Since much of the PA crop is into grain fill stages of corn development, foliar diseases start to become pretty important around now. If a crop was sprayed with a fungicide, you can expect the efficacy of that treatment to start breaking down about 2 to 3 weeks post application. Combined with current wet weather conditions, don't be surprised if corn that looked clean a week ago starts to show disease symptoms.
The typical diseases like gray leaf spot (Figure 1), and especially northern corn leaf blight (Figure 2), will be favored by increased humidity and our mild temperatures of late. I expect to see these both increase in our crops over the next few weeks. If you have late-planted corn that has not been sprayed, check its resistance to these diseases. A tassel spray may be warranted here if they don't carry good resistance and you are in a no-till and/or continuous cornfield.
Figure 2. Northern Corn Leaf Blight Photo credit: Alyssa Collins
Be on the lookout for two new diseases that have not yet been seen in PA this year: southern rust and bacterial leaf streak. Southern rust can look extremely similar to common rust, a disease we often see a bit of late in the growing season. Common rust is very rarely problematic because almost all hybrids carry good resistance to it. Southern rust, on the other hand, can significantly impact yield depending on when it arrives and the weather thereafter. Southern rust has not been detected in PA, but there are some suspicious samples in Ohio this week and recent weather systems may have brought spores in on their winds. See photos and learn more about southern rust at the Crop Protection Network.
Bacterial leaf streak is another new corn disease to watch for. This disease was seen in several states (Colorado, Illinois, Iowa, Kansas, Minnesota, Nebraska, Oklahoma, South Dakota, and Texas, but not PA) in 2016, and has been confirmed in many of the same areas this year. My colleague in Nebraska, Tamra Jackson-Ziems, reports that it seems to be "at least as severe and widespread as it was last year." The symptoms of this disease closely resemble grey leaf spot, but as the name implies, bacterial leaf streak is caused by a bacterium and fungicides would have no effect on it. We are still trying to learn about the spread, distribution, and possible control measures for this disease, so if you think you see it, please contact your extension educator for help in diagnosis. Visit the Crop Protection Network for pictures and much more info.
Keywords: disease, fungicide, corn, gray leaf spot, northern corn leaf blight, southern rust, bacterial leaf spot