Many different types of diseases affect wheat. They can be classified as seed-borne diseases, leaf and head blight diseases, crown and root rot diseases, and virus diseases.
Seed-Borne Diseases: Most problems resulting from seed-borne diseases have been eliminated by highly effective seed-treatment fungicides. Several seed-borne diseases are of concern to wheat growers. They are seed-borne scab, seed-borne Stagonospora (previously known as Septoria), common bunt (stinking smut), and loose smut. Seed-borne scab and Stagonospora are discussed later in the section on leaf and head blight diseases. Both diseases result in lightweight, shriveled kernels that may be moldy. Producers should follow the recommendations listed below for proper seed cleaning and planting to reduce seedling blight and stand losses resulting from planting diseased seed.
The two smut diseases, stinking smut and loose smut, can be particularly devastating. Stinking smut causes losses by giving the seed of diseased plants a foul, fishy odor, making the grain unfit for milling. Producers have been docked severely when attempting to sell smutty grain. Loose smut affects plants and yield by converting the grain and parts of the head to smut spores. Therefore, infected plants have no grain left to harvest. There are no varieties resistant to stinking smut, and numerous races of the loose smut fungus exist.
Note the following guidelines to control seed-borne diseases and seedling blights:
Leaf- and Head- Blight Diseases: Major diseases in this group are powdery mildew, leaf rust, Septoria tritici leaf blotch, Stagonospora (Septoria) nodorum leaf and glume blotch, and Fusarium head scab. All can cause major yield losses, but their occurrence is essentially weather-dependent. Cool, rainy weather from mid-April through the flowering period of the wheat plant in late May to early June favors the development of most of these diseases. They require either the leaf surfaces to be wet for a certain period of time or the relative humidity within the plant canopy to be near 100 percent. Powdery mildew and Septoria tritici leaf blotch are the first leaf diseases to occur in the spring. Both are favored by cool, humid, or wet weather. Leaf rust and Stagonospora nodorum leaf blotch require slightly warmer weather, thus they follow in mid to late May. Stagonospora nodorum glume blotch and Fusarium head scab become evident in June soon after flowering, especially if wet weather persists through this time.
The use of resistant varieties is the major control procedure for these disease management, leaf, and head blight diseases. Few varieties are resistant to all of these diseases. Determine which diseases cause most consistent problems in your area, and choose a variety based on its level of resistance. The level of fungal carryover from one wheat crop to the next is minimal if a two-to three-year rotation away from wheat is maintained. Rotation is a primary control measure for powdery mildew and the Septoria and Stagonospora diseases. Spores of leaf rust are blown up from the southern states in late May; therefore, crop rotation has little effect on the incidence of leaf rust. Adequate and balanced fertility, to provide optimum nutrition for hardy plants, helps lessen the adverse effects of foliage diseases. High rates of nitrogen will favor powdery mildew and Stagonospora leaf and glume blotch. Head scab is usually more severe when wheat is planted after corn because the fungus causing scab is the same one that causes Gibberella stalk rot. When possible, wheat should follow soybean or other legumes in the cropping sequence.
Fungicides are available to control most foliar diseases of wheat. However, the use of these fungicides should be based on sound economic decisions and the level of disease in the field. Fungicides have been profitable during years when foliar diseases have been severe on susceptible cultivars or in locations where diseases are a persistent problem. Obtain a copy of Extension Bulletin 735, Wheat Disease Control in Ohio or contact your local county Extension office for up-to-date information on fungicide recommendations for wheat or consult the Ohio State University Extension Website Ohioline (http://ohioline.ag.ohio-state.edu).
Crown and Root Rot Diseases: Two of the more important crown- and root-rot diseases are take-all and Cephalosporium stripe. Both of these diseases are soilborne, meaning that the fungi that cause these diseases reside in the soil. Take-all can be recognized by examining the roots and lower stems of prematurely killed plants. The base of the stem and roots will have a scurfy, black appearance. Cephalosporium stripe disease is characterized by alternating yellow and brown stripes that extend the entire length of the leaf blades. Both of these diseases are favored by planting wheat year after year in the same field. Usually a rotation sequence with a break of two years or more between wheat crops will eliminate fungal carryover. The exception to this is when no-till is used to produce all crops in the rotation sequence and wheat residues do not decompose before the next wheat crop is planted. The other exception to this is when perennial grass weeds, such as quack grass, become established in the field. Proper rotations, tillage and elimination of grass weeds have been highly effective in managing both diseases. Additional control of the stripe disease can be achieved by maintaining a soil pH above 6.2 by proper liming according to a soil test. Adequate soil fertility will also reduce yield losses from root diseases.
Virus Diseases: Wheat spindle streak mosaic (yellow mosaic) and barley yellow dwarf are the two most common virus diseases. Wheat spindle streak mosaic is a soilborne disease that is usually recognized in early May as the stems of the wheat plant begin to elongate. The upper leaves of affected plants will show short, spindle-shaped, yellow streaks. These symptoms may intensify if weather remains cool. The symptoms will tend to disappear as the weather begins to warm. Control of wheat spindle streak is achieved through the use of resistant varieties. A number of highly-resistant varieties are available.
Barley yellow dwarf virus is transmitted by aphids. Aphids arriving from the southern states transmit the virus to the newly-planted wheat crop in the fall. Severely affected plants may be stunted, have reddish or yellowish leaf tips and produce no heads. Yield losses greater than 50 percent have occurred when entire fields have been infected in the fall. Because no varieties have an acceptable level of resistance and early-fall infections cause the greatest yield losses, wheat planting should be delayed until after the Hessian fly safe date when aphids have ended their fall flights.