Tar spot is caused by the fungal pathogen Phyllachora maydis.
Under favorable conditions for disease, yield loss on susceptible hybrids can be severe.
Tar spot was first identified in DE at the end of 2023 and has been identified in all three countries for the 2025 season (Fig 1)

Fig 1: Tar Spot Distribution as of Aug. 2025 https://corn.ipmpipe.org/tarspot/

Fig 2: Tar spot on a corn leaf

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

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 teliospores can be mistaken for tar spot.
Rust spores or insect frass/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 tassels.

Management

Corn is the only known host for Phyllachora maydis. There are diseases caused by different pathogens that go by the same common name, particularly in ornamentals.
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.
Many fungicides are now labeled for tar spot, and the best economic return has been shown on a single application made between VT-R3.

References

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

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