Issue No. 7, Article 8/May 12, 2006
Foliar Wheat Disease: Is There Fungus Among Us?
Early-season virus diseases have been a concern in several areas of the state, but attention now should be turned to foliar disease. Reports of powdery mildew are widespread in wetter areas, and leaf rust is beginning to show up. Now is the time to begin scouting for fungal leaf diseases, including powdery mildew, leaf and stem rust, and Stagnospora leaf blotch.
With all fungal leaf blights, the most significant yield losses result from infections on the flag leaf and flag-minus-one leaves. Diseases may be present on the lower leaves without affecting yield. For future reference, remember that it is important to monitor weather conditions as plants approach flag-leaf emergence (growth stage 8). If it is warm and wet (60° and 75°F) and diseases appear to be spreading onto the upper leaves, a fungicide application may be warranted. Note that if the weather dries out, a fungicide is probably not needed, even with diseases on the lower leaves.
Powdery mildew: The fungus Erysiphe graminis causes powdery mildew of wheat. High humidity and temperatures between 60° and 75°F favor the development of powdery mildew. The disease starts as yellow chlorotic flecks on the leaves. The symptoms appear first on the lower leaves. The first diagnostic symptoms of powdery mildew are patches of whitish-gray, cottony growth on the leaf surfaces. If you turn over the leaf, you will see that tissue on the opposite side of the leaf becomes chlorotic. As the disease develops, small brown-to-black spherical fruiting structures (cleistothecia) develop in the cottony fungal growth. The fungus overwinters as cleistothecia on the straw of infested wheat and grasses. Dense growth and high nitrogen levels favor powdery mildew, so wider row spacing and a moderate balanced fertilizer program help reduce disease severity.
Stagnospora leaf blight: Stagnospora leaf blight from fall infections is present on the lower leaves in some fields. Look for small, tan, oval lesions on the lower leaves. When raindrops hit these mature lesions, spores are splashed to other parts of the plant or nearby plants in the dispersing raindrop. If rainy weather continues, the disease will continue to spread up the plant.
Leaf rust: Leaf rust, caused by the fungus Puccinia recondita f. sp. tritici, may be the most widely distributed wheat disease in the world. The development of rust-resistant wheat varieties has substantially reduced the yearly losses to rust. However, the appearance of new races able to infect previously resistant plants has allowed rust to remain a substantial threat to wheat. Some leaf rust occurs every year in Illinois, but variations in weather and wind patterns plus differences in the ability of the fungus to overwinter south of Illinois cause year-to-year variations in the level found in this state. Orange-colored spores, known as urediospores, are blown northward from Mexico and the southern United States to Illinois, where they settle on wheat plants and, if temperatures and moisture level favor disease development, begin to infect plants within 6 to 8 hours. A temperature range of 59° to 77°F and wet conditions (heavy dews, frequent light rains, or high humidity) are required for infection. A new generation of urediospores can be produced every 7 to 14 days.
Symptoms of leaf rust are small, round-to-oval, raised, orange-red, dusty pustules scattered primarily on the upper leaf surface or leaf sheath. Jim Donnelly, crops specialist with Ag View FS in Walnut, Illinois, reports leaf rust in Bureau County.
Close-up of a leaf rust pustule (courtesy of J. Donnelly).
Fungicides and thresholds: Fungicides are available to control fungal diseases, and the decision to use them should be based on yield potential and disease infection and weather. A rough way to determine yield potential is to count the number of tillers per square foot and multiply that by 1-1/2. Fungicide cost plus application will be approximately $16 to $20 an acre. Yield increases of seven to eight bushels per acre (at $3 per bushel) would be needed to break even. In this case, yield potentials should be greater than 60 to 70 bushels per acre for application to be cost effective. See Table 3 for Illinois fungicide recommendations.
Determining if wheat infected with powdery mildew, specifically, will need a fungicide application depends on the resistance of the variety planted as well. An Ohio State University bulletin, Wheat Disease Management, outlines this very well (see Table 4). You can also access this information online to help you determine probable yield loss.
To determine yield loss possibilities for other diseases, see (also from Ohio State University) a guideline for infection thresholds for fungicide applications based on number of lesions on the flag and flag-1 of our common diseases (Table 5). This information is available online to help with your application decisions.