Illinois Corn: Tar Spot – Conditions Unfavorable for Development

Tar spot symptoms on corn leaf. Photo: Purdue University

As many of you are aware, our tar spot coalition is working to refine a tar spot model developed by colleagues at the University of Wisconsin Madison this season.  This model, currently named Tarspotter, uses weather data to predict risk of tar spot disease on corn.

For more information on the model and fungicide applications click here

Tar spot risk map for 6/21/19

Much of the corn in Illinois is just emerging, but some is past the V6-V8 stage.  This is the point in time where paying attention to the weather, and future forecasts can help you determine if a fungicide application for disease management is needed.  Remember that there is an approximate 14 day latent period with tar spot, meaning that if your plants are infected today, you likely will not see the black tar spots on  for another 2 weeks.

Tar spot does not like the warm weather moving through much of the state, so I do not expect the values in the middle part of the state to change much, even with the rains forecasted over the next five days.

I have heard reports (unconfirmed) of common rust in parts of the state, and Southern rust appears to be starting up in Texas.   It is very important to be in your fields scouting for diseases prior to the VT stage to help you know if you need to pull the trigger on a fungicide this season.


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