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Northern Illinois Agriculture

University of Illinois Extension
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Will Tar spot overwinter in northern Illinois?

Posted by Russel Higgins - Weather

In earlier posts we shared that Tar spot, specifically caused by Phyllachora maydis, had been confirmed in Illinois. We are now trying to determine if the fungus will survive our winter months. A trial was established in cooperation with Dr. Santiago Mideros, University of Illinois plant pathologist. Corn leaves exhibiting Tar spot symptoms were collected and placed in mesh bags. Samples will be left both on the soil surface and buried to emulate corn residue incorporated by tillage. Next spring the samples will be collected and the survivability of the pathogen determined.

Despite the November date, our recent mild weather pattern has prevented our soil temperatures from dropping below 50 degrees. The November 4th Soil temperature measured under bare soil at the NIARC was 54.5 degrees. Current University of Illinois recommendations encourage fall applications of anhydrous ammonia to take place after the soil temperature at 4 inches reaches 50 degrees and is falling. Illinois soil temperatures can be accessed online at

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