Wheat can be affected by a number of foliar diseases caused by fungi, including Septoria leaf blotch, powdery mildew, and tan spot. There are also three different rust fungi that can infect wheat. Leaf rust, Puccinia recondita, is the most common rust disease and has the most destructive potential of the foliar diseases.
Many decisions regarding wheat disease control are made prior to planting. Resistant varieties are available for effective control of rusts, powdery mildew, and Septoria leaf blotch. Crop rotation and tillage will reduce the risk of Septoria leaf blotch and tan spot. Planting date influences the opportunity for rust and powdery mildew infections that may occur in the fall. It is important to scout wheat fields for diseases this year, in order to make decisions about next years crop.
But what about this years crop? If foliar diseases develop this year, there is a possibility of controlling them with a fungicide. Because of the expense, however, routine use of fungicides on wheat usually is not profitable. Several criteria must be evaluated to decide if a fungicide is warranted. First, the yield potential for the field must be high enough. Usually, 45 to 50 bu/acre yield potential is considered the minimum. Next, consider the susceptibility of your varieties. Susceptible varieties are obviously at higher risk for disease losses and, therefore, are better candidates for fungicide application. You should know something about susceptibility based on past experience, on the advice of seed company representatives, or on information from other states with greater wheat production, such as Kansas or Illinois. Most importantly, you can evaluate the need for a fungicide by scouting your fields. Begin scouting just prior to flag leaf emergence, when the stems are rapidly elongating. The flag leaf is very important in providing carbohydrates to the developing grain, so protect it. Select five spots for every 50 acre block, and examine 20 to 30 tillers per spot. Prior to flag leaf emergence, examine the lowest green leaves on the plant. After flag leaf emergence, examine the upper 1 or 2 leaves. Scout every 4 days or so.
Disease thresholds have not been determined for fungicide application in Iowa wheat, but other states have established thresholds. Fungicide application is warranted if there is an average of one leaf rust pustule/leaf, 5 powdery mildew pustules/leaf, or 25 percent of leaves with one or more Septoria leaf blotch lesions. If the threshold is met in three of the five spots in the field, spraying is recommended. If the threshold is met at two or less of the spots in the field, repeat scouting in about 4 days. You should consider rainfall forecasts as well as scouting information in your decision. Heavy rainfall favors more severe diseases. Also, intervals between scouting can be longer if weather is very dry.
The earliest effective fungicide application should occur at growth stage 8, or flag leaf emergence. This typically occurs around the first week of May, but development is a little slow this year. Several fungicides are available. The most commonly used are propiconazole (Tilt), triadimefon (Bayleton), and mancozeb (Manzate 200, Penncozeb, Dithane). Benomyl (Benlate) (mixed with mancozeb) and thiabendazole (Mertect) are also labeled. Tilt can be sprayed only at growth stage 8. The window for spraying Tilt is short and early, and this limits its application substantially. Disease symptoms may not reach thresholds until after the window for Tilt application. In this case, a tank mix of triadimefon and mancozeb would be the most likely choice. This can be applied as early as flag leaf emergence, but also as late as 26 days prior to harvest. Tilt has a longer residual activity, and is usually a little cheaper than the tank mix.
More than one application may be required to fully control diseases in a wet year. If only one application will be made, it is best to do this at growth stage 8 (if Tilt is used), or during or immediately after head emergence (if the tank mix is used). If one application is the limit, begin scouting just after flag leaf emergence. With the tank mix, a surfactant is recommended to increase effectiveness.
My thanks to Donald Hershman, University of Kentucky, and Robert Bowden, Kansas State University, for information gleaned from their articles.