Effective Timing for Fungicide Applications to Combat Tar Spot


Effective Timing for Fungicide Applications to Combat Tar Spot
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Tar spot is a growing concern for farmers, especially those cultivating corn. This fungal disease can significantly impact crop yields if not managed properly. Effective timing of fungicide applications is crucial for controlling tar spot and ensuring healthy crops.

Understanding Tar Spot

Tar spot, caused by the fungus Phyllachora maydis, manifests as black, tar-like spots on corn leaves. It thrives in humid, wet conditions, making it a prevalent issue during certain seasons. Early detection and timely fungicide application are essential to mitigate its effects.

Importance of Timing in Fungicide Application

Applying fungicides at the right time can make a significant difference in controlling tar spot. Here are some key points to consider:

  • Disease Monitoring: Regularly scout fields for early signs of tar spot.
  • Weather Patterns: Fungicide application is most effective during specific weather conditions.
  • Crop Growth Stages: Apply fungicides at critical growth stages to protect the plant.

Optimal Fungicide Application Stages

V6 to V8 Growth Stage

At this stage, the corn plant is developing rapidly. Early fungicide application can prevent initial infections from spreading.

VT to R1 Growth Stage

This is a critical period for fungicide application. The tasseling to silking stage (VT to R1) is when the plant is most vulnerable to tar spot. Applying fungicides during this window can significantly reduce disease severity.

Post-R1 Growth Stage

While earlier applications are more effective, a second application post-R1 can provide additional protection if the disease persists or weather conditions remain favorable for fungal growth.

Factors Influencing Application Timing

Weather Conditions

  • Humidity and Rainfall: High humidity and frequent rainfall create ideal conditions for tar spot. Monitor weather forecasts to time applications effectively.
  • Temperature: Moderate temperatures favor the growth of tar spot. Plan fungicide applications accordingly.

Field History

  • Previous Infections: Fields with a history of tar spot infections may require earlier and more frequent fungicide applications.
  • Crop Rotation: Rotating crops can reduce the risk of severe tar spot outbreaks.

Strategies for Effective Fungicide Use

Choosing the Right Fungicide

Select fungicides specifically labeled for tar spot control. Look for products that provide broad-spectrum activity and have proven efficacy against tar spot. Consult with agricultural experts or extension services to choose the most appropriate fungicide for your region.

Following Label Instructions

Adhere to the recommended application rates and intervals mentioned on the fungicide label. Over-application can lead to resistance, while under-application may not provide adequate protection. Ensure you are familiar with the product’s specific guidelines.

Using Proper Equipment

Ensure that application equipment is calibrated correctly to distribute the fungicide evenly. Proper coverage is essential for effective control. Regularly maintain and check your equipment to avoid malfunctions that could compromise application efficacy.

Tank Mixing and Rotation

Consider tank mixing fungicides with different modes of action to delay resistance development. Additionally, rotate fungicides with different modes of action between applications. This approach helps in managing resistance and maintains the long-term effectiveness of fungicides.

Case Study: Successful Tar Spot Management


A Midwestern corn farm experienced severe tar spot outbreaks in previous years, significantly affecting yields. The farmer implemented a strategic fungicide application program based on expert recommendations.


  • Monitoring: Regular scouting identified the initial presence of tar spot at the V6 stage.
  • Application: The first fungicide application was made at the V8 stage, followed by a second application at the VT stage.
  • Weather Considerations: Applications were timed before forecasted humid and rainy periods.
  • Rotation and Tank Mixing: Fungicides with different modes of action were rotated and tank mixed.


The strategic fungicide application program reduced tar spot incidence by 70%. The farm saw a significant increase in yield compared to previous years, highlighting the importance of timely and well-planned fungicide applications.


Effective fungicide application timing is crucial for managing tar spot and protecting your corn crops. By understanding the disease, monitoring weather conditions, and applying fungicides at critical growth stages, you can significantly reduce the impact of tar spot. Stay proactive and informed to maintain healthy and productive fields.

Additional Resources

For more information on fungicide application and tar spot management, consult your local agricultural extension office or visit reputable agricultural websites.

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