Cercospora Leaf Spot Treatment: Understanding and Managing the Disease

In 2025, Cercospora leaf blight (CLB) was responsible for yield losses of over 4 million bushels in the U.S. and was the second most damaging soybean foliar disease, just behind frogeye leaf spot.¹

Don’t let CLB limit your yield. Learn how to protect your crop with early identification and proactive management tactics.

When does Cercospora leaf blight appear?

Throughout the season, multiple species of Cercospora fungi (also responsible for purple seed stain) can remain dormant within plants, with leaf blight symptoms appearing in varying severity depending on the following factors:

• The amount of initial inoculum (present in overwintering infected residue and seed)
• Crop stress caused by drought and heat
• The number of disease cycles within a season
• High humidity (at least 92.5%) and warm temperatures (68 to 86 F)²
  

How to identify Cercospora leaf blight

• Foliar symptoms: Prominent from R5 to R6 in soybean when the combination of a fungal toxin and sunlight create reddish-purple bronzing on upper leaves, resembling sunburn.³
• Petiole symptoms: As the infection progresses, plants develop black ovate lesions on petioles and pods with subsequent defoliation in the upper canopy.
• Defoliation: In severe cases, Cercospora leaf blight can cause significant defoliation.
  

Why does Cercospora leaf blight continue to be a problem?

Understanding disease-inducing factors for Cercospora leaf blight can allow you to take appropriate measures to prevent and manage the disease.

• Overwintering in crop residue: Cercospora leaf blight is a residue-borne pathogen, meaning it can overwinter and infect crops planted in the same field the following season.
• Transmission via infected seeds: It can overwinter in seeds with infected seed resulting in diseased seedlings.
• Latent period: For standard soybean maturity groups, Cercospora leaf blight’s latent period is usually triggered around 100 to 110 days after planting.⁴ Leading up to visible symptoms, the disease is actively growing and spreading within the host plant without showing any symptoms.
• Resistance to traditional fungicides: Some populations of Cercospora leaf blight can tolerate the fungicides in FRAC groups 1 (MBC) and 11 (QoI) making it difficult to control with traditional fungicidal applications.⁵
  

Applying fungicides in a timely manner is critical. Waiting too long to apply a fungicide treatment may result in the disease progressing significantly, making it challenging to achieve successful control.

How to prevent and manage Cercospora leaf blight and purple seed stain in soybeans

Prevention can start with selecting high-quality seed, implementing cultural practices, and applying preventive applications of fungicide.

1. Select high-quality seed

   Seed quality and germination rate are affected by numerous factors including environment, storage, genetics, seed treatments and more.
   Soybean seeds should appear uniformly round, smooth and creamy yellow; these seeds usually have better germination and field emergence rates compared to smaller, wrinkled, green to dark brown seeds or soybean seeds infected by Cercospora fungi and exhibiting purple seed stain. Soybean seed lots should be tested for quality before planting.
   

2. Consider row spacing and canopy densities

   Narrow row spacing and dense canopies increase moisture and subsequently the spread of Cercospora pathogens. Adopting wider row spacing can enhance air circulation and sunlight penetration, promoting faster drying of foliage and soil surfaces, reducing conditions that favor pathogen development.
   

3. Preventative applications are key

   Revylok® provides season-long control of a broad disease spectrum.

   When should Revylok fungicide be applied to prevent Cercospora leaf blight?
   For maximum efficacy, apply Revylok fungicide during the R2-R3 soybean growth stage, prior to disease development.

4. Use crop rotation and tillage

   Cultural practices like rotating to a non-host crop like corn can break up the life cycle of Cercospora leaf blight and minimize pathogen build-up. Tillage operations can also speed up decomposition of infected residue, destroying the pathogen.⁶

Experts are available to help you make your decisions. Reach out to your seed retailer, a nearby extension office agent or a seed company professional like your regional BASF representative.

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Endnotes

1. Sikora, Ed, et al. “Soybean Disease Loss Estimates from the United States and Ontario, Canada — 2025.” Crop Protection Network, 6 Mar. 2026, cropprotectionnetwork.org/publications/soybean-disease-loss-estimates-from-the-united-states-and-ontario-canada-2025. Accessed 12 Sept. 2026.
2. Price, Trey, et al. “An Overview of Cercospora Leaf Blight of Soybean.” Crop Protection Network, 9 Jan. 2025, cropprotectionnetwork.org/publications/an-overview-of-cercospora-leaf-blight-of-soybean. Accessed 12 Sept. 2026.
3. “Cercospora Leaf Blight.” Crop Protection Network, 2020, severity.cropprotectionnetwork.org/crop/soybeans/cercospora-leaf-blight. Accessed 12 Sept. 2026.
4. Ticiano da Silva, Ernesto. “Hiding in Plain Sight: Disentangling the Biology of Cercospora Species Associated with Cercospora Leaf Blight of Soybean.” LSU Scholarly Repository, 24 Apr. 2025, repository.lsu.edu/gradschool_dissertations/6761/. Accessed 12 Sept. 2026.
5. Lux, LeAnn. “Soybean Diseases and Management.” NC State Extension Publications, 15 July 2025, content.ces.ncsu.edu/north-carolina-soybean-production-guide/soybean-disease-and-nematode-management. Accessed 12 Sept. 2026.
6. “Cercospora Leaf Blight - Soybean Disease” Soybean Research & Information Network, 1 May 2024, soybeanresearchinfo.com/soybean-disease/cercospora-leaf-blight/. Accessed 10 May 2024.