An application of fungicide to corn has become a popular input with many farmers in Iowa. The effect of fungicide on corn yield, however, can vary from year to year. Environmental conditions, such as rainfall and temperature, likely are the main factors for differences in how a fungicide affects corn yield because these factors influence disease development and crop growth. Because environmental conditions vary from one year to the next, it is difficult to predict how and when to use a fungicide. Compilation of trial data over many years could help identify factors associated with fungicide response in corn.