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Corn tar spot reported in southeastern Minnesota

corn tar spot
By Dean Malvick, University of Minnesota

Tar spot of corn was detected in southeastern MN in southern Fillmore County on June 29. The black tar spots were found on low levels on corn in growth stages V9-V11 in an area where tar spot had developed in previous years.  Although early infection can lead to significant levels of disease later in the summer, that depends on the weather.  Tar spot requires wet conditions to develop and spread. Dry conditions slow or stop its development.  

Tar spot is a fungal disease of corn that primarily infects and damages leaves. Signs of the disease are small (@ 0.1”), raised, irregular-shaped black spots. The tar spots are firm, appear mostly smooth, and do not rub off or break open (see photos below). Tar spot can also produce fisheye symptoms that appear as tan lesions surrounding the black spots. Corn tar spot is caused by the fungus Phyllachora maydis.

Since tar spot was first found in Minnesota in Fall 2019, the disease has been spreading in Minnesota and nearby states ( Corn fields in southern and central MN should be scouted for tar spot from now to the end of the season. I had thought that we could wait until late July to start scouting based on previous years, but this year is different!  Over the next few weeks, where there is adequate rain or irrigation, tar spot is most likely to develop in areas where it developed in previous years. After that, in-season spread and rain could lead to it developing in many different areas in Minnesota.  

Tar spot can cause significant yield loss. Multiple different fungicides applied at growth stages VT-R2 have been shown to be effective for reducing tar spot (  Some corn hybrids also vary in susceptibility.  

If you see leaves that you suspect to be infected by tar spot, please contact me ( and send information and photos if possible; or submit your information to the Digital Crop Doc system ( I would like to receive samples from anywhere tar spot is found, and especially from counties where the disease has not yet been confirmed in Minnesota.  We are working to track the spread and development of tar spot in MN, and your samples and observations will help us with that goal.

The above and below photos show different levels of tar spot development and severity on corn leaves (photos by D. Malvick).

corn tar spot

Additional information for tar spot of corn:

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