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Release Date: 11/03/2022

By: Dr. Marty Chilvers, Michigan State University

Tar spot was detected much later than usual this year. Although the first reports came in during mid-July, many counties still don’t have tar spot confirmations even though we are now approaching harvest. The drier than usual conditions limited the development of tar spot early in the season, which has slowed development of disease as there is not as much active inoculum as there has been the last few years.

To develop tar spot disease management tactics, field trials were established near Decatur, MI and East Lansing, MI, which include but are not limited to the following:

  • Fungicide timing – optimizing fungicide timing for tar spot management
  • Fungicide efficacy – comparison of foliar fungicide products for tar spot management
  • In-furrow fungicide efficacy – the potential role of in-furrow applications for disease management, including tar spot
  • Hybrid resistance to tar spot – trials were established with industry colleagues to assess the local lineup of hybrids for tar spot resistance
  • Effect of planting population on tar spot development – a trial in conjunction with Younsuk Dong to understand the mechanism of greater tar spot development at lower plant populations
  • Tar spot disease development dynamics – what is the rate of disease development and what are the environmental conditions that favor disease development
  • Spore trapping of tar spot fungus spores – to determine conditions that favor tar sot fungus development and subsequent disease

The project team is tracking disease development and spore release over the season. Although there has been little disease to track so far this season, this year’s data will be invaluable to refining risk models. Marty said this of the 2022 season, “every year is going to be different in terms of disease timing and epidemic development. Although 2022 was a low tar spot pressure year, understanding tar spot disease development and epidemiology, even in low pressure years, is critical to developing improved risk prediction models and in season management strategies for the future.”

Next steps: The project team has developed a real-time assay that is specific to tar spot. This assay can be used as a rapid diagnostic technique, and also as part of a method to examine spore trap samples. This technology will be shared with collaborators and used in future years to diagnose disease pressure more rapidly. The Chilvers lab has also sequenced and assembled a high quality genome sequence of the tar spot pathogen, Phyllachora maydis. The genome will be invaluable in better understanding fungal infection, plant resistance and pathogen biology. This research is a continuation of work completed in the last two years. Upon completion, data from both years will be analyzed for trends and the results will be published in the CMPM annual research report, which will be released in early 2023.

Figure 1: Tar spot distribution map as of Sep 20, 2022, which can be found at

Figure 2: PhD student Jill Check checking a weather station at our tar spot location

Figure 3. MSc student Peyton Phillips checking corn silage plots for tar spot

Figure 4: Corn seed prepared in seed plates for plot planting

Figure 5: Bill Widdicombe and Adam Byrne planting the 2022 corn trials