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. Tilt can be sprayed only at growth stage 8. The window for spraying Tilt is short and early, and this limits its application substantially.