Tar spot is caused by the fungal pathogen Phyllachoramaydis.
Under favorable conditions for disease, yield loss on susceptible hybrids can be severe.
Tar spot was first identified in PA at the end of 2020 and has been identified in 2 counties in MD and 1 suspected county in VA in 2022 (Fig 1).

Identification

Symptoms include small, black specks (stroma) on the upper and lower surface of corn leaves (Fig 2).
Mature common and southern rust pustules that transition from orange-red spores to production of black teliosporescan be mistaken for tar spot.
Rust spores or insect poop can be scraped away with a fingernail, but tar spots cannot be scraped off the leaf (Fig 3).
Tar spots can also form on leaf sheaths, husks, and tasslels.

Fig 1: Tar Spot Distribution in the US in on October 25, 2022 https://corn.ipmpipe.org/tarspot/

Fig 2: Tar spot on a corn leaf

Fig 3: Fungal structures of tar spot on a corn leaf

Management

Corn is the only known host for Phyllachoramaydis. There are diseases caused by different pathogens that go by the same common name.
The pathogen will overwinter in corn residue.
High relative humidity and prolonged leaf wetness favor disease. Epidemics that start earlier in the season result in greater chance for yield loss.
Some fungicides may reduce tar spot, but data efficacy and economic returns are still being generated.

References

Crop Protection Network (2022, Jan 24). Tar Spot of Corn. Retrieved from: https://cropprotectionnetwork.org/encyclopedia/tar-spot-of-corn

Authors: Madeline Henrickson and Dr. Alyssa Koehler

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