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. 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.