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Tar spot of corn in Minnesota: 2021 Edition

By Dean Malvick, Extension plant pathologist

Figure 1. Minnesota counties where corn tar spot has
been confirmed.
Tar spot is a fungal disease of corn that primarily infects and damages leaves. It produces small black spots on leaves and husks. Since tar spot was first found in Minnesota in 2019, it has been spreading in Minnesota and nearby states ( Tar spot has been confirmed in southeastern MN again this season, and is likely in many fields in that part of the state (Figure 1). Now and until the end of the season is a good time to scout for tar spot to determine how far this disease has spread in Minnesota.

Tar spot can cause significant yield loss. Although it has reached high levels of severity in some fields in Minnesota, there has been been little evidence of yield loss here. Once again, this year, dry weather in many areas is likely suppressing spread and development of tar spot. However, there has been adequate rain in parts of southern MN, and rain in the forecast along with longer nights and cooler weather may favor tar spot development other areas. We do not know how far this disease has spread in MN and it may not be restricted to southeastern parts of the state.

Symptoms of tar spot

Corn tar spot is caused by the fungus Phyllachora maydis. The tar spot fungus infects leaves (and sometimes husks) and produces small (0.1” – 0.2”) raised, irregular-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 (as do rust pustules). Tar spot can also produce fisheye symptoms that have black spots surrounded by tan lesions with dark borders.

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

Do you suspect tar spot?   

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 also like to receive infected leaf samples from anywhere tar spot is found in Minnesota, and especially from counties where it has not yet been confirmed. We are working to track spread and development of tar spot in MN, and your samples and observations will help us with that goal.

For more information

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