Northern corn leaf blight of corn
Northern corn leaf blight (NCLB) is caused by the fungus Setosphaeria turcica. Symptoms usually appear first on the lower leaves. Leaf lesions are long (1 to 6 inches) and elliptical, gray-green at first but then turn pale gray or tan. Under moist conditions, dark gray spores are produced, usually on the lower leaf surface, which give lesions a "dirty" gray appearance. Entire leaves on severely blighted plants can die, so individual lesions are not visible. Lesions may occur on the outer husk of ears, but the kernels are not infected. On hybrids that contain an Ht gene for resistance to the fungus, lesions are smaller, chlorotic, and may develop into linear streaks. These lesions rarely produce spores. "Cigar-shaped" northern corn leaf blight lesions. Image: D. Mueller
Hybrids resistant to northern corn leaf blight have smaller and less developed lesions. Image: A. Robertson
Northern corn leaf blight disease cycle.
The fungus overwinters in corn residue. Spores are dispersed by wind and splashing water. Disease development is favored by extended periods (>6 hours) of leaf wetness (rain or dew) and moderate temperatures (64-81°F). There are at least four races of the fungus, with race 1 being the most predominant.
Resistant hybrids are available and should be grown when disease is a potential problem. There are two types of resistance to NCLB, monogenic (Ht genes - resistance that is controlled by one gene) and polygenic (resistance that is controlled by many genes). Hybrids with an Ht gene are susceptible to some races of the pathogen. Polygenic resistance provides resistance to all races, but the resistance is not as absolute as Ht resistance. Crop rotation and surface residue reduction through tillage can decrease inoculum. There are several foliar fungicides that are labeled for NCLB.