Will It Be Year of Gray Leaf Spot?

This fungus seems to be the hot one so far.

Published on: Jul 19, 2010

Scouting corn is always a good idea. It's especially true this year since June and July have featured warm, wet weather across a good portion of the Corn Belt. And if you irrigate and create these conditions, it's also a good reason to scout.

Reports from the central Corn Belt indicate gray leaf spot lesions are already prevalent in many situations. They've showed up on corn brought to 40-H judging contests. Usually, 4-H'ers know to pick stalks without disease lesions. While most can find some stalks with little or no lesions, hardly no one has found three stalks where three are required that didn't have at least a few lesions on a leaf or two. Some have been speckled with the characteristic, rectangular, brown lesion. 

Dave Nanda, a crops consultant, Indianapolis, Ind., has been in the field and observed the same thing. There are also striking difference sin hybrids as to how much infection has developed so far, he notes. That relates back to susceptibility to the disease. Your seedsman can tell you which hybrids are most susceptible. Not all seed companies use the same rating scale. So a number for disease resistance for a particular hybrid doesn't mean anything unless you know the scale that the company is using.

Many fields have been sprayed with fungicides. In many cases it was probably justified. In some, farmers may have given in to the pressure of circumstances, such as having an aerial applicator in the area, and sprayed without checking. At $30 or more per acre to have fungicide applied, or 10 bushels of corn at $e3 per bushel, scouting can pay off if it turns out you don't need it.

If gray leaf spot is at ear leaf or above and the hybrid is still pollinating, that field is a good candidate for spraying, especially if the hybrid is relatively or very susceptible to gray leaf spot. There should still be an opportunity to have fields sprayed in most locations.

Another factor is at work now, however, Nanda says. If the corn has already pollinated and silks are totally brown, spraying now may do little good, especially if the gray leaf spot lesions are still below the ear leaf. The disease may continue to develop, but once the ear has formed, yield effects should be minimal. It could predispose the field to stalk rot later in the fall, Nanda concludes.