DISEASES OF SMALL GRAINS - SMUTS

Several types of smut can adversely affect wheat (and other small grain) production. The two most common types of wheat smut are loose smut (shown in this slide) and stinking smut (also called common bunt).

Loose smut is caused by the fungus Ustilago tritici, which overwinters in infected wheat seeds. As the infected seeds germinate and grow, the fungus grows along with the growing point. The fungus stimulates infected plants to head slightly earlier than non-infected plants, and to be slightly taller. At the time that non-infected plants begin to bloom, kernels of infected heads disintegrate, releasing loose smut’s characteristic black, sooty spores. The disease is most easily diagnosed at this time. Spores are blown to healthy plants and, acting like pollen, germinate and grow down the pistil where they infect the wheat embryo. Infected kernels are asymptomatic.

Loose smut can be controlled through use of wheat seed that has been certified as being smut free. Systemic seed fungicide treatments are also very effective in loose smut control. Systemic fungicides are those that are absorbed by the seed and transported internally. Protectant fungicides (those that are not absorbed by the seed, but simply coat the seed surface) are not effective because the fungus is already inside the seed at the time when a seed treatment would be made. Finally, selection of loose smut resistant wheat varieties may also aid in disease control.

Stinking smut is caused by the fungus Tilletia tritici. Fungal spores on the surface of the seed germinate and infect the wheat seedling as it emerges. The fungus grows along with the growing point of the plant and eventually colonizes and takes over the kernels as they develop. Infected plants may be stunted and have slender heads that tend to remain green longer than those of non-infected plants. The extent of infection in a field is typically most apparent at the time of harvest. At this time, infected kernels break apart releasing spores that coat healthy seeds. These spores serve as the source of inoculum for the following year’s infections.

To prevent problems with stinking smut, use seed that has been certified as being smut free. Also, chemical treatment of wheat seed has proven very successful in preventing infections. Both protectant and systemic fungicides are effective for stinking smut control. In addition to use of certified seed and seed fungicide treatments, smut resistant wheat varieties are also available for stinking smut control.