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Dwarf Bunt (Stinking Smut) of Winter WheatCrop Protection Factsheet
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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%.
Hosts:Dwarf bunt has also been reported on some grasses, rye and winter barley, but this is much less common than infection of winter wheat. Spring wheat is not affected. Dwarf bunt was observed in spelt in 1998 at Armstrong B.C. |  Bunt balls containing black spores |
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 ripens. 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. |
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 3-8øC) 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 (revised 1998):
The fungicide seed treatment "Dividend" (difenconazole) is now registered for control of dwarf bunt in Canada (as of July, 1998). Dividend is the only registered seed treatment that provides control of the soil-borne phase of dwarf bunt. The level of control provided by Dividend has been found to be excellent in BCMAF trials at Armstrong BC.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, seed treatments such as Vitavax or Anchor will protect against seed-borne smut and bunt diseases.
Canadian winter wheat varieties are all susceptible, however some varieties are more susceptible than others. For example, Norwin is more susceptible than Readymade. 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 in only two
locations, including the North Okanagan/Shuswap
area of B.C., and several counties in southern
Ontario. A 1993 BCMAFF survey found dwarf bunt
in 18/20 crops of winter wheat grown in the area
between Vernon and Tappen. The disease is also a
problem in Washington, Oregon and Idaho.
- Dwarf bunt is a disease of "quarantine
significance". Some countries, such as China,
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", and none have been recorded for decades. Although the days of smut explosions seem to be over, it would probably be wise to refrain from smoking when combining a heavily infected crop.
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.
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