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How to Manage Pests

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

This tomato spray forecast model, developed at UC Davis,

  • 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.

Using the spray forecast model

During a tomato growing season, you can expect to run TomatoPM many times to get updates to the spray forecast.

  1. Set up a "location," that is, a tomato field where you'll be monitoring and getting a forecast.
  2. Collect the weather data from the tomato field or know that close by, there's a tomato weather station for which data are collected and stored by UC IPM.
  3. Run TomatoPM to update the forecast using the most recent weather data.
  4. Repeat steps 2 and 3 throughout the season.

This program uses information that you supply about your tomato fields. To get a forecast, you may use weather data collected through the California Tomato Weather Network and stored in the UC IPM California Weather Database, or a file of weather data you collect in your field.

The first time: set up a location

  1. Start TomatoPM.
  2. Enter a name describing your location or field (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 forecast for this location.
  4. Enter an estimated harvest date for this location. You'll be able to update it later if you need to.
  5. Indicate whether you'll be using weather data from UC IPM's database or from a file you prepare.
  6. Click on the FORECAST button to complete setup and request a forecast.

Updating forecasts over time

  1. Start TomatoPM and select a location.
  2. Change the start date or harvest date if necessary.
  3. Add any fungicide sprays that you've made, but haven't entered before.
  4. Add the weather file, if you're monitoring your own data.
  5. Click on the FORECAST button to request an updated forecast.

TomatoPM will search find the appropriate weather data, calculate the daily values and values for the evaluation period, then display the forecast, recommendations, and a data summary on your screen.

Interpreting the forecast

The forecast display includes the following information:

Forecast Display
Last day of evaluation period "Day" is the last day of data used in the evaluation.
Recommended action From the disease risk evaluated for the most recent period, an action (spray, don't spray, reevaluate) is recommended.
Disease risk No or low disease, moderate disease, or severe disease is expected, based on conditions over the most recent period.
Summary for most recent 10 days Date for each day in the most recent period (usually 6 days), and the evaluation of the weather conditions for each day: Conducive, Moderate, Nonconducive. Spray recommendations are shown in red type, sprays made are in bold.
Data from (day) to (day) Dates indicate the total period for which data have been used for the forecast.
Location The name of the location.
Today is (date) Current date shows when the forecast was generated.

Caution: The spray forecast and recommended action provide you with additional guidance to use in making a decision about when (and if) you should spray a fungicide to control powdery mildew in your field. The applicability of the forecast to fields where no weather data are being collected is unknown; reliability will depend on the similarity of micrometeorological conditions between the field where the weather is monitored and other fields to which you are attempting to apply the data and forecast.

Effect of sprays on the forecast
When you spray a fungicide, enter the date on which the spray is made. The model assumes that fungicide sprays can reduce infection in progress and provide effective protection for a 10-day period, where the first day is the date of the spray.

As you update the forecast, the display will always inform you of the conditions for each day and the disease risk without fungicide protection. However, the forecast will also indicate whether the crop is protected by a recent spray.


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. For details of the research, see

  • 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.

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