Wisconsin Corn Tar Spot Update

Damon Smith, Extension Field Crops Pathologist, Department of Plant Pathology, University of Wisconsin-Madison, Roger Schmidt, Nutrient and Pest Management Program, University of Wisconsin-Madison

Damon Smith has posted a new item, ‘Wisconsin Corn Tar Spot Update – July 29, 2020‘. Here is a short summary.

Tassels and silks have been out in the southern portion of the state. We are now in the tail-end of the window of opportunity for a fungicide application if you feel the risk for disease, including tar spot, is warranted. While tar spot is slow to develop, we have seen gray leaf spot (GLS) developing in the lower canopy and moving up. We continue to also scout for southern rust, but have had no confirmations of this disease so far in Wisconsin.

Figure 2 shows the calculated risk from Tarspotter* for July 29, 2020, for various locations in Wisconsin. The action threshold for high risk is 75% using the updated Tarspotter model for 2020. As you can see, the present risk has continued to decline overall for the entire state with the exception of the far Northwest, where there is not a history of tar spot. Continued dryer and warm weather is less conducive for the development of tar spot, thus we see the risk continuing to decline. So while we can find tar spot in handful of fields, progress of the disease is slow.

No new 2020 detects of tar spot in Wisconsin this week on the national tar spot map. Only a few more counties are being added in Iowa, Illinois, Indiana, and Michigan.

*Tarspotter, the smartphone prediction tool for tar spot, is currently available only to research beta testers.