Keep an Eye Out for Tar Spot

Tar spot of corn

Tar spot on corn. Photo: Martin Chilvers, MSU.

By:  Stephanie Karhoff

As the season progresses, keep an eye for tar spot, a new corn disease caused by the fungus Phyllachora maydis. The pathogen originates from Mexico and Central America, but has made its way to the Buckeye State.

Tar spot of corn was first detected in 2015 in Indiana and Illinois, and was later observed in Florida, Iowa, Michigan, Ohio (2018), and Wisconsin. It thrives in wet, cool conditions. Since this is a newly emerging disease, it is important to be vigilant and scout your fields to track its spread and impact on corn yield.

Last season (2018) was the first year yield losses due to tar spot were evaluated in hybrid trials in Illinois, Indiana, Michigan, and Wisconsin.1 Across all trials and hybrid maturities, a 0.32-1.36 bushel per acre loss occurred per 1% increase in tar spot. Based on this, we would predict a field with 40-50% tar spot severity on the ear leaf by R5/6 would have a loss of 16.8-38.7 bushels per acre. In these trials, all hybrids experienced some level of tar spot.

When scouting, look for small, raised black spots called ascomata on leaves, leaf sheaths, and husks. These spots are often surrounded by a tan halo (see picture). It is easy to confuse these ascomata with saprophytes or insect frass. The key difference is that ascomata cannot be rubbed off the leaf surface with your fingers. If you suspect it is tar spot, call the Williams County OSU Extension Office at 419-636-5608.

You can find real-time tracking of tar spot occurrence in 2019 here


1Telenko, DEP, MI Chilvers, N Kleczewski, DL Smith, AM Bryne, P Devillez, and et al. 2019. How Tar Spot of Corn Impacted Hybrid Trials During the 2018 Midwest Epidemic. Crop Protection Network.

MI Chilvers. 2019. Corn Tar Spot Outlook for 2019. Michigan State University Extension.

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