Johnston, Iowa – In just seven years, corn tar spot disease in the United States has grown from a relatively minor disease with little economic impact to a major crop-stealing problem.
The disease was discovered more than a century ago in Mexico, but it wasn’t until 2015 that it was found in the US corn belt in Illinois and Indiana and has continued to spread to the states of Iowa, Wisconsin, Michigan, Ohio, Missouri, Minnesota, Pennsylvania, New York, Kentucky, Nebraska, and southern Ontario. .
In 2022, the tar slick has made a significant westward expansion, with its presence confirmed for the first time in several eastern Nebraska counties, as well as a few counties in northeastern Kansas.
The tar slick began to accumulate with a large-scale outbreak of the disease in 2018, when severely affected areas reported a 30% to 50% decrease in yields compared to yields in 2016 and 2017.
“The tar slick, though new to the Midwest, is going to be around for a while.”
– Mary Gomes, director of agricultural engineering, Pioneer
Tar spot reduced U.S. corn yields by about 231.3 million bushels in 2021 — more than any other disease, according to the Crop Protection Network, a collaborative effort between land-grant universities in the United States and Canada.
Production losses in the United States increased from 64 cents an acre in 2020 to $13.69 an acre in 2021, amounting to nearly $1.247 billion in crop losses, according to the latest Crop Protection Network statistics.
Tar spot is the physical manifestation of the round-shaped, tar-stained fruiting bodies of fungi, called ascomates, that develop on corn leaves.
Initial symptoms are small brown lesions that darken with age. Leaf texture becomes bumpy and uneven when fruiting bodies are present.
Under favorable conditions, tar spot spreads from the lowest leaves to the upper leaves, leaf sheaths, and eventually the scales of developing ears.
Severe infection can cause leaf necrosis. Affected ears can have low weight and soft kernels, and the kernels at the tip of the ear may germinate prematurely.
Kevin Fry, a leading field agronomist, provided tips on how to look for tar spot and identify the disease to help determine when to apply fungicides.
“As for scouting, it starts with looking in the canopy and with a sheet exposed to sunlight that’s well lit, looking for shadows on the underside. If you see a spot, for example, you wipe it with your finger and it doesn’t come off, you need to check further,” Fry said. .
“Peel off the paper in question, lay it on a flat surface and put a little water on the stain and let it soak for 10 to 15 seconds to see if it erases. If it does wipe, it won’t pass the tar stain test.”
“Vigilant scouting is essential. When corn begins to reach its reproductive stage, that’s when tarspot can appear, especially when wet and wet conditions persist.”
“The tar slick, though new to the Midwest, will be around for a while,” said Mary Gomes, director of Pioneer Agricultural Engineering.
Disease management begins with choosing the right hybrid.
“We have plenty of data and plots showing how different hybrids handled heavy tar slick pressure and your agronomist or sales professional can lead you to the best recommendations for your local area,” said Gomes.
“You want to choose a hybrid that shows tolerance to tar spot, as well as generally good health with leaf diseases and environmental conservation.”
She also recommended a plan to use a fungicide if tar slick is present in the area.
“If the weather has been favorable to infection for a long time or your stress is very severe, you may even want to plan on using fungicides, especially if you’re watering,” she said.
“Keep in touch with your agronomist as we learn more and more about this new disease and better ways to manage it.”