Washington State University

Common Smut

Wahington State University Bulletin
SP0004 -- 1993
Diseases of Washington Crops.
Otis C. Maloy and Debra Ann Inglis
Small Grains Home * Wheat Diseases * Barley Diseases
Cause
Hosts
Symptoms and Signs
Disease Cycle
Control
1997 PNW Pesticide
Recommendations
Other interesting
Common Smut sites
Common smut or common bunt is one of the so-called stinking smuts. Until the mid-195Os, it was considered the most serious wheat disease in Washington. It was not uncommon to see large, black clouds of smut teliospores billowing up behind combines when infected fields were harvested. Workers in such fields were often covered with these black, foul-smelling spores. The disease not only reduced yields greatly but also caused losses by contaminating clean wheat. The foul, fish oil-like odor comes from the presence of trimethylamine in the teliospores. Ignited by sparks during harvest, this compound started fires in the fields and caused combines to explode.

 

Cause
Either Tilletia caries or T. foetida cause common smut. T. caries is important in Washington, and is the more widely spread of the two species. T. caries produces reticulate teliospores, while T. foetida, more common in the central and eastern United States, has smooth teliospores. Both fungi are basidiomycetes that have many pathogenic (physiologic) races.

Hosts
Wheat is the primary economic host, but both fungi also infect rye and some Agropyron species.

Symptoms and Signs
Plant stunting may not be noticed. The first apparent symptoms occur after heading. Smutted heads remain green longer than healthy heads but are more of a gray-green. The kernels are replaced by "smut balls" which are more rounded than normal kernels, and which cause the glumes to spread abnormally, giving the heads an unsymmetrical appearance (Photo 5). Since infection occurs in the seedling stage before tillering and is systemic, a) tillers on a plant are diseased.

Disease Cycle
Teliospores released from the smut balls during harvest either remain in the soil or are carried on the seed. Soilborne inoculum is an important factor to consider in the control of this disease. The teliospores germinate in moist soil to produce a short hypha (or promycelium) on which long, thin basidiospores (also called primary sporidia) develop. Opposite mating types of basidiospores fuse, and produce secondary basidiospores. Temperatures of 40 to 60F favor the production of secondary basidiospores. The basidiospores germinate near the seed and infect the developing plant coleoptile before it emerges from the soil (seedling infection).

The mycelium then develops in meristematic tissue of the growing plant and is carried upward with plant growth. At flowering the fungus colonizes the ovaries and replaces the kernel with teliospores to produce easily ruptured bunt (smut) balls. The teliospores are only viable in the soil for about 2 years.

Control
Chemical.
The dramatic event that brought common smut under control was the use of the protectant fungicide, hexachlorobenzene (HCB), as a seed treatment. This chemical, effective only against common bunt, had great advantage over other fungicides (such as the mercurials) because it controlled not only the teliospores carried on the seed but those in the adjacent soil as well. Since seedling infections resulting from soilborne inoculum constituted the major problem, this was a major breakthrough. Other fungicides have replaced HCB, which is no longer available.

Resistance. Even though most commercial varieties have resistance, researchers still advise seed treatment to delay development of pathogenic races and to prolong the effective life of these cultivars. Plant breeders estimate that a newly released resistant cultivar will last only 5 to 8 years before a fungal strain develops that can attack it.

Reference
Hoffmann, J.A. 1982. Bunt of wheat. Plant Disease 66:979-986.

Other Interesting Common Smut sites:
University of Nebraska-Lincoln, NebGuide, Bunt or Stinking Smut of Wheat -- description and picture of grain.

Purdue University, Crop diseases -- picture and description.


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Edited and reviewed by Ed Adams, WSU Extension Plant Pathologist
Comments and questions: adamse@wsu.edu

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