CFIA). Refer to the CFIA Policy Directive linked below for more information.

Grain or straw produced in a regulated area can be moved to a non-regulated area if it is sampled and tested free of (negative for) dwarf bunt. Growers should request a CFIA inspector to sample the grain if they are considering moving it out of B.C. or to a non-regulated area of B.C.

dwarf bunt regulated areas Regulated areas for dwarf bunt in British Columbia are shown bordered in red. Click on map to open a larger copy.

 

Symptoms:

Wheat plants infected with dwarf bunt tend to be shorter than healthy plants, typically one-quarter to one-half the normal height. They may also have an increased number of tillers. Infected plants produce heads that contain bunt balls in place of the seed. Bunt balls are roundish in shape, and consist of a thin pericarp (skin) which is filled with black, powdery spores. The skin is initially green, but turns brownish as the crop matures. Some heads may contain both kernels and bunt balls, but generally the whole head is infected. Bunt spores have a strong and unpleasant fishy odour.

Bunt is not very conspicuous in the field, because bunt balls are hidden inside the heads, and infected, dwarfed tillers may be hidden in a canopy of taller, healthy heads. Bunt balls are often large enough to push the glumes apart, causing the infected heads to have a ragged appearance. Awns are sometimes malformed. Diseased heads become easier to see as the crop matures and bunt balls begin to break open, revealing the black powdery spores. Dark spore clouds may be produced at harvest when infection levels are high.

Dwarf bunt-infected tiller of winter wheat. Winter wheat head with dwarf-bunt infection

Other smut diseases:

There are other smut and bunt diseases which are similar in appearance to dwarf bunt. Common bunt is virtually identical in appearance, expect the infected tillers are closer to normal height. Common bunt may also affect spring wheat. It is most commonly spread by contaminated seed, and is easily controlled by seed treatments. Loose smut is another seed-borne disease. It does not survive in the soil. Loose smut causes the whole head (rather than individual kernels) to be replaced by a mass of dark brown spores. The spores are blown away early in the summer, leaving an empty rachis (central stalk) behind. Because the spores are blown away long before harvest, loose smut does not cause grain contamination problems.

Life cycle:

Dwarf bunt survives for a long period of time in the soil (over 10 years). The disease may be spread on seed, and also at harvest when spores contaminate the soil and may be blown to neighbouring fields. Cultivation equipment could also spread contaminated soil from field to field.

When winter wheat is planted in infested soil, the crop may become infected over the winter months. Dwarf bunt spores germinate only at low temperatures (optimum 3-8°C) over a period of several weeks. This normally happens under snow cover on unfrozen ground, between December and February. Conditions are not favourable for dwarf bunt every winter, which explains why the disease is not a problem every year.

When plant growth resumes in the spring, the fungus grows systemically within the infected plants. Heads are colonized by the fungus as they develop, and bunt balls are formed instead of kernels.

Control:

The fungicide seed treatments "Dividend" and "Cruiser Maxx" (containing the active ingredient difenoconazole) are registered for control of dwarf bunt in Canada, providing excellent control of both soil-borne and seed-borne dwarf bunt.

Other seed treatments are not effective against soil-borne dwarf bunt, because they do not persist long enough in the soil to protect the plant through the winter. However, there are many seed treatments that will protect against seed-borne loose smut and common bunt diseases.

Canadian winter wheat varieties are all susceptible to dwarf bunt. Resistant varieties may be available in the future.

Grain grading and bunt:

Bunt contamination of grain may result in dockage. Wheat having a smutty odour and/or heavy smut or bunt infection will be downgraded to "sample". Grain having no distinct odour but containing bunt balls may be specially cleaned by aspiration.

Bunt contamination may also result in rejection of malting wheat.

Interesting facts about dwarf bunt:

  • In Canada, dwarf bunt occurs only in certain interior valleys in B.C. and in Ontario east of Thunder Bay.
  • Dwarf bunt also occurs in several U.S. states including Washington, Oregon, Idaho, Colorado, Montana, Wyoming, Utah, Indiana, Michigan, and New York.
  • Some countries have zero tolerance for dwarf bunt spores in imported grain. Contamination of exported wheat could jeopardize export markets.
  • Clouds of bunt spores are flammable. In the early 1900's there were hundreds of threshing machine fires in the Pacific Northwest. Equipment losses alone in 1914 and 1915 were estimated at $1,000,000. Modifications to threshing equipment gradually eliminated the problem of "smut explosions".

Health effects of smuts and bunts:

Smut and bunt spores are highly allergenic. Bunt may cause respiratory allergy symptoms similar to pollen and dust.

Bunt is not toxic to livestock. However, individual animals may have allergic type reactions to large amounts of fungal spores, such as smuts, bunts or rusts.

Resources

Policy directive D-99-01: Barley, Oats, Rye, Triticale and Wheat - Phytosanitary Requirements on Import, Transshipped, In-Transit and Domestic Movement - Canadian Food Inspection Agency.

Prepared by:
Gayle Jesperson, Plant Pathologist
Ministry of Agriculture, Kelowna B.C.

 

Revised January, 2011