Scout every 4 days or so. Disease thresholds have not been determined for fungicide application in Iowa wheat, but other states have established thresholds. 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. Tilt can be sprayed only at growth stage 8.