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University of Minnesota Extension

Tar Spot in Corn


Tar Spot in Corn


By Katie Drewitz, University of Minnesota Extension 

PRESTON and CALEDONIA, Minn. (07/12/2022) — Tar spot is currently developing throughout Fillmore and Houston counties again this season. First identified in Fillmore County in 2019 and Houston County in 2020, tar spot is a corn disease that has spread dramatically since its first identification in the US in 2015. Given the relatively recent nature of this disease, there is still a lot that we do not know. However, given that potential yield losses can range from no losses to 50 bushels an acre, this is a corn disease to watch.

Tar spot is a fungal disease of corn and primarily infects and damages leaves. Symptoms include irregularly shaped black structures on leaf surfaces. The black structures are firm, appear mostly smooth on the surface and the spots do not rub off or break open. Tar spot can also produce fisheye symptoms that have black spots surrounded by tan lesions with dark borders. The easiest way to differentiate tar spot from insect frass or other diseases is to wet the area and then rub vigorously with your fingers. Tar spot should not rub off, whereas other diseases that mimic tar spot should come off onto your hand. 

Tar spot prefers relatively cool temperatures and humidity to develop and spread. If we begin to see hot dry weather, tar spot may slow or stop. You should scout fields for tar spot from now until harvest. Pay close attention to those areas that have had tar spot in the past. 

Tar spot can have a significant impact on yields. When applied at the correct time, rate and combination, fungicides can reduce this impact. Corn hybrids also vary in their susceptibility which will be something to consider when choosing next year's seed if you have fields with known tar spot populations. Both crop rotation and tillage only play a minor role in reducing the risk of tar spot in fields. However, with both the length of rotation and the degree of which tillage helps, it is still a relative unknown.

If you suspect you have a field with tar spot, please contact your local County Extension office or Dean Malvick at dmalvick@umn.edu. Residents in Fillmore and Houston counties can email wins0115@umn.edu or call 507-765-3896 or 507-725-5807 for assistance.

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