Be Aware of This Corn Disease in 2022

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Man in examining corn leaves

At this point in the season, the thought of crop disease might be far from your mind. However, if you’re not careful, diseases can sneak up and take a bite out of your corn or soybean yield at the end of the season.

In 2021, one disease was particularly tough on Midwest corn farmers: tar spot. Tar spot is capable of causing yield loss of up to 50 bushels per acre, and experts say it’s liable to make trouble again this year. So here’s what you need to know to protect your corn yield in 2022.

What Is Tar Spot?

Tar spot is a relatively new disease to the Midwest and has quickly become an agronomic and economic concern for corn farmers. It is not an entirely new disease, however, having been first identified in Mexico in 1904. Since then, tar spot has been limited to high elevations in cool, humid areas of Latin America until spreading to the southern United States and, eventually, farther north.

When tar spot was first found in the Midwest in 2015, it was thought to be a minor cosmetic infection. In 2018, farmers realized it was much more serious when a widespread outbreak led to significant yield losses. Last year, tar spot hit the Corn Belt hard again, catching farmers by surprise in states such as Illinois, Indiana, Iowa, Michigan and Wisconsin. 

Will Tubbs is a market development specialist for Corteva Agriscience, based in Iowa. He says: “Factors such as hybrid tolerance, previous tar spot infection, timing of infection, environmental conditions, overall plant health and fungicide applications can all factor in together to impact yield losses in corn.”

Tubbs says farmers in all Midwest states should keep an eye out for it going forward.

“Tar spot is likely here to stay in the Corn Belt, but it is difficult to predict how it will impact our growing season. It is a new disease that university and industry researchers are still learning more about each year,” Tubbs says. “Many experts predict that its footprint will likely expand in the future due to its ability to reproduce and disperse so rapidly. Tar spot is likely going to appear on some level each year for the foreseeable future, but it will be difficult to predict how severe infection will be until symptoms appear in our fields.”

Watching for Tar Spot

Tubbs says tar spot manifests as small, raised stromata on plant leaves. The stromata look like small black spots that turn into black ovular or circular lesions. These symptoms will start on the lowest leaves, spreading upward to upper leaves, leaf sheathes and the husks of developing ears. Eventually, the disease can cause premature plant death.

When it comes to conditions for development, Tubbs says tar spot favors the following:

  • Cool temperatures, about 60 to 70 F
  • High relative humidity (>75%)
  • Frequent cloudy days
  • Seven-plus hours of dew at night

“Rainy conditions and high humidity levels release the spores from stromata to then be dispersed by wind or other environmental transport methods,” Tubbs says. “Fields with a history of tar spot have been known to allow the pathogen to overwinter on surface residue after harvest.”

Preventing Tar Spot

Knowing the impact tar spot can have on corn yield, Tubbs says it’s important to not only watch for the disease but also take steps to prevent it. 

“Select hybrids with the best genetic tolerances establish a healthy stand at planting to ensure optimum plant health, scout fields for disease presence and manage crop residue after harvest,” Tubbs says. “When it comes to scouting, I suggest starting early in the season and checking fields on a weekly to bi-weekly basis. I also suggest looking for symptoms on lower leaves first, since the disease manifests at the bottom of the plant and spreads upward from there.” 

Tubbs also recommends a timely fungicide application to prevent tar spot. Aproach® Prima fungicide has two powerful modes of action that can provide preventive and curative action against the disease. Even if tar spot hasn’t hit your state or area yet, he says it’s a good idea to take precautions sooner rather than later.

Corteva Agriscience market development specialist Jason Gibson discusses tar spot and white mold in soybeans and how to control them:

Tar spot isn’t the only disease you’ll want to watch for this season. Download the Corn and Soybean Disease ID Guide from Corteva to see how you can protect your fields and yield in 2022.

Aproach® Prima is not registered for sale or use in all states. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your state. Always read and follow label directions.