University of Illinois

No. 14/June 27, 1997

Gray Leaf Spot

Gray leaf spot (GLS) is becoming increasingly more important with the continued adoption of reduced tillage. This pathogen is most common as a leaf-blighting fungus after tasseling, but it can appear earlier in the season if weather conditions are favorable. I attended a meeting of Midwest pathologists last week, and the potential early appearance of this pathogen was a major topic of conversation. With the change to much more favorable weather for leaf blights, producers may wish to begin scouting fields to anticipate if this disease is becoming a problem.

Gray leaf spot also overwinters in crop residues, with spores scattered by the wind. Lower leaves become infected first, and the disease then moves up the plant. If significant infections occur before tasseling to about 10 to 14 days after tasseling, then economic losses can be expected. The disease is always more severe when corn follows corn or if plantings are made in low-lying fields, such as along a river bottom, where humidity tends to be much higher for a longer period of time each day.

Sorting out the potential damage from gray leaf spot is not easy. A number of corn hybrids appear to possess some tolerance to this pathogen; and, although they appear to be severely diseased, they still yield quite well. However, others may become diseased and have true economic losses.

Control of gray leaf spot requires some advance planning as to hybrid selection and planting area, tillage system, and determination of whether or not to use a fungicide. This pathogen also requires scouting, particularly if you plan to use a fungicide. Tilt fungicide (Novartis) can provide excellent protection and yield improvement if the disease is found at or above the ear leaf before tasseling. It cannot be applied later than midsilk stage.

Now would be the time to begin devising a disease management plan for leaf blights. Although corn is still very young and probably not in any immediate danger, it is not too early to consider your options for the coming weeks, particularly if we continue to have this kind of weather pattern. It's much easier to have all options in place rather than to find out that leaf blights have already passed the ear leaf before tasseling and you now need to quickly determine a management plan.

H. Walker Kirby, Extension Plant Pathology, (217)333-8414