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Northern corn leaf blight 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.
For more information on northern corn leaf blight, see the Crop Protection Network article here.
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