Pecan scab is one of the most significant disease threats to pecan production in Louisiana, particularly during the warm, humid months of the growing season. Managing this disease effectively requires timely fungicide applications—but how do you know when to spray?

A weather-based tool known as “scab hours” offers a practical solution. By tracking periods of high humidity and temperature—conditions that favor scab development—growers can better time fungicide applications based on actual disease risk. This approach not only improves disease control but can also reduce the number of sprays needed each season, saving time and resources.

What Are Pecan Scab Hours?

“Scab hours” refer to the number of hours when environmental conditions are favorable for the development of pecan scab—a fungal disease that thrives in warm, humid weather. Specifically, scab hours accumulate when:

• Relative humidity exceeds 90%, and
• Temperatures are above 70°F

These hours are counted from the time of your last fungicide application and reset after each new spray. The more hours that accumulate under these conditions, the greater the risk of scab infection.

Because different pecan varieties have varying levels of susceptibility to scab, researchers have developed thresholds to help guide fungicide timing. When the number of scab hours reaches the threshold for a given variety, it’s time to reapply fungicide to prevent infection.

Understanding Variety Susceptibility and Scab Hour Thresholds

Not all pecan cultivars are equally vulnerable to scab. Some varieties—such as Desirable or Caddo—are highly susceptible and can develop infections after only a short period of favorable weather. Others, like Elliott or Kanza, are more resistant and can tolerate longer exposure before symptoms appear.

To help growers make informed decisions, researchers have established scab hour thresholds based on variety susceptibility. These thresholds indicate how many hours of warm, humid conditions a variety can endure before a fungicide application is recommended.

By aligning your spray schedule with your orchard’s specific cultivars and local weather conditions, you can improve disease control while potentially reducing the number of fungicide applications.

Pecan Scab Hour Thresholds by Variety Susceptibility

If your orchard reaches the threshold within 14 days of the last fungicide application, it’s time to reapply. If not, you may be able to delay the next spray—saving time, money, and reducing chemical use.

How You Can Track Scab Hours

Tracking scab hours using local weather data is one of the most accurate ways to assess disease risk in your orchard. With just a few tools, you can monitor conditions and make informed decisions about fungicide applications.

What You’ll Need:

• A weather station or sensor that records:
  • Hourly temperature
  • Hourly relative humidity
• A spreadsheet (Excel or Google Sheets) or a simple logbook to record and calculate scab hours

How to Calculate Scab Hours:

1. Record hourly data for temperature and humidity.
2. Identify hours where:
   • Temperature is above 70°F, and
   • Relative humidity is above 90%.
3. Count each of those hours as one “scab hour.”
4. Reset the count after each fungicide application.
5. Compare the total to your variety’s scab hour threshold.

Tip: Automating this process with a spreadsheet formula or script can save time and reduce errors.

What If Scab Hours Don’t Hit the Threshold?

Even if scab hours don’t reach the threshold in a single 14-day period, risk can still build up over time—especially for susceptible varieties.

Here’s what to watch for:

• Multiple near-threshold periods: If you’ve had two or more 14-day windows where scab hours reached 50–90% of your variety’s threshold, the fungus may still be gaining ground.
• Example: For a variety with a 20-hour threshold, two periods with 10–15 scab hours each could still pose a serious risk.

What You Can Do:

• Track cumulative scab hours across several intervals (3–4 weeks).
• Scout your orchard for early signs of scab, such as leaf or nut lesions.
• Consider spraying if cumulative hours approach or exceed your threshold—even if no single period triggered it.

Where Did This Model Come From?

The scab hour model was developed by Dr. Jackie Lee at the University of Arkansas, building on earlier research conducted at Oklahoma State University. Through extensive field trials across multiple seasons, Dr. Lee adapted and validated the model to reflect real-world orchard conditions.

Her work translated complex environmental data into a practical, grower-friendly tool that helps pecan producers make more informed, cost-effective decisions about fungicide applications. This model represents a collaboration between academic research and on-the-ground agricultural practice—bringing science directly into orchard management.

Conclusion: Smarter Spraying Starts with Smarter Tracking

Pecan scab remains a persistent challenge for growers in Louisiana, but tools like the scab hour model offer a more precise, data-driven way to manage it. By understanding your orchard’s varieties, monitoring local weather conditions, and adjusting your spray schedule based on actual disease risk—not just the calendar—you can improve control while reducing unnecessary fungicide use.

Whether you’re using a weather station or simply keeping a close eye on conditions, tracking scab hours puts you in control. Over time, this approach can help you fine-tune your management strategy for your unique orchard environment—leading to healthier trees, better yields, and more sustainable practices.

Jackson is cultivar that is moderately susceptible to scab. The black lesions are scab on the shucks.

Elliot is a cultivar that is resistant to scab.

Range of nut scab severity.