Scouting for and Managing Gray Leaf Spot

July 30, 2019

Gray leaf spot (GLS) is a fungal disease that can dramatically reduce corn yields without the proper protection. One of the best ways to help prevent a dramatic yield reduction from the disease is to plant a corn product with some genetic resistance to the disease. Channel® products within the 100 to 120 relative maturity (RM) group are screened and rated for GLS resistance by our pathology department to give our growers a starting point on how to manage GLS. Products rated 5 are considered to have average resistance to GLS in a year that has normal levels of disease pressure. Not all products offer the same level of disease protection and this should be taken into consideration when selecting products to plant, especially if fungicide application is not desired or feasible on those acres.

Conditions that favor GLS are temperatures from 75 to 85° F along with greater than 90% relative humidity.1 Foggy mornings with excessive dew favor GLS building out in the field. There are some tricks to scouting for GLS, which is generally difficult to gauge pressure due to its latency period. The latency period for GLS is about 2 weeks, which means one lesion may not seem like much disease is present (Figure 1), but in 2 to 3 weeks many lesions could be working upwards on the plant (Figure 2).

The goal of scouting for GLS is to be able to manage the disease with a fungicide and potentially get the timing of the application to be more precise. Go to the same spot in the field each time so that you can mark the lesion on the plant. Come back in a week and make another mark and then the following week to see the progression of the disease upwards in the plant canopy. Using some type of marking system can provide a better perspective on the spread of the disease. Our Seedsman can document disease spread in Seedsman 360, our scouting app, to share with you on your Climate FieldView™ platform account. Depending on the stage of the crop, keeping the disease below the ear leaf is critical. A well-timed Delaro® fungicide application per labeled recommendations can help manage GLS.

In years where conditions favor rapid disease development, being proactive in scouting and recognizing the disease early are critical for the best return on your fungicide investment. If the weather continues to favor disease development, it’s possible that even a corn product with good resistance could benefit from an application. If the conditions turn less favorable, such as hot and dry, then the disease is likely to slow down. This is where keeping track of disease progression could be a benefit in the years where disease development is more variable.


1. Wise, K. 2010. Gray leaf spot. Diseases of Corn. BP-56-W. Purdue University.

Web source verified 5/28/19.