There have been a number of reports of wheat fields in which flag leaves are becoming brown and developing symptoms of tip dieback. In some cases this brown discoloration is very obvious from a distance giving the field a scorched appearance.
Some growers were wondering if these symptoms might be caused by chemical injury from recent fungicide applications. While this is possible, I do not believe the evidence supports this conclusion, because the symptoms are appearing in both fungicide treated and untreated fields.
I think that these symptoms are being caused by bacterial leaf blight. This disease is not well described, but is associated with bacterial infection of the leaves following periods of warm humid weather. In my experience, the symptoms of bacterial leaf blight often are most severe along the margins of a field following intense thunderstorms that bring high winds that pick up sand and other debris from roadways. This wind-driven sand creates small wounds that allow the bacteria to enter the plants and stimulate the bacterial infections. The disease is often most intense in areas of the field that lodge during these storms simply because the canopy in those areas remains wet for extended periods of time, further favoring the disease.
Obviously, losing leaf area at this point in the grain development is not a good thing; however, there are no official estimates for potential yield loss caused by bacterial leaf blight. There are no treatment options for the disease.