Apple Scab Infection Risk Model

This app estimates apple scab infection risk. You choose a place and date, and the server looks up the weather data and runs the risk model. This app then shows you the risk index values classified into risk categories. You can use these categories to adjust management actions, such as intensity of scouting, fungicide selection and fungicide application intervals.

This app is a newer, streamlined interface for the existing model, which is also available through MyPest Page.

Disease Information

Apple scab (from Pacific Northwest Plant Disease Management Handbook).

Using this App

Select the "Inputs" tab, search for weather stations near you, and choose one either from a list or a map. Be aware that some weather stations have poor quality or missing data. Our software estimates the quality of each station, but you may wish compare results for several nearby stations.

Optionally, you may choose a start date and a span (number) of days. If you leave the start date blank, the end of the output will be five days in the future, using forecast weather data, and the app will count back to a start date in the recent past. The weather forecast data is from the National Weather Service's "NDFD" forecasts.

Then, select "Graph" or "Table" to run the model. Your output will appear after 10-20 seconds depending on server load.

Interpreting the results

Apple scab infection can be predicted by measuring temperature and leaf wetness, assuming the presence of inoculative ascospores. This model (based on Mills 1944 and adapted by Len Coop) predicts risk of infection by accumulating degree-hours conducive to infection during periods of leaf wetness, which we estimate from temperature, dewpoint, and windspeed. The result (a number 0 – 350, with 195 being the threshold for infection) is classified into a risk class:

No scab risk
the index is well below the threshold; no infection risk.
Scab cycle nearing
the index is below but nearing the threshold. There may be some risk if conditions in the field differ sufficiently from those at the weather station.
Scab cycle starting
the index exceeds the threshold; there is risk.
Scab cycle ongoing
the index is well above the threshold, and the risk increases accordingly.

Like most plant disease risk models, this one assumes that inoculum is present locally and that you have not recently applied a protective fungicide.

Technical information about apple scab and this model is available on our disease risk model documentation page.

Disclaimer: The index is intended to inform your decisions about management actions, such as choice and timing of control measures and intensity of scouting. It should supplement, not replace, the other factors you consider in making these decisions. Use at your own risk.

Email Subscription

Automated email delivery of the disease risk index outputs displayed in this app is available at no cost. An email subscription offers model results for this and several other plant disease models, for up to three weather stations, on a schedule that you select. To subscribe, you will need a account.


Next, use one of these buttons to run the model and see the output in the form of your choice.

Risk Model

To get this information by email, log in to or sign up for email notifications. To see the model output together with relevant weather inputs, go to MyPest Page.

Risk Model

To get this information by email, log in to or sign up for email notifications. To see the model output together with relevant weather inputs, go to MyPest Page.