Common Bunt (Stinking Smut) in Wheat

Common Bunt (Stinking Smut) in Wheat

July 22, 2015

This year there are reports of outbreaks of common bunt of wheat, also known as stinking smut, in a wide area from western Nebraska into eastern Colorado.  This article summarizes information about common bunt and its management.

Causal Organisms

Common bunt in wheat

Figure 1. Bunt balls containing spore masses of the common bunt (stinking smut) fungus.

Common bunt is caused by two closely related fungi, Tilletia caries (also known as Tilletia tritici) and Tilletia laevis.  These fungi survive on the surface of the seed and in soil.  The most important source of infection is contaminated seed.

Disease Cycle and Favorable Environmental Conditions

Infection of wheat occurs during germination and is favored by cool, wet conditions. The fungi penetrate and infect the coleoptile (tissue sheathing the first true leaf) before seedling emergence.  Mycelia (tubular fungal filaments) grow within the plant and inhabit the terminal meristematic tissues and eventually displace all tissues in the ovary. 

Black spores known as teliospores form and fill the kernels.  These kernels are dull gray brown and are known as bunt balls (Figure 1).  During harvest, the bunt balls are broken and release teliospores which contaminate the grain and soil.  Some bunt balls remain intact and are mixed with healthy grain. In heavily affected fields, dark clouds of bunt spores can be seen during harvest.

Economic Importance

Grain contaminated with bunt spores has a darkened appearance and gives off a pungent, fishy smell (hence the name stinking smut). Common bunt reduces both grain quality and yield. Contaminated grain usually is discounted at the elevator and can be rejected altogether.

Management

  1. The most effective management strategy for common bunt is to treat seed with fungicide before planting. It is preferable to buy certified, fungicide-treated seed or have it cleaned and treated by a commercial seed conditioner.  If seed is treated on-farm, it is essential to clean it first using grain cleaning equipment before treating. To maximize the effectiveness of the seed treatment, ensure thorough, uniform coverage.  Always read and follow label instructions when applying a fungicide. A list of fungicide seed treatments for wheat is provided in Table 1.
     
  2. Early planting when soil temperatures are warm and unfavorable for infection can provide partial control of common bunt.  However, planting early increases the risk of other diseases such as wheat streak mosaic virus and barley yellow dwarf.
     
  3. Avoid using farm-saved grain from previous crops as seed for the next crop.
     
  4. There are no current wheat cultivars with good resistance to common bunt.

Use of Common-Bunted Grain

The common bunt fungi do not produce toxins harmful to livestock.  Therefore, common-bunted grain that has been rejected by the elevator can be used as livestock feed.  All classes of livestock can feed on it without negative health effects.  However, some livestock may refuse to feed due to the pungent smell.  This refusal can be alleviated by mixing common-bunted grain with healthy grain to reduce the proportion of bunt balls in the feed.

More Information

Additional information on common bunt of wheat, also known as stinking smut, is available in these publications:

Stephen Wegulo
Extension Plant Pathologist