Reports of tar spot beginning to increase

Damon Smith and Roger Schmidt
UW Madison
The tar spot fungus produces raised, black fungal structures called
ascomata on the surface of corn leaves.

Last week brought our first detect of tar spot in Wisconsin for the 2020 field season. This week we are seeing more counties being added now in Iowa and Indiana, with some additional non-confirmed reports from southwest Wisconsin. Figure 1, show the location of the confirmed detections, which are all in the previously recorded range of the tar spot pathogen.

Using the calculated risk from Tarspotter (a smartphone prediction tool for tar spot) the action threshold for high risk is 75% using the updated Tarspotter model for 2020. 

Tar Spot risk for various locations in Wisconsin for July 14, 2020.

As you can see, the present risk is elevated in much of the southern and southwestern portion of the state, the areas where reports are coming in. This is due to the fact that the weather continues to be relatively wet and humid for the past 30 days with decent rainfall. Cooler conditions this week are also contributing to the rising risk.

Tar spot is favored by persistent temperatures between 60 and 70 F and high relative humidity averaging above 75% for a 30-day period, accompanied by extended periods of leaf wetness caused by dew, rain, or irrigation events. 

Tar spot risk for the southern and south-central zones of Wisconsin for July 14, 2020.


Tassels are starting to peek or are out in the southern portion of the state. We are now in the window of opportunity for a fungicide application if you feel the risk for disease, including tar spot, is warranted.

Do some scouting and check weather reports. If it is dry in your area and has been hot, then no disease may be present. You might be able to hold off on that fungicide application. If it has been humid and rainy and you have some disease present in the lower canopy, then a fungicide application might be warranted.