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How to Manage Pests

Interactive Tools and Models: About the Fire Blight Risk Assessments

The Fire Blight Risk Assessments are computer representations of the Zoller (degree-hour) and Schroth (mean temperature) disease models for controlling fire blight, Erwinia amylovora, in apple and pear crops in California.

Summary of the degree-hour model


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.

Model computation

This degree-hour fire blight model assesses actual conditions for Erwinia amylovora bacterial growth and infection. It also indicates when treatment is unnecessary. It takes into account early bloom and periods of continuous cool weather, allowing adjustments in treatment timings.

The model was dveloped from five years of field observation on the development of Erwinia amylovora in Bartlett pear blossoms. It is based on the correlation between the number of accumulated Fahrenheit-degree-hours and the incidence of random new blight infections per holdover infection during an 11 year period.

Use of the degree-hour method requires a recording thermograph to obtain a continuous thermometer reading in the orchard beng managed.

  • One degree-hour equals 1 degree above 65°F for 1 hour. For example, a temperature of 70°F for 2 hours generates 10 degree-hours.
  • Accumulate degree-hours each hour of the day unless 3 consecutive days below 66°F occur. In this case, the accumulation of degree-hours is then reduced to zero until temperatures again exceed 65°F.
  • The accumulated degree-hour total is not reduced by continuous cool temperatures if the total has surpassed 400 degree-hours and has coincided with precipitation or simultaneous warm, humid infection periods of at least 57°F and 90% relative humidity.
  • If the orchard is being irrigated, the humidity threshold is reduced to 80% relative humidity as measured outside the orchard.

If possible, growers should start the season with a full soil water profile so irrigation during bloom can be avoided.

Spray considerations

Growers can track the degree-hours for your orchard and take into account the spray applications. The UC IPM Pest Management Guideline for Fire Blight on Apple and Pear explain the current recommendations for applying the results of the model in a spray program.

  • In the Sacramento Valley, treat within 24 hours preceding rain if 1 to 150 degree-hours have accumulated.
  • In the North Coast region, treat within 24 hours preceding rain when more than 150 degree-hours have accumulated.
  • Treatment for both areas is recommended every 3 to 4 days when accumulation exceeds 150 degree-hours (Sacramento Valley) or 250 degree-hours (Lake County).
  • Alternate day treatments are recommended in the Sacramento Valley whenever more than 500 degree-hours occur in conjunction with major bloom periods.

Summary of the Schroth "UC" or "mean-temperature" model


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.

Model computation

Prediction of flower colonization by Erwinia amylovora bacteria is based on the daily mean temperature rising above a line drawn from 62°F (16.7°C) on March 1 to 58°F (14.4°C) on May 1. Subsequent applications should be made every five days until the end of bloom.

Spray considerations

The UC IPM Pest Management Guideline for Fire Blight on Pear explains the current recommendations for applying the results of the "UC" or mean-temperature model in a spray program.

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