Description and Symptoms
Northern corn leaf blight (NCLB) occurs often in Iowa. 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 gives lesions a “dirty” gray appearance on the surface. 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 the Ht gene for resistance to the fungus, lesions are smaller, chlorotic, and may develop into linear streaks. These lesions rarely produce spores.
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.
The best time to scout is V15 through R4; earlier in seed production fields. 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 the 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.
It is likely that the hybrids planted in the fields varied in susceptibility to NCLB, and those fields with more disease were planted to a susceptible hybrid. In corn-on-corn fields, NCLB was present in the lower and mid-canopy, while in corn-on-soybean fields, lesions usually occurred in the mid-canopy. This should not be surprising since the fungus that causes NCLB survives in corn residue.
On susceptible hybrids, a foliar fungicide application at tasseling/silking is likely necessary. Farmers and agronomists are advised to scout fields, especially all fields planted to NCLB-susceptible hybrids. If the disease is present on 50 percent of the plants in the field (one or more lesions per plant) at tasseling, a fungicide application may be necessary to protect yield. Severe NCLB development during grain fill can result in yield losses of 30 percent of more. Fields planted to more resistant hybrids should be scouted on a weekly basis to monitor disease development. A fungicide spray may not be necessary.
Photo by Daren Mueller