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 Tar Spot of Corn

Pest Background

  • 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 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 2: Tar spot on a corn leaf
Fig 3: Fungal structures of 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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