Circular to elongate, golden brown to cinnamon brown pustules that turn brownish-black late in the season. Common rust pustules commonly form on both sides of the leaf and are more sparse than those of southern rust. Occurs wherever corn is grown. Moderate temperatures (60 to 77 degrees Fahrenheit) and high relative humidity (greater than 95 percent for at least 6 hours) favor infection. The disease is easily controlled by resistant hybrids. Fungicides are not recommended due to limited yield loss from this disease.
Taken from Corn Production Handbook, KSU Extension
Pest photo source
Diagnosing Corn Problems in Kansas, KSU Extension