Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu
Corn in many parts of the state is growing rapidly and tasseling or approaching tassel. Now is a good time to scout fields to see if there are any disease issues that may need attention. With recent wet weather diseases such as Gray leaf spot (GLS) and Northern corn leaf blight (NCLB) may flare up, particularly if you have a highly susceptible hybrid in your field. We have had reports of both diseases over the past week in grower fields. It is a little early for NCLB, but this year the disease has appeared earlier throughout many growing regions.
GLS and NCLB are similar in many aspects but can be distinguished in the field fairly easily (Figure 1; Table 1). Symptoms may be more pronounced in areas such as tree lines or bottomlands, where humidity levels may persist for longer periods of time. Hybrid selection plays a key role in management of these diseases (Figure 2) as does residue management and crop rotation. In some situations a fungicide application between VT and R3 may be profitable in high risk fields where these diseases are detected on the 2nd or 3rd leaf below the ear leaf at VT. Losses can occur when these diseases reach the ear leaf prior to grain fill and tend to be more problematic in late planted fields. If the diseases are restricted to the upper canopy or do not reach the ear leaf until 5-6 weeks post tassel, one can expect negligible yield loss.
Figure 1. Symptoms of Northern corn leaf blight (top) vs Gray leaf spot (bottom) on corn. Northern corn leaf blight lesions are grayish to olive in color and often pointed at the ends, giving the lesion a cigar or football shape. Lesions can range in size from 1-7 inches in length. Gray leaf spot lesions are rectangular and limited by the leaf veins. Old lesions turn gray over time.
Table 1. Similarities and differences between Gray leaf spot and Northern corn leaf blight.
| Gray Leaf Spot | Northern Corn Leaf Blight | |
| Source of disease | Corn residue | |
| Optimal temperature | 75-85°F | 64-80°F |
| Favorable environment | Prolonged wet weather, heavy morning dew, overcast | |
| Infection to symptom | ≈14 days+ | 7-10 days |
| Lesion shape | Rectangular | Oblong |
| Lesion length | 1-3 inches | 1-7 inches |
Figure 2. A side by side comparison of a hybrid with good resistance to Northern corn leaf blight (right) vs a susceptible hybrid (left). Selecting a hybrid with good resistance to Northern corn leaf blight and Gray leaf spot are essential, particularly in fields with high amounts of corn residue and a history of these diseases.