Source: Crop Watch News Service
University of Nebraska-Lincoln Extension, Institute of Agriculture and Natural Resources
http://cropwatch.unl.edu/archives/2006/Crop18/wheat.htm

Figure 1. Wheat grains containing spore masses of stinking smut.

Figure 2. Loose smut on a wheat head..

Figure 3. Wheat grain contaminated with ergots.

Figure 4. Wheat heads infected by Fusarium head blight (left) and black chaff (right).

Figure 5. Wheat grain discolored at the embryo region by black point. (Photo credit: John E. Watkins)

This year there have been reports of wheat with stinking smut in several counties in southern Nebraska. Stinking smut, also known as common bunt, derives its name from a strong fishy odor associated with infected wheat kernels (Figure 1). It is caused by two closely related fungi whose spores contaminate grain during harvest.

Stinking smut is one of several seed transmitted fungal diseases of wheat. Other diseases include loose smut (Figure 2), ergot (Figure 3), Fusarium head blight or scab (Figure 4) and black point (Figure 5). Black chaff (Figure 4), a bacterial disease, is also seed transmitted.

Economic importance of seed transmitted diseases

Seed-transmitted diseases reduce yields as well as grain quality. Grain contaminated by stinking smut can be rejected at the elevator, often resulting in total loss. The pungent odor associated with contaminated grain causes livestock to reject it, significantly reducing its value as a feed. In addition, the compound responsible for the odor, trymethylamine, has potential to cause explosions in combines and elevators when present in appropriate concentrations.

Although loose smut does not affect grain quality, yield losses of up to 40% can result from the disease.

Incidence of ergot in wheat is relatively low; however, sclerotia or ergots (compact masses of the ergot fungus) can lower grain quality. In addition, the ergots contain toxic alkaloids that can cause ergotism and death in humans and livestock.

If seed heavily contaminated with Fusararium head blight fungi is sown, seedling emergence can be reduced by up to 80%. Significant yield losses also can result from floret sterility and poor seed filling. More importantly, the Fusarium species that cause head blight produce mycotoxins that can contaminate grain, causing it to be less palatable to livestock. If contaminated grain is used as feed or in food products, the mycotoxins can cause health problems in both livestock and humans.

Black point can significantly reduce the bread-making quality of wheat. Heavy contamination of grain with black point can result in discolored flour. Millers can reject grain on this basis.

Black chaff can cause up to 40% reduction in yield. Germination of seed that is severely infected by the black chaff bacterium can be significantly reduced.

Management of seed transmitted diseases

Three strategies can be used in combination to manage seed-transmitted diseases of wheat: seed treatments, use of clean seed and planting resistant varieties.

Using seed treatments

Seed treatments are important for several reasons.

1. They control seed transmitted pathogens that may be surface-borne on the seed or internally seed-borne. Additionally, they control soil-borne pathogens such as Pythium and Rhizoctonia that cause seedling damping off and root and crown rots. Systemic seed treatments also provide additional protection against fall season foliar diseases.
2. In addition to controlling diseases, fungicide-insecticide combination seed treatments also control insect pests such as wireworms, Hessian fly, and fall season aphids.
3. By controlling seedling damping off, seed treatments improve stand establishment and result in healthy, vigorous seedlings. Such seedlings build natural defenses that provide additional protection from disease and insect pests.

It is preferable to buy certified 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 before treating. Cleaning seed before treating is especially necessary if the seed transmitted diseases listed above were present in the field during the growing season. For the seed treatment to be effective, ensure thorough, uniform coverage. If possible, use a broad spectrum systemic fungicide or fungicide-insecticide combination product. For a list of seed treatment fungicides for control of seed transmitted diseases of wheat, see the UNL Extension NebFact, Wheat Disease Fact Sheet No. 6: Management Program to PreventSmut Diseases of Wheat (NF03-587). A more comprehensive list of seed treatment products for control of diseases and insect pests of wheat can be found at the Nebraska Department of Agriculture Pesticide Registrations Web site.

Fungicide seed treatments are not effective against ergot and black chaff. The best strategy for managing these diseases is using certified, pathogen-free seed.

For more information see the UNL Extension Circulcar, Diseases Affecting Grain and Seed Quality in Wheat (EC97-1874)

Planting resistant varieties

Resistant varieties can be used in combination with seed cleaning and seed treatments to more effectively manage seed transmitted diseases of wheat. The Wheat Varieties Virtual Tour Web site lists the wheat varieties grown in Nebraska and information on each variety, including disease resistance and susceptibility. It also contains information on how to select wheat varieties adapted to your area. Another useful resource is the Nebraska Crop Improvement Association's online Seed Book.

Using clean/certified, treated seed

The importance of using clean/certified, treated seed cannot be overemphasized. Clean/certified, treated seed optimizes the chances of obtaining high yields. Using farmer-saved or bin-run seed is likely to result in poor variety purity, a low germination percentage, poor stand establishment, and disease, insect, and weed problems. The result will be reduced yield and poor grain quality. Total loss may result if grain is heavily contaminated with fungal spores. Additional costs may be incurred in treating for diseases, insects, and weeds resulting from use of poor quality seed. In terms of dollars, the return from using clean/certified, treated seed will almost always exceed the cost of the seed. Using clean/certified, treated seed is a form of insurance that is definitely worth the cost.

Stephen Wegulo
Extension Plant Pathologist, Lincoln

Source: Crop Watch News Service
University of Nebraska-Lincoln Extension, Institute of Agriculture and Natural Resources
http://cropwatch.unl.edu/archives/2006/Crop18/wheat.htm