August 28, 2013

Common bunt (also known as stinking smut) is a disease of wheat that was recognized in the 18th century. It occurs worldwide and is of major economic importance. In Nebraska, common bunt occurs frequently but to varying extents, especially in the eastern half of the state.

Causal Organisms and Disease Cycle

Figure 1. Bunt balls containing spore masses of the common bunt (stinking smut) fungus. Common bunted grain is usually discounted and can be rejected at the elevator.

 

Common bunt is caused by two fungi: Tilletia tritici and T. laevis. Spores of these fungi survive on seed or in soil. In the fall following wheat planting, the spores germinate in response to moisture to form hyphae (tubular filaments of a fungus, also known as mycelia). The hyphae penetrate (infect) the coleoptile (protective sheath around the first true leaf) before seedling emergence.

Germination and production of infectious hyphae are favored by cool, wet weather. In an infected wheat plant, the fungus grows and inhabits the terminal meristematic tissues (dividing cells responsible for growth), particularly the flower primordia (first tissues to form) of the head. In a susceptible cultivar, mycelium proliferates in the developing ovary, displacing all tissues in the pericarp (outer layer of the ovary) and eventually forming black spores known as teliospores.

Symptoms

Infected plants may appear stunted but are not readily distinguished until head emergence. Bunted heads are slender and retain their green color longer than healthy heads. Glumes and awns spread apart exposing bunt balls (“kernels” full of black spore masses). The bunt balls (Figure 1) are dull gray to brown and resemble kernels but are more rounded. They remain on the head and give off a strong pungent, fishy odor. At harvest, bunt balls rupture, releasing fishy-smelling teliospores. In severely affected fields, dark spore clouds form during harvesting.

Economic Importance

Common bunt reduces both wheat yield and grain quality. Affected grain has a pungent, fishy smell, is discounted in value, or can be rejected at the elevator. This year (2013) and in previous years, a number of wheat growers had their grain rejected at the elevator due to common bunt. If large areas of fields or entire fields are severely affected, a significant economic impact can result from failure to sell grain.

Management

This fall you can take the most important steps to manage common bunt:

• Plant certified, pathogen-free seed.
• Treat seed with a fungicide before planting.
• Choose resistant cultivars.

For more information on common bunt, see NebGuide G1978, Loose Smut and Common Bunt of Wheat.

Are Common Bunt or Other Wheat Smuts Toxic to Animals?

Conventional wisdom is that common bunt is not toxic to animals, but individual animals may experience allergic reactions when exposed to fungal spores. Animals may refuse to eat rations containing wheat contaminated with common bunt. That opinion, or parts of it, appears in several publications about smutty wheat without reference to a source for the opinion.

Toxicological assessment of animal exposure to common bunt is complicated by the various types of wheat smuts and the names given to them. The risk of adverse effects depends on the specific fungus infecting the ingested feed. Smuts occur regionally depending upon which pathogenic organisms can survive in the regional climes.

There is limited university information to support the opinion that ingestion of smut-contaminated wheat is not harmful to animals. We found no credible evidence that wheat smut is toxic to animals. Indexes of veterinary toxicology textbooks do not include “smut” or “bunt” in their listings. That supports the opinion that the risk of adverse health effects to animals ingesting smut-contaminated feed is minimal. Since smut is a fungal disease and some fungi produce mycotoxins, it is logical to wonder if smut fungi might produce such compounds. We found no indication that smut fungi produce mycotoxins.

Recommendations for Feeding Smut-Contaminated Wheat To Animals

Even though there is no apparent risk of feeding smut-contaminated wheat to animals, it is safest not to feed it. Otherwise, follow these general guidelines. 

1. Dilute the smut-contaminated grain with other components of the ration. Blend in the contaminated component as thoroughly as possible so the animals cannot selectively avoid eating that component. The required dilution may need to be determined by trial-and-error. (See items 2 and 3.)
    
2. The taste or odor of feed containing smut-contaminated wheat may cause feed refusal by some animals. Sufficient dilution may mitigate this.
    
3. Nutritive content of smut-contaminated wheat should be assessed so its impact on the nutritive value of the total ration is considered during ration formulation.

It is safer to avoid feeding it to breeding stock, young animals, and lactating or gestating animals.

Smut-Dust Fires

The historical record includes dust explosions and fires associated with smut-infested grains. The risk of such incidents today is likely significantly different because of current scientific knowledge, safety, and fire protection policies and practices. But smut-infested wheat can produce a cloud of spores when it is agitated, so precautions against a dust fire or explosion should be taken when handling the grain.

Stephen Wegulo, Extension Plant Pathologist
Michael Carlson, Diagnostic Toxicologist/Analytical Chemist