Gray leaf spot is now very common in southern and central Iowa corn fields. This disease has become the most prevalent foliar disease in southern Iowa, and last year it could be found throughout the state. It has had an impact on corn production over the past two years and is threatening again this year. We have seen symptoms earlier in central Iowa this year, compared to last year. This could mean greater losses, depending on the rate of disease development from now on.
Gray leaf spot symptoms.
Gray leaf spot can be recognized by the rectangular tan lesions that develop first on the lower leaves. They gradually move up the plant as the season progresses. Spores of the fungus, Cercospora zeae-maydis, are produced on corn residue, and they splash or are windblown to the leaves. It takes several weeks from the time of infection until the symptoms appear. Spores are then produced on the infected leaves.
On hybrid field corn, there are typically no chemical control recommendations. The disease can be reduced by crop rotation, tillage, and the use of moderately resistant hybrids. In some cases, it may be profitable to control gray leaf spot with a fungicide, but the most effective registered fungicide, Tilt, can not be sprayed after 50 percent silking. At this point, most fields are beyond this stage, and it is too late to spray Tilt. Now the most important consider-ation is yield loss. It is too early to estimate losses in general, but fields with gray leaf spot lesions on or near the ear leaf at silking are at risk for yield loss.
Fields should be scouted for gray leaf spot at least two weeks before tassel-ing if a fungicide application is being considered. We do not have a threshold established for spraying, but fields in SE Iowa that are corn on corn, reduced tillage, and planted to a susceptible hybrid are the best candidates for a fungicide application if the disease is present before or at tasseling.
Fields should be scouted a few weeks after silking to determine whether the disease will be severe enough to reduce yield, and to estimate the disease risk for next year. Fields that have gray leaf spot should not be planted to corn next year. If corn must be planted, a more resistant hybrid should be selected. Plowing or disking the infested residue also will reduce or eliminate the fungus from the field. However, even if there is no corn residue in a field, the disease can blow in from adjacent fields.
This article originally appeared on pages 147-148 of the IC-476(21) -- August 5, 1996 issue.