Wheat can be affected by a number of foliar diseases caused by fungi, including Septoria leaf blotch, powdery mildew, and tan spot. There also are 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 of the 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 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 year's crop.
But what about this year's crop? Powdery mildew is appearing in some fields; patches of cottony white or tan mycelium can be seen on the leaf surface. Rust can be expected soon. If foliar diseases develop this year, there is a possibility of controlling them with a fungicide. Because of the expense, however, it is not generally profitable to use fungicides routinely on wheat.
There are several criteria that must be evaluated to decide if a fungicide is warranted. First, the yield potential for the field must be high enough. Usually, 45-50 bu/acre yield potential is considered the minimum. Next, you should consider the susceptibility of your varieties. Susceptible varieties are obviously at a higher risk for disease losses and therefore better candidates for fungicide application. You should know something about susceptibility based on past experience, seed company representatives, or obtain this information from other states with greater wheat production, such as Kansas or Illinois.
Most importantly, the need for a fungicide can be evaluated by scouting your fields. Scouting should begin 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 it must be protected. Select five spots for every 50 acre block, and examine 20 to 30 tillers per spot. Prior to flag leaf emergence, examine the upper two leaves. After flag leaf emergence, examine the flag leaf. Scouting should be done every four 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 per leaf, five powdery mildew pustules per 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 four days. Consider rainfall forecasts as well as scouting information in your decision. High rainfall favors more severe diseases. Also, intervals between scouting can be longer if the weather is very dry.
The earliest effective fungicide application should occur at growth stage 8, or flag leaf emergence. There are several fungicides available. The most commonly used are propiconazole (Tilt), triadimefon (Bayleton), and mancozeb (Manzate 200, Penncozeb, Dithane). Benomyl (Benlate) (mixed with mancozeb) and thiabendazole (Mertect) also are 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. With the tank mix, a surfactant is recommended to increase effectiveness.
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 to be used) or during or immediately after head emergence (if the tank mix is used).