Small Grain Smut and Bunt Diseases

The smuts and bunts are a group of related fungi that infect developing grain. These pathogens can cause significant yield losses and may cause hazardous conditions for combine operators. Dense concentrations of spores suspended in the air can be explosive. In Utah, four types of smut and bunt are of economic importance. These are loose smut, covered smut, common bunt or stinking smut, and dwarf bunt.

LOOSE SMUT

Loose smut (Ustilago nuda, U. avenae, U. tritici) is found on barley, oats, and wheat. This disease is carried in embryos of infected seed. Plants grown from infected seed become evident soon after "boot" stage when the developing head appears as a mass of dark brown spores with a gray paper-like membrane covering. This membrane ruptures as healthy plants begin to flower, and windblown spores infect the embryos of developing seed. By harvest, smutted heads are left with only a bare, sooty rachis.

Control
The best control for loose smut is to plant certified smut-free seed. Do not plant seed from fields with even a small amount of loose smut without treating it. Carboxin or Carboxin plus Thiram provides good control of the loose smuts. Triadimenal (Baytan) will give fair control.

COVERED AND SEMI-LOOSE SMUT

Covered (U. hordei, U. nigra) and semi-loose smut (U. koller) are found on barley and oats, respectively. Kernels in the head are replaced by masses of dark brown spores. A white membrane covers the spore mass and usually remains intact until harvest. The spores are dispersed during threshing, resulting in contaminated, healthy grain. No foul odor is present when the spore mass is crushed.

Control
Seed should be treated at planting. On barley, Carboxin or Carboxin plus Thiram provide good control. Captan, Mancozeb, Pentachloronitrobenzene (PCNB), Baytan, or Thiram give fair control. On oats, Carboxin or Carboxin plus Thiram provide good control and Captan and PCNB give fair control.

COMMON BUNT OR STINKING SMUT

Common bunt or stink smut (Tilletia caries, T. Foetida) are found on wheat. There are no obvious symptoms until the grain heads emerge. The kernels are replaced with smut balls filled with dark spores. Glumes on the infected heads are spread wide, exposing the plump smut balls. When the smut ball is crushed, it has a foul, fishy odor. Infection occurs either from spores on the seed surface or from spores in the soil near the seed.

Control
To control common bunt or stinking smut, you should plant resistant varieties. One of the following seed treatments should be used. PCNB provides the best control. Baytan, Captan, Thiram, and Carboxin give fair control.

DWARF BUNT

Dwarf bunt (T. controversa) is found on wheat. Infected plants are stunted in growth when compared to healthy plants. The kernels are replaced with smut balls which have dark spores. When the smut ball is crushed, a foul, fishy odor is present. Infection of the seedlings occurs under the snow in January or February. The spores will survive for up to 10 years in the soil.

Control
Plant resistant winter wheat varieties or avoid the disease by planting spring grains. Resistant varieties have been released by the Utah Agricultural Experiment Station. Thiabendazole (TBZ) may be effective as a seed treatment if seeding is delayed.

Sherman V. Thomson/Extension Plant Pathologist
Scott C. Ockey/Plant Disease Diagnostician

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