Factors previously reported as having strong linear relationships with gray leaf spot severity (5) and which could be determined prior to planting were used as input variables. These included maize surface residue (SR), planting date (PD), and genotype resistance (GLSR). Genotype maturity (MAT) and previous crop (PC) were also used. Since gray leaf spot severity was shown to vary onsiderably among locations in Iowa planted with the same genotype and having similar cropping practices, and because hours of gray leaf spot-favorable weather conditions were observed to have a strong east to west variation across the state (5), longitude (LON) was also used as in input variable for model development.