Common bunt of wheat
Common bunt is also known as covered smut or stinking smut, and reduces yield and grain quality. It is caused by Tilletia caries and T. laevis. Though historically important, current management practices typically prevent major losses. Evidence of common bunt is rarely present before ripening, but some stunting may occur. As heads ripen (FGS 10.5.3), spikelets may develop an off-green discoloration and appear greasy. Glumes start to open as if “overstuffed” as large spore masses (bunt balls) form within spikelets. Bunt balls grow within the seed coat, which becomes fragile and eventually ruptures to reveal large masses of black spores. Wheat with common bunt will not be severely stunted, and bunt balls will be roughly the shape of a wheat kernel, not globular. Spores give off a distinct fishy odor. In awned varieties, awns of infected heads may be smaller or are completely missing. Infected heads tend to be thinner and may not lose green coloration as quickly. While not problematic for consumption, heavily infected wheat is greatly discounted due to odor and discoloration. Heads symptoms begin to appear during flowering.
Common bunt is favored when infected seed is planted into cool (<68°F), moist soil. While spring wheat can be infected, common bunt usually occurs in fall planted wheat, when the environment is most favorable for infection. Snow cover is not required for the fungi to infect seedlings.
Some resistant varieties have historically been available, but due to pathogen resistance, alternative management practices are typically used today. Resistance is critical for organic production of winter wheat. Plant early (winter wheat), or late (spring wheat) to avoid giving the fungus an opportunity to colonize the plant early on in its development. This may prevent the fungus from reaching the wheat heads and forming bunt balls. Fungicide seed treatments are labeled and effectively manage common bunt.
Gallery images: M. Burrows and C. Grau.