Wheat plants are slow to get started this year, but now is the time to start thinking about controlling leaf diseases if necessary. Conditions have been wet enough to promote significant leaf disease development.
Wheat can be affected by a number of foliar diseases caused by fungi including Septoria leaf blotch, powdery mildew, and tan spot. Three different rust fungi also can infect wheat. Leaf rust, Puccinia recondita, is the most common rust disease and has the most destructive potential of the foliar diseases.
Powdery mildew on wheat leaves.
Leaf rust pustules.
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 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.
These practices can reduce the risk of future disease development, but what about the current crop? Powdery mildew is usually the first to appear and it can be found now; patches of cottony white or tan mycelium can be seen on the surface of the lower leaves. Rust can be expected to follow soon after.
If foliar diseases develop this year, it is possible to control them with a fungicide. Because of the expense, however, it is generally not profitable to use fungicides routinely on wheat. There are several criteria that must be evaluated to decide if a fungicide is warranted.
Yield potential and price. Usually, a 45-50 bu./acre yield potential is considered the minimum for profitable fungicide use. However, when wheat prices are high, the likelihood of profitable fungicide use increases.
Susceptibility. Susceptible varieties obviously are at a higher risk for disease losses and, therefore, are better candidates for fungicide application. You should know something about susceptibility based on past experience, seed company representatives, or from university publications. The 1996 Iowa State University Wheat Yield Test Report has ratings for powdery mildew and leaf rust. This type of information also is available from other states with greater wheat production, such as Kansas. Look for this information on ISU Extension's Plant Pathology home page on the World Wide Web at: http://www.exnet.iastate.edu/Pages/plantpath/
Disease severity. Scouting should begin just prior to flag leaf emergence, when the stems are rapidly elongating. Usually this means early May. 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 20-30 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.
Thresholds. Disease thresholds for fungicide application have not been determined in Iowa wheat, but other states have established thresholds. Fungicide application is warranted if there is an average of one leaf rust pustule/leaf, five powdery mildew pustules/leaf, or 25 percent of the leaves with one or more Septoria leaf blotch lesions. If the threshold is met at three of the five spots in the field, spraying is recommended. If the threshold is met at two or fewer 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 weather is very dry. The earliest effective fungicide application should occur at growth stage 8, or flag leaf emergence.
Fungicides. 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 until 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).
For more information, check your county extension office for a copy of the 1995 Integrated Crop Management Conference Proceedings. See the article, "Integrated Wheat Disease Management," by Pat Lipps at Ohio State University.
This article originally appeared on pages 60-61 of the IC-478 (8) -- May 12, 1997 issue.