Field Facts: Tar Spot

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Widespread outbreaks of severe tar spot in recent years have proven that this disease can cause a significant economic impact to corn growers across the Midwest. Proactive management practices will help your customers overcome tar spot when environmental conditions are favorable.

  • Common name: Tar spot
  • Scientific name:Rhytisma acerinum
  • Fungal pathogen: Phyllachora maydis
  • Symptoms: Tar spot is the physical manifestation of fungal fruiting bodies, the ascomata, developing on the leaf. 
    • The ascomata look like spots of tar, developing black oval or circular lesions on the corn leaf.
    • These black structures can densely cover the leaf and may resemble the pustules of rust fungi but cannot be scratched off.
  • Conditions for development: Tar spot favors cool temperatures (60 F to 70 F), high relative humidity, frequent cloudy days, and seven or more hours of dew at night. Tar spot can rapidly spread through the corn canopy under favorable conditions, causing premature leaf senescence. 

Fast Facts on Tar Spot:    

  • Tar spot spreads from the lowest leaves to the upper leaves, leaf sheathes and, eventually, the husks of developing ears.
  • Tar spot reduces the photosynthetic capacity of leaves, causing rapid premature leaf senescence. This leads to reduced stalk quality and yield loss.
  • Disease pressure is typically higher in areas closer in proximity to the Great Lakes, river bottoms and irrigated corn acres, as these locations provide environmental conditions conducive for disease development.1
  • University corn hybrid trials suggest tar spot can cause yield losses of up to 39 bu./A under heavy infestations, with later-maturing hybrids suffering greater yield loss.2
  • Tar spot is polycyclic, meaning the disease in one season will have multiple cycles, producing spores that can spread to new plants under favorable environmental conditions.
  • Tar spot has been proven to survive for at least one year (more suspected) in corn residue. However, managing corn residue doesn’t eliminate the risk of tar spot. Spores can still come from neighbors’ fields or from fields counties away.

Watch this video to learn more about the life cycle of tar spot and how to best manage against this disease in cornfields.

Control Tips:

Proper diagnosis and early detection of tar spot is critical for successful disease management. Customers can further protect their yield by implementing the following practices: 

  • Use tolerant hybrids. If tar spot is a problem in your area, consider tolerant hybrids as a first defense against the disease.
  • Scout vigilantly. Scouting the lower canopy will help determine if a fungicide application is warranted. If signs of the disease are present on the upper canopy, the disease has already gained access to the plant and spraying a fungicide will have little to no effect.
  • Time fungicide applications wisely. To improve efficacy and reduce risk of fungicide resistance, encourage customers to watch for favorable disease conditions and be ready to react if the season changes. Applying too early or too late will likely only result in wasted product. Encourage customers to use a fast-acting fungicide to control tar spot when favorable disease development conditions arise.
    • Aproach® Prima fungicide offers dual modes of action, providing both preventive and curative benefits. It is tough against fungicide-resistant diseases and rainfast within one hour to help reduce the impact of yield-reducing diseases like tar spot.

The best tar spot management program will be specific to each customer and, in most cases, vary by each field. These resources can help you guide customers in making cost-effective disease management decisions.


1 Quinn, D., and D. Telenko. 2022. Tar Spot of Corn: What to Know and New Research

2Telenko, D. 2010. New and Emerging Diseases in Field Crops: Tar Spot of Corn


Aproach® Prima is not registered for sale or use in all states. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your state. Always read and follow label directions.