Smuts and Bunt

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 Biology | Damage description | Economic threshold | Diagnosis | Management strategy

covered smut - Ustilago hordei
false loose smut - U. nigra
loose smut - U. nuda

covered smut - U. kolleri
loose smut - U. avenae

stem smut - Urocystis occulta

common bunt (stinking smut) - Tilletia caries, T. foetida

loose smut - Ustilago tritici
common bunt (stinking smut) - Tilletia caries, T. foetida


All cereals are attacked by smut, but each crop is host to specific species of smut fungus. Each smut species has a number of races that differ only in their ability to infect certain cultivars of that crop host.

Covered and false loose smut of barley, bunt of wheat and loose and covered smut of oats will be dealt with as a group because their disease cycle is similar. These fungi overwinter as spores on the seed surface. When infested seeds are sown, smut spores germinate, penetrate the seedling and grow within the cereal host until the heads develop. Smut fungi replace all or most of the grain head and form masses of black smut spores instead of seeds and chaff. These spores are released at grain harvest and contaminate the surface of other healthy kernels.

True loose smut of barley and wheat differs from the above cycle in that the fungi overwinter within the embryo of the seed. The fungus grows within the plant and produces loose black smutted heads. Wind-borne, dust-like spores infect healthy cereal flowers, where they penetrate the developing seed and infect the germ (embryo).

True loose smut infection is favored by cool wet weather in July, which slows and prolongs the flowering period and allows more time for infection.
All other smuts are favored by seeding into cold soils during the late fall or early spring.

Damage Description

Most smut fungi attack and replace the internal tissues of the grain with dark brown or black smut spores.

  • Covered smut (barley, oats)
    The plant may be slightly stunted with hard, compact, upright, smutted heads. The smut balls are covered in a membrane that remains intact along with the awns and chaff.
  • False loose and true loose smut (barley)
These fungi have a very thin covering membrane around the spore masses. The brown spores are blown or washed away and leave bare spikes in the ripening crop. Losses from loose smut in barley average less than 1 per cent, but losses of up to 40 per cent have been recorded.
A laboratory examination is needed to tell them apart.
  • Bunt (wheat and triticale)
    Infected heads may be wholly or partially smutted. The heads may remain green longer and glumes in the infected head may be spread apart. Awns may be reduced or fall out as the heads ripen. Bunt balls are rounder than wheat kernels. When these balls rupture, the loose black powdery spores have a distinctive fishy odor.
Smuts attack the grain directly, replace the contents and cause a 1:1 direct yield loss. A 10 per cent bunt infection, if the heads are completely infected, results in a 10 per cent yield loss (aside from quality considerations). The presence of covered and false loose smut and bunt also results in a drastic reduction in grain quality because of the visible contamination with black smut spores.

Smutted grain should be stored separately from clean grain. Heavily smutted or bunted grain will not be accepted at the elevator and may even be difficult to sell as feed owing to respiratory or feed refusal problems that might result. Smuts and bunts are not toxic to livestock.

Economic Threshold

The maximum percentage of smutty barley kernels (true loose smut) permitted under the Canadian Seeds Act, July 1, 1987, are as follows:

Maximum % of smutty barley kernels
Canada Foundation #1
Canada Foundation #2
Registered #1
Registered #2
Certified #1
Certified #2
Common #1
Common #2
The fishy odor from bunted wheat is detectable at less than 0.01 per cent thus rendering grain unsuitable for food even at this very low level of contamination.


Use the following formula to calculate losses from loose smut and bunt in barley, wheat and oats. The percentage of infected ears is directly proportional to grain yield loss.
    Loss (kg/ha) = Pr x Pa / 100-Pr
    Pr = percentage infected heads
    Pa = actual yield (kg/ha)
    This is done during heading (Feekes G.S. 10.1-10.5) for loose smut and true loose smut and during ripening (Feekes G.S. 11) for bunt. Assess 50 plants along a diagonal at sites selected at random. Calculate the percentage of infected heads. Thus a 10 per cent infection in a 3000 kg/ha crop = 333 kg/ha yield loss.
Management Strategy
  • Use resistant cultivars. See Varieties of Cereal and Oilseed Crops for Alberta.
  • Use smut free seed.
  • In southern Alberta bunt of wheat may be soil-borne. Avoid continuous cropping of winter wheat and do not seed winter wheat within 2 km of a known bunt infected field of either spring or winter wheat because the spores are wind blown.
  • Use a hot water seed treatment for loose smut. Dip seeds in 20 degrees Celsius water for 5 hours, drain and then dip in 49 degrees Celsius water for 1 minute, then 52 degrees Celsius water for exactly 11 minutes, then immediately in cold water until seeds cool off. Some seed may be killed by hot water treatment.
  • Use systemic seed treatments that contain carbathiin for loose smut control in wheat and barley. All other semi-loose, covered smut and bunts in cereals are controlled by any of the registered fungicide or fungicide combinations.

True loose smut in barley (systemic seed treatments for control).

Covered smut in barley emerging at the boot stage.

Covered smut of barley.

True loose smut of barley.

True loose smut of barley infection of healthy embryo's (seeds) takes place at flowering.

Loose smut infection inside a barley embryo (the bran).

Covered smut of oats. (Ustilage kolleri). A seed borne fungus controlled by seed treatment fungicides.

Stem or stalk smut of fall rye. A seed-borne smut that can be controlled by a systemic fungicide or smut resistant varieties.

Bunt of wheat. The bunt bulls or bunted grain release masses of black spores that contaminate healthy grain. A 2 % bunt infestation because of the fishy smell and black soot colours can downgrade #1 wheat to sample.

Loose smut in oats. (Ustilago avenae).

Bunt clouds in wheat fields during the thirties and earlier caused by very heavy infection of the wheat crop by this seed-borne disease. Resistance and fungicide seed treatments now have this disease under control.

Loose smut in wheat.

Common bunt of wheat (Tilletia foetida) . Healthy head right diseased head left. All the grain in the diseased head has been replaced with fishy smelling black bunt balls.
This document is maintained by Shelley Barkley.
This information published to the web on November 16, 2001.
Last Reviewed/Revised on November 23, 2011.