Northern corn leaf blight causes large (1/2-inch up to 6-inch) grayish-green to tan spots on corn leaves. Southern corn leaf blight causes small (1/2-inch by 1/4-inch) tan spots with reddish-brown borders on the leaves. Corn leaf blights occur in the eastern part of North America. The infection does not usually kill the plant, but weakens it, making it more susceptible to other diseases. Young plants are most susceptible. These blights are favored by warm, wet weather. The fungus overwinters on infected plant debris in the soil.
Plant resistant varieties, and purchase only certified disease-free seed. Rotate crops and clean up garden debris at the end of the season.
Photo courtesy of Keith Weller, USDA Agricultural Research Service, Bugwood.org