Photo taken on April 27, 2018 at dryland wheat field in Wichita Co (Photo courtesy of Dr. Emi Kimura)
As we received some rain last weekend, leaf rust started to show up on the flag leaf of already headed wheat. This field was about 4-5 weeks before harvesting depending on the weather, but the most fungicide (The list of fungicide available here) need to be applied prior to Feekes 10.5 (head emergence). Although flag leaf is very important and must be protected for maintaining grain yield and test weight, at this stage of wheat maturity, yield potential may already have been set and the fungicide application may not be economically justifiable.
Thank you to Dr. Ed Bynum for identifying the black dots you see in the photo. They are the Brown wheat mite (More about brown wheat mite). Preharvest interval for most miticides with chlorpyrifos is 28 days. Good hard rain can crash the mite numbers; however, if we see increasing numbers of mites on flag leaves, miticide application may be necessary.