Agronomy Talk

Agronomy Talk: Tar Spot in Corn

Published on Tuesday, October 23, 2018

Click here for a downloadable version of this Agronomy Talk update

Tar Spot is a new phenomenon in the US. It is caused by a fungus called Phyllachora maydis, native to Central America. Tar Spot had only been identified in very isolated geographies in the U.S. until the summer of 2018. In Central America, the yield-robbing form of Tar Spot forms a complex with two other plant pathogens, neither of which have been documented in the U.S. It is unknown whether the Tar Spot organism is forming a pathogenic complex with other species present in the Midwest. 

This disease is so new that pathologists are still working to identify all of the species involved and mechanism by which Tar Spot is spreading. The current theory is that spores are moved on weather events. It is not known how, or if, the disease overwinters in the Midwest, though the impacted geography has spread considerably from the general area where it was first identified in the U.S. in 2015.

Field Conditions:

Most of what we know about Tar Spot comes from its native Mexico. Cooler weather (between 50-70° F), high humidity and prolonged periods with wet leaves seem to be good conditions for the disease to form. 


  • Small raised black lesions on the leaf that cannot be rubbed off
  • May be present on dead or living tissue
  • May or may not be surrounded by a tan halo

Severe Tar Spot lesions are raised, black lesions on the leaf surface.  Photo: Mike Hannewald

Tar Spot lesions with a halo.  Photo: Jon Skinner

Tar Spot lesions without a halo.  Photo: Mike Hannewald


Tar Spot decreases the leaf area available for photosynthesis, and may eventually lead to premature death of leaf tissue, reducing the resources available to the plant during grain fill. If the leaf is infected early in the season, it will run out of sugars to fill the kernels, resulting in shallower kernels and lower yields. The net result often is that the plant cannibalizes resources away from the stalk, paving the way for stalk rots to move in. 

The disease has also been seen as a secondary infection, exploiting leaves that are already weakened by other diseases like Gray Leaf Spot or by limited fertility. 

We do know that Tar Spot is polycyclic, meaning it can infest, form spores, reinfect and spread all within one growing season (like many other leaf diseases of corn). In years with amenable weather patterns, the disease appears to rapidly spread by wind-blown spores.


Due to weaker stalks, prioritize harvest of Tar Spot infested fields when possible. Until more is known about the life cycle of this disease, practice good plant health management: rotate away from corn in infected fields, consider tillage to bury residue, and plan to invest in fungicide. There are currently no fungicides labeled for control of Tar Spot in the US, however fungicide applications have helped slow the infection in some situations.

Fungicide Trial - Sprayed at R3 in August 2018 (Photo taken September 7, 2018)

A fungicide study from Martin Chilvers at Michigan State illustrates the protection afforded by timely fungicide applications.
Shared on Twitter by @MartinChilvers1 on Sept. 11, 2018

Healthy plants are better able to mitigate losses from Tar Spot. Be sure that you plan for adequate fertility so that plants are strong and healthy in June and July, when Tar Spot seems to move in. 

Planting a diverse hybrid mix is a good strategy to spread risk. Hybrids with good stay green tend to tolerate Tar Spot infection better in comparisons, but again, we do not know enough to make hybrid-specific recommendations. 

Moisture increases the spread of Tar Spot. Some areas in Michigan saw increased Tar Spot under pivots compared to the corners, but the corners were lower-yielding because water was more limiting than the Tar Spot infestation. 

Thus far, mycotoxin production is not associated with Tar Spot infection. Continue to test grain if you plan to use it as feed after storage. 

We need YOU!

There are so many unknowns with this new disease. The best way to advance our understanding is to submit samples from your affected fields to a National Plant Diagnostic Network university diagnostic lab for diagnosis. Your Beck’s representative can help to identify a suitable lab.


Click here for a downloadable version of this Agronomy Talk update

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Samantha Miller

Samantha Miller

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