There’s a New Disease in Town, and Its Name is Tar Spot


Tar Spot is a relatively new disease in the U.S. 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 produces 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.

Spores are moved on weather events by wind and rain. Tar Spot is known to overwinter in the U.S. in two fungal forms of the same species: stroma and ascospores. While some of these fungal bodies do survive, the overwintering survival rate is unknown.

Field Conditions

Disease development is fostered with temperatures are 60 to 72°F and when a 75% average relative humidity occurs over a 30-day period. When seven hours of leaf wetness are added to these conditions, infection increases.


Tar Spot releases most of its spores at night when the relative humidity is more than 85%. Lesions then form on the upper and lower leaf surfaces. In some situations, lesions can form on the husk and leaf sheaths. Tar Spot has a latent period (between sporulation and infection symptoms) of approximately 14 days.

Tar Spot Lesions


Tar Spot decreases the leaf area available for photosynthesis, and it may eventually lead to premature death of leaf tissue, reducing the resources available to the plant during grain fill. The disease colonizes the vascular bundles of the plant, disrupting the flow of water and nutrients by obstructing the xylem and phloem. 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.

We do know that Tar Spot is polycyclic, meaning it can infest, form spores, re-infect and spread in approximately 21 days, 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, it’s important to prioritize harvest of Tar Spot-infested fields when possible. Practice good plant health management: rotate away from corn in infected fields, consider tillage to bury residue, and plan to invest in fungicide. Trivapro® fungicide is PFR Proven and labeled for control of Tar Spot. Numerous other fungicides are recommended to control Tar Spot through the FIFRA 2(ee) program.

Fungicide Trial

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.

Genetics are a significant factor in limiting the severity of Tar Spot. Look for hybrids with strong plant health characteristics. Hybrids with good stay-green rankings tend to tolerate Tar Spot infection better in comparisons.

Spores are spread by wind and splashing from rain. 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.

There are 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. As we continue to learn more about Tar Spot, stay in touch with your local Beck’s representative to learn more and to monitor this disease’s presence in your local area.

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