Stacy Campbell
K-State Research & Extension

Common bunt (stinking smut) on wheat

Each year wheat growers will save some of their best yielding varieties wheat seed to plant in the fall. This management practice works well in reducing seed costs by not having to purchase new certified seed wheat each year. However sometimes too much of a good thing can back fire. If growers continue to save seed wheat from the same seed lot each year for several years in a row, problems can eventually occur with seed borne disease of loose smut and common bunt or stinking smut.
Common bunt (stinking smut) occurs somewhere in Kansas almost every year, resulting in significant price penalties or rejected loads of grain. This fungal disease causes deformation of wheat kernels, and infected kernels often have a gray color. The infected kernels are filled with black powdery spores as opposed to the normal white starches of healthy kernels. The fungus produces volatile chemicals that have a strong fishy odor. This odor is readily detected in loads of grain and may persist through the milling and baking process.
Common bunt is a seed-borne disease. The disease persists between seasons on seed that was contaminated with the fungus during harvest or subsequent grain handling. The spores will survive on the outside of the kernels until fall, when they germinate and infect the developing seedlings shortly after planting. This infection process is favored by cool and wet fall conditions. Survival in the soil is considered rare in Kansas, because summer rainfall is normally sufficient to trigger germination of the fungal spores left in the field after harvest. The potential for survival in the soil is greater when soil conditions remain dry.
Unfortunately, there do not appear to be many options for using the rejected grain. Saving this grain for seed will increase the chances of having problems with bunt in following years. In some situations, I have heard of growers working with local feed lots to move rejected grain. The availability of this option will likely vary regionally within the state.
Management options for common bunt:
Fungicide seed treatments: Seed treatment fungicides are the first line of defense against seed-borne fungal diseases like common bunt. Even low levels of bunt can become a serious problem the following year and growers that have detected bunt or smut should use fungicide seed treatment for all their wheat. Products such as Dividend Extreme, Raxil MD, and Charter are all highly effective at controlling seed-borne diseases like common bunt and loose smut. Thorough coverage of the seed with the fungicide is very important and growers should consider having their seed commercially treated were possible. When used properly these seed treatments are able to protect more than 98% of the kernels from infection. Even at this high level of control, it may take several years of persistent use of the seed treatment fungicide to completely eliminate the disease.
Disease free seed: Common bunt is most likely to be a problem when wheat has been saved for seed for 2 or more years. Renewing the seed supply with certified seed every few years will greatly reduce the risk of future common bunt problems. On farms with a history of bunt problems, it is a good idea to have the new seed treated with a fungicide seed treatment. This seed treatment will reduce the risk of contamination from equipment (trucks, augers, grain charts used to move and handle the new seed) and eliminate the risk of infection from soil-borne sources of the fungus.
Crop rotation and volunteer wheat: It is also important to clean up volunteer wheat in fields with a history of common bunt. Any grain that passed through the combine during harvest is likely contaminated with the spores of the bunt fungus. If weather conditions are right, these volunteer plants could become infected with common bunt and negate the efforts to use clean seed or fungicide seed treatments. Because the infection is favored by cool and wet conditions, the late flushes of volunteer wheat are most at risk for infection. Therefore, crop rotation in fields with a history of bunt is probably the best option. Where rotation is not possible it is important to remain vigilant about volunteer control prior to planting the next wheat crop. Wheat varieties with the Clearfield technology that allows use of the herbicide Beyond to control volunteer wheat and feral rye may also be an option for some growers.
Common bunt can be a frustrating problem, but the disease can be controlled with a few adjustments to the production practices. Remain persistent and pay attention to details so that you receive maximum benefit for your efforts.
Information provided by Erick DeWolf, K-State Extension Plant Pathologists