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Tar Spot of Corn Found in Minnesota in 2020

By Dean Malvick, Extension plant pathologist

Tar spot of corn was found last fall for the first time in Minnesota. In 2020, tar spot is again developing in corn fields in southern Minnesota. Tar spot was recently confirmed at low levels in two fields in southern Fillmore County. This disease was also found near those sites in 2019.

Now is a good time to scout for tar spot in fields in Minnesota to determine how far this disease has spread. The dry weather in many areas of Minnesota in late July and early August likely suppressed tar spot, but recent rains may have favored development of this disease. Although tar spot was found only in southeastern Minnesota in 2019, we do not know how far this disease spread and it may not be restricted to southeastern parts of the state.

Identifying tar spot

Tar spot of corn. Photo: Dean Malvick
Corn tar spot is caused by the fungus Phyllachora maydis. The tar spot fungus infects leaves (and sometimes husks) and produces raised, small (0.1” – 0.2”) raised black structures on the leaf surface. The black structures are firm, appear 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 appear as black spot surrounded by tan lesions with dark borders.

Tar spot of corn.
Tar spot can result in significant corn yield losses, depending on weather, severity, and timing of disease development. Risk of disease is highest where the disease has occurred previously. Thus, even if the levels of disease are low, we want to know where it has spread in part to help us understand risk in future years. 

If tar spot starts developing to high levels well before maturity, fungicides have been shown to be effective to manage it, although optimal timing of applications must to be determined.

Please contact us

Please contact me (Dean Malvick, University of Minnesota) through the Digital Crop Doc program (, where you can send photos of symptoms if you suspect that you have found tar spot of corn in Minnesota.

Additional information 

Background information on tar spot and disease management:

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