Are there brownish gray elongated lesions on your corn leaves? If you are not sure, have you seen any crop dusters flying in your neighborhood, spraying your neighbor’s cornfields? If you are still not sure, it may be time to pay a scouting visit to your corn to ensure it is healthy and working hard to produce a girthy ear.

One of the problems that may be appearing in your cornfield is the frequently found fungus known as gray leaf spot. Depending on your weather, the problem could extend from very light to very severe. While gray leaf spot appears periodically, it usually does not appear quite this early in the season, says Tamra Jackson, Plant Pathologist at the University of Nebraska. In her latest newsletter Jackson warns there are indications for “the potential for a severe gray leaf spot epidemic in 2009 if conducive conditions persist. Foliar fungicide applications will likely be required in many high risk fields.”

Gray leaf spot is caused by a fungus that lives over the winter in corn stalks and other infected crops and then appears as the proper climatic conditions develop. Its spores are produced in the crop residue and as rain hits the ground the spores splash up to the new plants and infect the lower leaves of a corn stalk. If humidity exceeds 90% within the corn canopy or if rain or irrigation water is present, the spores will germinate and infect the corn plant. Gray leaf spot does well in temperatures from 70 to 90 degrees Fahrenheit.

Once the spores have infected the plant, they produce lesions and those produce other spores that spread further on corn leaves and stalks with a 14-28 day reproduction cycle that will continue as long as environmental factors are conducive to its growth. Your concern is the fact that the lesions reduce the photosynthetic capacity of the corn leaf and that is translated into a reduction of grain, which Jackson says can approach 50%.

Corn hybrids carry varying degrees of resistance, and while some will be quite resistant to GLS, others can be devastated by the fungus. Genetic resistance fights the fungus by limiting lesion development on the leaves and retards its spread on the plant. If your hybrids are not highly resistant to gray leaf spot, your alternative would be the application of a fungicide that will retard the spread of the fungus. Much of your decision on whether to spray for gray leaf spot will depend on how far the lesions have spread. Jackson says that since the leaves above the ear contributed 70% to its development, those are more important than lower leaves. If the upper part of the canopy is infected at this early stage of crop development, then the decision is easier to make.
If you are about the pull the trigger on having your fields sprayed, ensure that you have gray leaf spot compared to other fungi that may be present, but may not damage a crop as severely. Spraying should be a priority for the fields with the greatest severity of infection, followed by the susceptibility of the corn hybrid. If your weather conditions have been conducive to fungal growth and reproduction, scout for problems that you may have, before you spend the $25-30 per acre for the fungicide and its application cost. At current prices of corn, that will cost 6-7 bushels.

Unfortunately, fungicides deteriorate within three weeks and lose their potency to protect against lesion development and spore reproduction. Since GLS is appearing so early in the season, there may be a necessity for more than one application of a fungicide for sufficient protection to get the crop through the grain fill stage of development.

For more information:
Ohio factsheet
Kansas State factsheet
Purdue factsheet
Illinois research report on fungicides

Summary:
Depending on weather conditions and susceptibility of hybrids, gray leaf spot could be taking a toll on corn yields. Since this is unusually early in the growing season, it gives more time for the upper leaves to be infected, which are the workhorses for putting grain on the ear. Scouting for GLS should be conducted to ensure that is the fungus that might be present, and to have a fungicide applied as needed.