Scouting should be done every four days or so. Disease thresholds for fungicide application have not been determined in Iowa wheat, but other states have established thresholds. 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. 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.