Figure 1. Scabby wheat with kernels infected by the scab (Fusarium head blight) fungus. Severe seedling blight can result if this grain is used as seed.	Figure 2. Bunt balls containing spore masses of the stinking smut fungus.

August 10, 2007

After a Year of Disease Problems

Exceptionally wet weather this year has favored development of many wheat diseases. Notable among them are seed-transmitted diseases. These diseases lower both yield and grain quality and can cause up to 100% economic loss, especially if contaminated grain is rejected at the elevator.

Seed-Transmitted Diseases

Figure 3. Loose smut on wheat heads.

Figure 4. Darkening of glumes caused by black chaff.

Figure 5. Ergots in wheat grain.

Seed-transmitted diseases of economic importance in wheat production include seedling blights caused by the scab (Fusarium head blight) fungus (Figure 1), common bunt or stinking smut (Figure 2), loose smut (Figure 3) and black chaff (Figure 4). The first three diseases are caused by fungi whereas black chaff is caused by a bacterium.

In addition, various other seed-transmitted diseases caused by fungi can cause varying degrees of economic loss. These include ergot (Figure 5), black point (Figure 6) and some foliar diseases such as tan spot and Septoria leaf and glume blotch.

Economic Importance

Grain contaminated by the scab fungus often is discounted at elevators because it contains deoxynivalenol (DON), a toxin produced by the fungus. Depending on the level of the toxin, grain can be rejected altogether. The toxin causes grain to be less palatable to livestock and can be harmful to both livestock and humans. If scabby grain (Figure 1) is used as seed, up to 80% reduction in seedling emergence can occur.

Stinking smut can cause grain to be graded as smutty, thus lowering its price or causing it to be rejected at the elevator. The pungent smell produced by the stinking smut fungus can be intolerable. The smell is due to an organic compound, trimethylamine. When present in appropriate concentrations this compound can cause explosions in combines during harvesting or in storage at the elevator.

Loose smut and black chaff each can reduce yield by up to 40%. Black chaff also can significantly reduce germination of infected seed.

Figure 6. Black point causes discoloration of the embryo region of wheat grain. (Photo by John E. Watkins)

Although incidence of ergot in wheat is relatively low, the ergots (sclerotia) produced by the fungus lower grain quality and contain toxic alkaloids that can cause death in humans and animals.

Contamination of grain by black point can result in discolored flour and can significantly lower the bread making quality of wheat. Heavy contamination can cause millers to reject grain.

The Importance of Treating Seed

Treating seed with fungicide before planting controls many seed-transmitted diseases caused by fungi. Soil-borne fungi such as Pythium and Rhizoctonia that cause damping off and root and crown rots also are effectively controlled by treating seed. Thus, establishing a good stand of healthy, vigorous seedlings can optimize the chance of obtaining high yields. If systemic fungicides are used to treat seed, additional protection from fall season foliar diseases is provided.

Black chaff and ergot are best managed by using certified, pathogen-free seed. Seed treatments are not effective against these two diseases.

Treating Seed

It is best to buy certified treated seed or use a commercial seed conditioner to clean and treat seed. Seed treated on-farm should be cleaned before treatment. Thorough coverage maximizes effectiveness of the seed treatment. For a list of seed treatment fungicides for control of residue-borne, soil-borne, and seed-borne diseases, see UNL NebGuide G1671, Management of Residue-Borne Diseases of Wheat.

Due to the many seed-transmitted diseases that occurred this year, it is highly recommended that certified, treated seed be planted for next season's crop. If farmer-saved or bin-run seed must be used, it should be cleaned and treated with fungicide before planting.

Stephen Wegulo
Extension Plant Pathologist