Reports of gray leaf spot have been numerous this week, after apparently little activity in the previous weeks. The disease has been observed throughout southern Iowa and as far north as Grundy County. Although it is not yet severe in most fields, the warm, humid weather and frequent rain are ideal for the disease this year, and I expect that it will be worse than in the past two years.
In southwest Iowa, gray leaf spot has apparently become severe enough in some hybrid fields to warrant a Tilt application. In seed corn, Tilt applications have been common this year.
Fields should be monitored closely over the next few weeks. Although Tilt cannot be applied after silking, in seed fields it may be advisable to follow the Tilt with a mancozeb product after silking (preharvest interval of 40 days). In hybrid corn, we have not seen adequate benefits to post-silking mancozeb applications, so the best strategy will be to wait and see. Disease severity at the dough stage is the best predictor of yield loss, although losses are variable, depending on the hybrid and site conditions. If the disease has killed more than 10 percent of the ear leaf by the dough stage, yield loss will occur and a more resistant hybrid should be used the next time corn is planted in the field. Rotation and tillage options also should be considered in this situation. For more information on gray leaf spot management, see ISU Extension publication Corn Gray Leaf Spot (IPM 49). The cost for this publication is $0.50 and it can be ordered from any county extension office or from the ISU Extension Distribution Center  by calling 515-294-5247.
This article originally appeared on page 147 of the IC-480(19) -- July 20, 1998 issue.