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How to Manage Pests

Interactive Tools and Models: About the Fire Blight Risk Assessments

These fire blight risk assessment models, developed at the University of California. The models

    Analyze daily (Schroth) and hourly (Zoller) weather data.
  • Assess daily conditions for bacterial infection and growth.

Using the assessment models

During an apple or pear growing season, you can expect to run the Fire Blight Risk Assessments many times to get updates to the assessments.

  1. Set up a "location," that is, an apple or pear orchard where you'll be monitoring and getting the assessments.
  2. Determine a nearby weather station for which data are collected and stored by UC IPM.
  3. Run the Fire Blight Risk Assessments to update the assessments using the most recent weather data.
  4. Repeat steps 2 and 3 throughout the season.

This program uses information that you supply about your apple or pear orchards. To get the assessments, you select weather data collected through the California PestCast Disease Network and stored in the UC IPM California Weather Database.

The first time: set up a location

  1. Start the Fire Blight Risk Assessments.
  2. Enter a name describing your location or orchard (Davis W1, for example). You'll be able to set up as many locations as you wish.
  3. Enter the start date, or first day of weather data you want to use in the assessments for this location.
  4. Enter an estimated end (harvest) date for this location. You'll be able to update it later if you need to.
  5. Click on the FORECAST button to complete setup and request an assessment.

Updating assessments over time

  1. Start the Fire Blight Risk Assessments and select a location.
  2. Change the start date or end (harvest) date--usually not necessary.
  3. Click on the SUBMIT button to request updated assessments.

The Fire Blight Risk Assessments will search for the appropriate weather data, calculate the daily values and values for the evaluation period, then display the assessments, a list of significant events, and a data summary on your screen.

Interpreting the assessments

The model display includes the following information:

Plot: Degree-hour model (Zoller)
See Fire Blight on Pears or Apples for complete information on analyzing results in a spray program.
Accumulated degree-hours 1 degree-hour (DH) equates to 1 degree above 65°F for 1 hour. The color-shaded areas represent accumulated degree-hours. The accumulattions are reset by continuous cool temperatures unless the total has surpassed 400 degree-hours and has coincided with precipitation or simultaneous warm, humid infection periods of at least 57F and 90% relative humidity.
Early-bloom period action threshold In the Sacramento valley, this degree-hour range occurs from 1 to 150 DH.
In the North Coast region, this degree-hour range occurs from 100 to 250 DH.
If rain predicted within 24 hours, the recommended action is that crop protection is required (spray).
Increased action threshold In the Sacramento valley, this degree-hour range occurs from 150 to 500 DH.
In the North Coast region, this degree-hour range occurs from 250 to 600 DH.
If rain or warm, humid weather is predicted where the temperature is at least 57F and humidity is at least 90%, the recommended action is to repeat treatment every 3-4 days with treatment in the 24 hours prior to predicted conducive weather.
Prior to rain action threshold In the Sacramento valley, this degree-hour range occurs above 500 DH.
In the North Coast region, this degree-hour range occurs above 600 DH.
When Predicted rain or warm weather occur where the temperature is at least 57F and humidity is at least 90%, the recommended action is to treat every other day during major bloom in the Sacramento valley, and, in the North Coast region, possibly treat every other day during major bloom.
Plot: Mean air temperature model (Schroth)
Mitigation action threshold Initiate treatment when the mean daily temperature rises above the action threshold, represented by a line drawn from 62°F. on March 1 to 58°F on May 1.
Mean air temperature When the mean daily temperature rises above the action threshold, initiate treatment.
Maximum relative humidity Reference information for both plots.
Precipitation Reference information for both plots. The units are in inches. The largest rainfall value is annotated in red.
List: Legend of significant events
Start of conducive period The accumulation of degree-hours has been reset by the model.
(--region--) treat within 24 hours preceding rain when below this threshold. The accumulation of degree-hours has crossed the "Prior to rain action threshold."
(--region--) 3-4 day treatment recommended when above this threshold. The accumulation of degree-hours has crossed the "Early-bloom period action threshold."
(--region--) alternate day treatments are recommended when above this bloom period threshold. The accumulation of degree-hours has crossed the "Increased action threshold."
No reduction in DH threshold. The condition for reseting the accumulated degree-hours has more stringent weather condition requirements, and, thus, is less likely to occur.
Daily mean air temperature crossed Schroth prediction line. Crop protection treatment is recommended when this condition occurs.
Table: Hourly weather data
A summary of the daily weather values used in making the fire blight risk assessments is presented in a chronological table.

Caution: The applicability of the assessments to orchards where no weather data are being collected is unknown; reliability will depend on the similarity of micrometeorological conditions between the orchard where the weather is monitored and other orchards to which you are attempting to apply the data and assessments.

References

The Fire Blight Risk Assessments are computer representations of the Zoller and Schroth disease models for controlling fire blight in Apple and Pear crops in California. For details of the research, see

  • Gubler, W.D, Lindow, S., Zoller, B., and Duncan, R. 1999. Pear Diseases in Production and Handling of California Pears. University of California, Division of Agriculture and Natural Resources Publication.
  • Zoller, B.G. 1990. Controlling Fire Blight of Pear Using Heat Summation to Predict Blossom Blight. Proceedings for Pear Short Course. University of California Cooperative Extension, Konocti Harbor Resort, November 27-29.
  • van der Zwet, T., Zoller, B. G., and Thomson, V.S. 1988. Controlling Fire Blight of Pear and Apple by Accurate Prediction of the Blossom Blight Phase. Plant Disease 72: 464-472.
  • Thomson, S.V., Schroth, M.N., Moller, W.J., and Reil, W.O. 1982. A Forecasting Model for Fire Blight of Pear. Plant Disease 66:576-579.

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