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Posted 20 July 2009. PMN Crop News.

Gray Leaf Spot

Source: University of Illinois Press Release.

Urbana-Champaign, Illinois (July 13, 2009)--Producers are beginning to report some evidence of gray leaf spot in our area, and this is of little surprise since conditions have been ripe for its return.


Gray leaf spot is a fungal disease of corn that prefers warm, humid weather similar to the muggy conditions we have experienced as of late. The fungus overwinters on corn debris and is moved to the plant either through wind or rain, which means that the disease appears on the older or lower leaves first.

Once infected, the plant takes a little while to show symptoms of infection, but when they do show, one-half inch to two inch long pale rectangles that are about 1/8 to 1/2 inch wide form on the leaves. With time, these regions can grow and form large necrotic areas on the leaf tissue. Yield reductions are minimal if the leaf spotting does not reach above the ear leaf. However, yield losses can surpass a quarter on susceptible hybrids when spotting reaches above the ear leaf by milk stage, and can be double digit at early dough. Losses taper as the crop passes hard dough.

Scouting should begin now, if it has not already, and should continue until two weeks after tasseling. For future reference, U of I Extension recommends that producers keep careful track of weather conditions and start scouting the first week of July. Scouting earlier planted fields should already be underway.

Gray leaf spot is not an easy problem to solve. Resistant hybrids are the first step. Rotation also helps to eliminate some of the overwintering spore problems, and plowing, where soil conservation issues permit, will also aid the situation. If the problem begins too near the ear leaf, fungicide applications can help.

University of Illinois Extension recommends that producers pay particular attention to the lower three leaves of the corn plant from the period just before tasseling to two weeks after tasseling.

Matt Montgomery