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Tar Spot

Jul 07, 2025

Tar Spot

Don't Let Disease Sneak in

Crop and weather conditions are aligning for a potentially strong year, making this a season worth investing in. The majority of corn and soybean fields are showing excellent growth and yield potential. However, the consistently high relative humidity over the past few weeks has created ideal conditions, not only for crop development, but also for disease pressure, particularly tar spot. When tar spot reaches 50% severity on the ear leaf during grain fill, yield losses of 20-60 bu/ac have been reported. 

What to Know About Tar Spot

  • Caused by the fungus Phyllachora maydis

  • Looks like small, raised black circular lesions on stalks, leaves, and husks
    (These spots cannot be rubbed off)

  • Most frequently appears between silking and late grain fill

  • Can overwinter on infected residue left on the soil surface

Favorable Conditions for Tar Spot

  • Cool temperatures

  • High humidity

  • Prolonged leaf wetness

  • Irrigated fields are especially susceptible

Confirmed presence in PA, IN, IL, IA, MO, KS, NE, and now WI means this disease is active and spreading. See where tar spot has been reported here: https://corn.ipmpipe.org/tar-spot/

We recommend evaluating your fields now, especially if you’re nearing V10 or have a history of Tar Spot pressure. If you’d like help scouting or building a fungicide plan, our agronomy team is ready. Contact your Premier agronomist today to get started. 

Greg Ballweg

Agronomist

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