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How to Manage Pests

Interactive Tools and Models: About the Spray Forecast for Tomato Powdery Mildew

TomatoPM is a computer representation of the spray forecast model for tomato powdery mildew, developed by R. Guzman-Plazola, R. M. Davis, and J. J. Marois at the University of California, Davis, and sponsored by the California Tomato Research Institute and California Tomato Board. This Web version replaces the Excel version published earlier.

Summary of the research


  • Guzman-Plazola, R. A. 1997. Development of a spray forecast model for tomato powdery mildew (Leveillula taurica (Lev) Arn). Ph.D. Thesis. University of California, Davis.
  • Davis, R. M. and J .J. Marois. Development of a spray forecast model for powdery mildew of tomato. California Tomato Research Institute 1995 Final Report. pp. 52-59. In conjunction with R. Guzman-Plazola.

The model is based on results from experiments under controlled conditions (growth chamber) and field monitoring of disease and weather data. A linear discriminant function was generated to classify daily environmental conditions of tomato canopies according to their conduciveness for disease. Cross-validation of the discriminant function and tests with independent data sets yielded percentages of correct classification of 74-96% and 87%, respectively.

The model was tested during two consecutive years (1995 and 1996) in ten tomato fields (fresh market and processing) in the northern San Joaquin and southern Sacramento valleys of California. After following the recommendations of the model, it was possible to save 2 to 5 fungicide sprays with no significant impact on fruit yield and quality. Disease levels at harvest time in plots managed according to the model have been equal to or only slightly higher than that of plots where fungicide applications were made every 14 days.

In generating a spray forecast, the model

  • Analyzes hourly weather data.
  • Classifies the conditions for each day in terms of conduciveness for disease.
  • Evaluates the daily conditions over the most recent period (usually 6 days) to assess the risk of low, moderate, or severe disease.
  • Recommends management action based on the conditions over the period.

This model assumes the following:

  • Unlimited inoculum of Leveillula taurica is present in the field.
  • The tomato cultivar in use is susceptible to Leveillula taurica.
  • There was no disease risk before the first day of data to be evaluated by TomatoPM.
  • Fungicide sprays can reduce infection in progress and provide effective protection against the disease for 10 days.
  • Location of sensors in the field and sensor measurements are representative of tomato canopy microclimate in the rest of the field.

While the results are very promising, remember that

  • If using the model in new areas, it should be tested for one or more seasons under local conditions to verify that it will work in your location. Monitor fields closely for disease while testing the model.
  • Sensors must be calibrated to give accurate results.
  • The model uses microclimate data from tomato canopies, so proper placement of the sensors is very important for correct classification of daily conditions.

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