Here is an excellent article that discusses when to spray for fursarium head scab.  Sure does a better job of explaining it than I could.  

Please note the following comment from that article.  It goes to show how critical timing is for head scab control.

" In a normal year, that translates into about a 36-hour window for optimum timing; treating prior to or after that ideal window usually results in about a 10 to 20 percent loss in control for each day the window is missed."





https://www.pioneer.com/home/site/us/agronomy/library/fungicide-timing-wheat-scab/