Late in the 2021 growing season, many areas in Iowa and southeast Minnesota developed Tar Spot in corn. This disease reduces maximum yield potential.

We have received many questions about the potential for this disease in 2022, today I’m going to provide an overview of Tar Spot.

Once the disease has reached your farm, future infections are possible as spores for Tar Spot overwinter in the soil and can emerge under the right conditions. Research has confirmed this disease is soil borne, not seed borne.

Tar Spot 3

Tar Spot is ranked from 1-5

Here’s what we have learned that can be useful in working to manage the disease:

  • Conditions that cause infection:
    • Infection generally occurs in June
    • 10 to 20 cloudy days preceding infection
    • Moderate temps 60 to 69 degrees Fahrenheit
    • Relative humidity above 75%
    • Seven consecutive hours of dew or free moisture on leaves multiple times in June growing period
  • What infection looks like:
    • 14- to 20-day latent period after infection before disease symptoms show
    • “Fisheye-like lesions” appear on surface of leaves with dark center or just dark, greasy looking lesions
    • Lesions or spots do not rub off and may smear
  • Scout areas that have had known Tar Spot infection
    • Focus on the ear leaf
    • Begin around V10 and continue through R5 growth stage
    • Look for lesions like in the photo or that you can find online
  • Apply high quality, multi-mode of action fungicide if needed
    • Apply at VT/R1 time frame; timing is critical
    • Watch fields through R5 and be prepared for a second fungicide application

Corn plants were ahead on grain fill when disease set in heavily in 2021. With our late planting dates in 2022, delayed kernel fill could result in more yield loss to the Tar Spot infection. It’s possible that more widespread later planting dates this season could shift the infection to later in the calendar. All we can do is watch vigilantly.

Latham’s Product Team is looking for signs of infestation and will share with you what we’re learning throughout the season. For additional tips on Tar Spot check out this article on The Field Position.