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Release Date: 10/24/2023

Project leader: Marty Chilvers

Tar spot continues to be detected in Michigan corn fields despite lower than anticipated reports of disease over the last two seasons. In 2023, initial tar spot development was limited by drought conditions experienced early in the season. As a result, the majority of tar spot reports didn’t come through until August (picture 1). As those initial disease levels were so low it hampered disease development later in the season.

There are multiple management options available to farmers to control tar spot. Optimizing management decisions for this pathogen is critical to achieve adequate control and maintain profitability. The Corn Marketing Program of Michigan (CMPM) has funded research on tar spot since 2019. For 2023, this research takes the form of fungicide efficacy and application timing trials, rapid detection of tar spot spores, corn hybrid and inbred resistance screening, and impact of plant population and fertility on disease.

Foliar fungicide trials were established with commercial hybrids in a commercial field near Decatur and at the Plant Pathology Farm in East Lansing, MI to assess fungicide efficacy and timing of application. Trials were established to optimize fungicide timing, which is being assessed by tracking disease, spore sampling and monitoring of weather conditions. These trials are ongoing and disease ratings are currently taking place through the end of the season.

To better understand the factors driving tar spot spore release, a spore trap study was established at the Decatur site and the Plant Pathology farm comparing different spore trap styles (picture 2). Spore traps were also deployed across the state with help from Chris Creguer (Pioneer) in Kingston, MI (picture 3), Missy Bauer (B&M Crop Consulting) in Coldwater, MI and Monica Jean (MSUE) in Merrill, MI.

The research team has assisted Dr. Addie Thompson’s program by planting a corn diversity panel at the Decatur location to screen for tar spot resistance. Commercial hybrids and inbreds are also being screened for resistance. In addition, the project team is continuing efforts to better understand the effect of plant population on tar spot disease. Due to observations of higher disease incidence at lower planting populations, a series of sensors were established in the field at the Plant Pathology farm to monitor environmental conditions within the canopy.

A research article from the CMPM funded work is currently going through final edits on the effects of planting and nitrogen fertility on tar spot. The citation is:

Click the following link to access the abstract In case you missed any MSU Extension articles about tar spot this year, click on the links below to access the content.

  1. Tar spot confirmed in Monroe County: Should you be concerned? Michigan Farm News Aug 3, 2023
  2. Identifying corn tar spot management strategies. Morning Ag Clips. Mar 20, 2023.
  3. What Indiana Corn Growers Can Do Now to Get Ahead of Tar Spot in 2023. Interview with Hoosier Ag Today

Next steps: This research is a continuation of tar spot work from last year. Upon completion, data will be analyzed for trends and the results will be published in the CMPM annual research report, which will be released in early 2024.

Tar spot distribution map as of Sep 26, 2023, which can be found at

Comparison of five spore trap types at the MSU Plant Pathology farm.

Spore trap at a corn field in Kingston, MI.