Northern corn leaf blight.
There are three different fungal diseases (southern corn leaf blight, northern corn leaf blight, and northern corn leaf spot) that are often collectively referred to as "Helminthosporium leaf blights."
Practically speaking, all three diseases have similar pattern of disease developmentare treated in the same way. However, there are differences among the three in terms of symptoms and in the availability of host-plant resistance. Proper identification of the different leaf symptoms will help the grower select the appropriate resistant hybrids in the future.
Southern corn leaf blight lesion symptoms range from minute specks to spots of 1/2 inch wide and 1-1/2 inches long. They are oblong, parallel-sided, and tan to grayish in color. A purplish to brown border may appear around the lesions, depending on the genetic background of the plant.
Northern corn leaf blight is recognized by long, elliptical lesions that are typically cigar-shaped. Lesions may be as large as 3/4 inch in width and 2 inches in length.
Northern corn leaf spot lesions vary depending on the race present. Race 1 lesions are tan, oval to circular with concentric zones, and are commonly 1/2 inch in width and 1 inch in length. Race 2 lesions are oblong, dark brown to blackish in color, 1/8 inch in width, and 1 inch in length. Race 3 lesions are the most common in the Corn Belt. These lesions are narrow and linear in shape, with lengths less than 1 inch and widths less than 1/8 inch. Lesion shape and size may vary with the genotype of the plant. Lesions are grayish tan and surrounded by a pigmented border.
The fungi overwinters in corn debris as spores and mycelium. Spores are spread by wind or splashing water to the lower leaves. Under favorable weather conditions, primary infection lesions produce more spores, and the disease progresses upward until, in severe cases, nearly all of the leaves are infected.
Crop rotation and clean tillage (for example, plowing) help reduce the risk of disease by reducing the amount of inoculum (spores) in the immediate area. Whenever feasible, grow resistant sweet corn hybrids. It is important to note that even resistant hybrids may show some flecking or small lesions but seldom result in economic loss. Where susceptible hybrids are used, it is important to control foliar diseases during the period from fourteen days before tasseling to twenty-one days after tasseling by applications of fungicides.