A field study was conducted at two locations in southern Illinois from 2008 through 2010 to determine if changes in corn yield potential associated with insecticide seed treatments (IST) and foliar fungicides could modify the optimum economic fertilizer N requirement. Corn was treated with either Poncho 250 or 1250 IST in a factorial without and with Headline foliar fungicide in whole plots. Subplots consisted of nitrogen (N) rates of 0, 67, 134, 202 and 269 kg ha^-1. Increasing N rates tended to increase leaf diseases, but this effect was significantly reduced as were leaf diseases in general with the application of fungicide. When averaged across N rates and IST treatments and across 6 site-years, the fungicide application increased corn yield by 376 kg ha^-1, but at the economic optimum N rate (EONR) this difference was 627 kg ha^-1. IST had few effects on yields at either location, except the higher IST at BRC significantly reduced the EONR in 2008 and grain yield in 2010. It appears that high demand for N during grain fill tends to rob leaves and stalks of tissue N. This seems to create conditions of increased stalk rot at lower N rates, especially without the use of fungicides, but increased leaf diseases at higher N rates, again without the use of fungicides.