JOHNSTON, Iowa — In just seven years, tar spot disease in corn has grown in the United States from a relatively minor disease with little economic effect to a major yield-robbing problem.
The disease was discovered more than a century ago in Mexico, but wasn’t found in the US Corn Belt until 2015 in Illinois and Indiana and continued to spread to Iowa, Wisconsin, Michigan, Ohio, Missouri, Minnesota, Pennsylvania, New York, Kentucky, Nebraska and southern Ontario.
In 2022, tar spot made a substantial expansion westward, with its presence confirmed for the first time in numerous eastern Nebraska counties, as well as a few counties in northeastern Kansas.
Tar spot began building up with widespread outbreaks in 2018, when severely impacted areas reported yield reductions of 30% to 50% compared to 2016 and 2017 yields.
“Tar spot, even though it’s new to the Midwest, is going to be around for a while.”
— Mary Gumz, agronomy manager, Pioneer
Tar spot slashed US corn yields by about 231.3 million bushels in 2021 — more than any other disease, according to the Crop Protection Network, a collaborative effort of US and Canadian land-grant universities.
US production losses soared from 64 cents an acre in 2020 to $13.69 per acre in 2021, totaling nearly $1.247 billion in crop losses, according to the latest Crop Protection Network statistics.
Tar spot is the physical manifestation of circular-shaped, tar colored fungal fruiting bodies, called ascomata, developing on corn leaves.
Initial symptoms are small brown lesions that darken with age. The texture of the leaf becomes bumpy and uneven when the fruiting bodies are present.
Under favorable conditions, tar spot spreads from the lowest leaves to the upper leaves, leaf sheaths and eventually the husks of the developing ears.
Severe infection can cause leaf necrosis. Affected ears can have reduced weight and loose kernels, and kernels at the ear tip may germinate prematurely.
Kevin Fry, Pioneer field agronomist, provided tips on how to look for tar spot and identifying disease to help determine the timing of fungicide applications.
“For scouting, it begins with looking up in the canopy and with a well-lit sunlight exposed leaf you’re looking for shadows on the underside. If you see a spot, for example, wipe it with your finger and it doesn’t come off, you need to do a little more examining,” Fry said.
“Take the leaf off in question, put it on a flat surface and put a little water on the spot and let it soak for about 10 to 15 seconds and see if it wipes off. If it wipes off, it did not pass the test for tar spot.
Vigilant scouting is necessary. When corn starts reaching reproductive stage, that’s when tar spot can set in, especially when wet and humid conditions persist.”
“Tar spot, even though it’s new to the Midwest, is going to be around for a while,” said Mary Gumz, Pioneer agronomy manager.
Managing the disease begins with selecting the right hybrid.
“We have a lot of data and plots showing how different hybrids have handled heavy tar spot pressure and your agronomist or sales professionals can lead you to the best recommendations for your local area,” Gumz said.
“You want to pick a hybrid that shows tolerance to tar spot, as well as having overall good foliar disease health and stay-green.”
She also recommended a plan to use a fungicide if tar spot is in the area.
“If the weather stays conducive for infection for a long time or you have very heavy pressure, you may even need to plan on two fungicide applications, especially if you’re irrigating,” she said.
“Keep in touch with your agronomist as we learn more and more about this new disease and better ways to manage it.”