Introduced in 1765, soybean is the second largest crop in North America and is grown over a diverse agroecosystem that covers a geographic area of 2,300 km in north-south (N-S) and 2,400 km in east-west (E-W) directions (latitude 28°33 to 48°33′N, longitude 74°W to 103°02′W). The diversity and the distribution ranges of soybean fungal diseases in this agroecosystem were examined and several geographic patterns were identified. Disease diversity measured by the number of diseases per latitudinal breadth had a gradient that decreased as latitude increased. Diseases with ranges centered at higher latitudes had greater geographic ranges in the N-S direction. Disease diversity measured by number of diseases per longitudinal breadth decreased from E-W. Significant linear relationships between disease diversity and planting area were found. A region centered around the Delta Area (approximately 36 to 39°N and 88 to 91°W), which is a transition area of three rainfall regime regions of the continent and is an area of confluence of the Mississippi, Missouri, and Ohio rivers, had the highest disease diversity in this agroecosystem.