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How the Smith-Kerns model can help maximize dollar spot control

By Matt Giese, technical services manager, Syngenta, and Paul Koch, Ph.D., University of Wisconsin-Madison

Originally published in Golf Course Industry magazine on May 22, 2019


With the recent introduction of a model that reliably predicts when dollar spot will first appear in a specific geography, it’s important to familiarize yourself with how it can be best used to treat your course.

Many of the first spring dollar spot applications are based on a calendar day or some combination of calendar day and weather conditions. Until recently, it’s largely been a guessing game.

In 2018, a research paper authored by Damon Smith, Ph.D., associate professor and extension specialist at the University of Wisconsin-Madison, and Jim Kerns, Ph.D., associate professor and extension specialist at NC State University, described a weather-based warning system for when to make fungicide applications to control dollar spot on turfgrass. They tracked dollar spot symptoms over a six-year period at six different turf research locations across the U.S. and found that relative humidity and average daily air temperature were the most predictive conditions for disease development. Using this data, a moving five-day average was inserted into a logistic regression equation, and the Smith-Kerns Dollar Spot Prediction Model was born.

The Smith-Kerns model does not tell a user when to spray; it makes predictions of dollar spot occurrences. Based on research done in Wisconsin with creeping bentgrass, fungicide applications made when dollar spot probability would exceed 20% tended to be the best benchmark for providing effective disease suppression.

Learning your threshold to protect your turf

Once a 20% threshold is established, observe when the model output crosses the threshold. This will be the indicator to apply a fungicide.

If applying a 28-day interval fungicide, don’t put too much stock in the model output during this time since the fungicide should be controlling the disease. After that interval expires, watch the model output for when it rises above the 20% threshold and then make a subsequent application.

In the example provided in Figure 1, a gap exists between the first and second applications that exceeds the 28-day interval.


Figure 1. An example showing how the Smith-Kerns dollar spot prediction model can be implemented.

Because the model output changed with the weather patterns, an application wasn’t necessary at the expiration of the 28-day interval, but rather a few days later when the model output exceeded the threshold. This process is repeated throughout the season until the dollar spot risk subsides in the fall.

Research conducted at the University of Wisconsin-Madison found that spring and fall were the two seasons where the model was most beneficial for the correct timing of fungicide applications for dollar spot. Additional uses of the model are currently being researched at the University of Wisconsin-Madison, specifically around identifying risk thresholds for when to shorten application intervals and/or increase fungicide rates when disease pressure ramps up. Knowing when to shorten application intervals as disease pressure intensifies during the season maintains better disease control and avoids less effective curative treatments.

To implement this strategy at your course, Syngenta has incorporated the Smith-Kerns model and local weather data into a user-friendly format that allows customization by zip code and alerts that can be delivered by email or text message. Simply select your spray interval (seven, 14, 21 or 28 days) and which day of the week to receive your alerts. The alert also:

  • Provides a risk index for the past 25 days

  • Predicts risk for the next five days based on your weather forecast

  • Has the capability to select historical risk from a custom-defined date range to view past dollar spot risk by zip code



The beauty of this model for predicting dollar spot risk is that you only make applications when you need them, not because the calendar says so. With the same products and the same rates, you can increase your control without increasing the amount of product used. It’s possible you may even save a fungicide application or two this season. For more information on using this model, check out this video from Jim Kerns.

Ensure you have the tools and knowledge to stay ahead of dollar spot. For free trial maps, videos from the experts, webinars and more, visit the Syngenta Dollar Spot Solutions webpage.

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