Tar spot has become a significant threat across the Corn Belt since appearing in the United States in 2015. Your best defense combines choosing resistant hybrids, to slow down disease development, with well-timed fungicide applications. Understanding Tar Spot’s disease cycle and watching for conditions conducive for disease development can help better time fungicide applications

What Makes Tar Spot a Unique Threat?

There is a combination of factors unique to Tar spot that make it more aggressive in nature than other disease when the right conditions exist.

Figure 1. Observed Tar Spot Range as of November 2025

• Limited genetic resistance, more tolerant hybrids only slow disease development
• Lack of visible symptoms for scouting until 2-3 weeks after initial infection
• Limited benefits from crop rotation or deep tillage due to wind born spores infesting plants in season from many miles away
• Multiple infection cycles can occur in single growing season, reintroducing later
• Impossible to control pre-existing infections with fungicide (no curative treatments)
• Rapid disease progression when conditions are favorable for infection.

What Causes Tar Spot?

Tar spot observed in the United States is caused by a fungus referred to as Phyllachora maydis.¹ In Latin America where P. maydis was first observed, it has been known to form a complex with a secondary fungal pathogen, Monographella maydis. The combination of the two fungi are referred to as tar spot complex and known to cause more severe yield loss when both pathogens are present.² Monographella maydis and its association with P. maydis has not yet been observed in North America, although significant yield loss is frequently observed.

Identifying Tar Spot in Your Fields

Figure 2. Phyllachora maydis, the fungus causing tar spot, with and without lesions forming around stromata.

Scout for Tar Spot, when conditions are favorable for infection (typically early to mid-July) and look for:

• Infection starting on lower leaves and moving upward on the plant
• Raised black spots that feel bumpy
• Spots that cannot be rubbed off
• A "fish-eye" appearance
• Bumps can be present on upper and lower leaf surfaces, as well as on healthy or dead plant parts

Sometimes confused with:

1. Insect frass, which should easily wipe off leaf
2. Late season Common and southern rust pustules as they switch from producing orange-red spores (urediniospores) to black spores (teliospores). However, rust pustules may easily be scraped from the leaf.
3. Saprophytic organisms that break down dead plant tissue late in the season, however they will not exhibit a bumpy texture.

Five Key Tar Spot Risk Factors Influencing How Much Yield Loss Can Occur

1. Hybrid tolerance to tar spot
2. Presence and quantity of inoculum in a field
3. Environmental conditions for infection and development are present
   • Temperature: Cool nights (60–70°F average)
   • Moisture: Periods of extended leaf wetness
4. Growth stage of corn when it is infected and when lesions form. Infections at earlier growth stages can be more detrimental.
5. The timing and effectiveness of your management decisions, especially fungicide applications, can mean the difference between minimal and severe yield losses.

 Best Management Strategy for You?

Step 1: Choose resistant hybrids to assist in slowing disease development

• Hybrids differ in susceptibility
• Understand your hybrid tolerance or susceptibility (Table 1)

Step 2: Consider crop rotation and tillage 

• Tar spot can survive on residue, implying benefits from rotation and tillage
• However, due to spores’ ability to travel long distances with wind, rotated and tilled fields remain highly vulnerable to in-season Tar Spot introduction.

Step 3: Use Crop Disease Forecasting tools to assess risk and assist with fungicide timing:

• Since infection occurs weeks before symptoms and fungicide effectiveness declines when applied after infection, predicting when infection may occur is important.

• Applications after symptoms appear may not be as effective
• VT/R1 through R3 stages most common
• Early vegetative stage timings may be considered in fields with history of disease and environmental conditions that are conducive for earlier than normal development

Fungicide Application Watch-Outs and tips

• Don’t wait for visible lesions to treat with fungicides
• Early fungicide applications at or before first signs of development have been effective at reducing tar spot in previous trials.³ Don’t rely on Late-season curative fungicide applications
• Scouting, along with using a disease risk forecasting app can help fine-tune spray application timing
• Sequential fungicide applications could be beneficial when more than one of the following exist       

1.   susceptible hybrid was planted
2.  high risk of continued disease development present
3.   multiple growth stages remaining before grainfill is complete.

• Fully registered Syngenta fungicide options to manage tar spot include Trivapro® and Miravis® Neo.
• CIt is important to also consider risk and benefits of managing other diseases riskssimiltanously. Potential for other diseases like gray leaf spot, Northern corn leaf blight, Northern corn leaf spot and rust can help simplify justify the decisions to treat fields with a broad-spectrum fungicide that can manage multiple diseases simultaneously

Key Takeaways: The most important factors for tar spot management are hybrid selection and fungicide timing. Remember that prevention is critical - once symptoms appear, it's often too late for effective control. Monitor field conditions closely and be prepared to act quickly when conditions favor disease development.

References
¹Crop Protection Network. 2019. Corn Disease Management, Tar Spot, CPN-2012, https://crop-protection-network.s3.amazonaws.com/publications/tar-spot-filename-2019-03-25-120313.pdf

²Hock J. J.Kranz, and B.L. Renfro. 1995. Studies on the epidemiology of the tar spot disease complex of maize in Mexico. Plant Path. 44:490-502.

³Bajet, N.B., B.L. Renfro, J.M. Valdez Carrasco. 1994. Control of tar spot of maize and its effect on yield. International J. of Pest Man. 40:121-125.

All photos are either the property of Syngenta or used with permission.
Syngenta hereby disclaims any liability for third party websites referenced herein.
Performance assessments are based upon results or analysis of public information, field observations and/or internal Syngenta evaluations. Product performance assumes disease presence