Description and Symptoms
Northern corn leaf spot (Bipolaris zeicola) is occasionally seen in the lower canopy during periods of high humidity and moderate temperatures. There are five known races of this fungus. Races 2 and 3 are the most common in the Midwest. Race 2 causes oblong, somewhat rectangular brown spots (1/4 to 1/2 by 1 inch) on leaves and a black mold on kernels. Race 3 causes long, narrow, light tan lesions (up to 1 inch) that are surrounded by a darker border on leaves, sheaths, and husks. Resistant hybrids and inbreds are available. Crop rotation and tillage reduce inoculum survival. Foliar fungicides labeled for northern corn leaf spot are available.
The best time to scout for northern leaf spot is VT through R4; eariler in seed production fields, more often in the lower canopy. May be more prevalent in fields with infected corn residue and when unusually high rainfall occurs in late summer.
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 Alison Robertson