For the past two years, it has been the leading yield-reducing disease in corn, taking more than 230 bushels in 20211 and 116 million bushels in 2022.2
It’s clear from the current county-level tar spot map at the Regional IPM Center’s tracking site that anticipating and dealing with the disease must remain an ongoing priority for Midwest farmers. Despite the hot and dry conditions across the Midwest the last two seasons, the disease footprint continues to spread. If the next season is moist, growers should be prepared for infections to surge.
Tar spot infections in corn can damage:
- Leaf tissue, which impedes photosynthesis, potentially causes poor grain fill, aborted kernels, and decreased yield.
- Stalk integrity, causing late-in-season lodging and hampering harvest.
Scouting for and identifying tar spot
Tar spot is named for the black specks it causes to appear on corn leaves, which look like small flecks of tar. These black lesions produce spores that spread the infection. Tar spot is a “spore-shooter,” meaning that it not only starts near the soil, but also launches spores into the air that can drift on the wind and infect plants from the top down.
Usually, a tan area of dead leaf tissue will spread outward from the black spot, creating a “fisheye” lesion. Tar spot lesions can resemble other conditions, such as the masses caused by common or southern rust, but while rust spores or other similar-appearing spots may be rubbed away, black tar spot lesions cannot be rubbed or scraped off leaves.
Suspected tar spot-infected corn leaves in Edgar County, IL (Left), showing late-stage infection; and McLean County, IL, showing early-stage infection (Right).
Timing and treatment for tar spot
Tar spot favors cool temperatures (60–70 °F), high humidity, and wet field conditions. If leaves remain wet for a period of about 7 hours, especially in cooler nighttime temperatures, infections can take hold. Tar spot won’t be visible to the naked eye until about two to three weeks (14–20 days) after infection, so if you learn about an infection in a neighboring field, treat immediately. Another challenge in dealing effectively with tar spot is that it can infect corn at different stages. While infection will most often show up between corn silking and R6, scouting should begin as early as V8 or V10.
It’s crucial that fungicide applications are completed before infection can set in. Keep scouting throughout the season, since awareness of infection in your fields, even if it’s too late to treat, can help inform your decision-making about next year’s crop.
Fungicide application timing is typically recommended between tasseling (VT) and blister stage (R2). It has been reported that little benefit is offered by applications in later stages3, but Ohio University Extension suggests that if you didn’t spray at R1 and the forecast is favorable for tar spot (several days of rain and wetness), a fungicide application at milk stage (R3) may be beneficial, especially with a susceptible hybrid.4
BASF internal photograph/Featuring a comparison between successfully treated corn infected with tar spot and untreated cornfields.
Improving your tar spot results this year
Set your fungicide spraying plan. Talk with your retailer about options or speak with your BASF representative for recommendations about spray timing and treatments that are proven effective in the control of tar spot, such as Veltyma® Fungicide.
The best way to protect your crop from multiple stressors is to implement a planned, preventative fungicide application. Utilizing Veltyma® fungicide not only helps stops disease, but also increases growth efficiency and environmental stress tolerance to protect your hard work and help maximize yields.
Always read and follow label directions. Veltyma is a registered trademarks of BASF. Copyright 2023 BASF Corporation. All rights reserved.
1. “Corn Disease Loss Estimates From the United States and Ontario, Canada — 2021,” Crop Protection Network, 2022. https://cropprotectionnetwork.org/publications/corn-disease-loss-estimates-from-the-united-states-and-ontario-canada-2021
2. “Corn Disease Loss Estimates From the United States and Ontario, Canada — 2022,” Crop Protection Network, 2023. https://cropprotectionnetwork.org/publications/corn-disease-loss-estimates-from-the-united-states-and-ontario-canada-2022
3. “Should I apply a late season fungicide for tar spot?” Iowa State University Extension and Outreach, 2022. https://crops.extension.iastate.edu/blog/alison-robertson/should-i-apply-late-season-fungicide-tar-spot
4. “More on Tar Spot: Mid to Late R-Stage Fungicide Application,” Ohio State University Extension Agronomic Crop Network. https://agcrops.osu.edu/newsletter/corn-newsletter/2022-27/more-tar-spot-mid-late-r-stage-fungicide-application
