Much of Indiana has experienced mild and humid weather over the past few weeks, which has influenced disease development. Gray leaf spot levels have been at low to moderate levels in most areas, but northern corn leaf blight has developed rapidly across Indiana. The high humidity and heavy dews have contributed to disease development, and in some fields the disease is at concerning levels.
Although current corn economics may make some farmers hesitant to spray fungicides, it is important to scout fields and determine if northern corn leaf blight is present in susceptible hybrids in high-risk fields for disease development, such as those that are in minimum or no-till or continuous corn. Although there are not robust thresholds in place for how much disease needs to be present to justify a fungicide application, threshold considerations and fungicide recommendations are discussed in a previous article at http://extension.entm.purdue.edu/pestcrop/2014/issue13/index.html#foliar.
There are several races of the fungus that causes northern corn leaf blight, and race type can influence symptom expression. We have not examined what races are currently problematic in Indiana, but if you are seeing the disease in hybrids that are considered resistant to northern corn leaf blight, please let us know.
For more information on northern corn leaf blight, please read Purdue Extension bulletin BP-084-W: http://www.extension.purdue.edu/extmedia/BP/BP-84-W.pdf.