Channel Field Check Up Series: Field evaluations critical to identify tar spot

A new corn pest pathogen may be floating in the wind toward your farm. A phenomenon in the United States since 2015, tar spot has appeared in some isolated cornfields and is concerning because the disease can reduce yield potential.

In 2018, the impact on yield by the tar spot pathogen was realized. That season, cool, wet weather and high humidity along with extended periods of leaf wetness persisted in the Corn Belt and were favorable to tar spot.

Since the discovery of tar spot in Illinois and Iowa, it has been found in Michigan, southern Wisconsin, Ohio, Pennsylvania and Florida. Although the disease has moved slowly, tar spot can overwinter on corn residue.

Tar spot affects yield by reducing the amount of leaf area available for photosynthesis and leads to the premature death of leaf tissue. To compensate, the corn plant may cannibalize resources during grain fill, producing shallower kernels and making the plant vulnerable to stalk rots.

Regardless of prior disease presence in Illinois, field evaluations are critical. Appropriate timing for fungicide application could depend upon disease onset, severity and incidence. With several fungicides now labeled for tar spot, early identification could improve our ability to manage the pathogen during the growing season.

Managing crop stress can help reduce the appearance of diseases like tar spot. Your Channel Seedsman can help you determine the correct inputs to support your Channel corn products, including planting at the right population, managing for balanced soil nutrients, and applications of fertilizer and fungicide.

On corn plants, tar spot often appears as small, raised, irregular-shaped black lesions scattered across the surface of leaves. Tar spot symptoms can look similar to other pathogens like common and southern rust, although leaf lesions cannot be rubbed off and may be surrounded by a tan halo.

For now, tar spot remains somewhat of a mystery as researchers work to determine how the disease moves and what options farmers have for controlling infection. Visit the Channel.com Agronomy Library for articles about managing corn diseases like tar spot.