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Agronomic Crops Network

Ohio State University Extension

CFAES

Corn Fungicide Trials

Fungicide applications, along with hybrid selection, crop rotation, and in-season scouting, is part of an integrated management plan for corn diseases. Treatments were not applied to these plots, but put your scouting knowledge to the test and look for the following three foliar diseases. Plots feature either resistant or susceptible hybrids with or without tillage. 

Gray leaf spot: symptoms of GLS include tan, rectangular lesions up to 2-4 inches long that first appear on lower leaves. Lesions are bordered by leaf veins and can turn gray later in season. Infection requires dew or foggy conditions and high relative humidity in the canopy.

Figure 1. Gray leaf spot symptoms

Tar spot:  this relatively new foliar disease prefers wet weather and moderate temperatures and can progress rapidly under the right conditions. The main signs of tar spot are raised, black spots called stroma that cannot be rubbed off and appear on both upper and lower leaf surfaces. Learn more about how to differentiate tar spot from insect frass (poop) and other diseases by clicking here. In general, fungicides with multiple active ingredients (AI) are more effective against tar spot and should be applied between VT and R3 in a high disease environment (>5% severity).

Figure 2. Progression of tar spot from a few lesions (stromata) to leaf blighting.

Northern corn leaf blight:  keep an eye out for tan, “cigar-shaped” lesions that will expand up to six inches in length across leaf veins. NCLB is favored by high relative humidity and wet, but slightly cooler conditions than those that favor the development of GLS.

Figure 3. Northern corn leaf blight lesion.