Research and IPM

Models: Diseases

Crop: Lettuce

Disease: Downy Mildew
Pathogen: Bremia lactucae

Note: Before using a model that was not field tested or validated for a specific location, the model should be tested for one or more seasons under local conditions to verify that it will work in this location. See "Validation Work" below.

Model developer and citation

Scherm, H., and van Bruggen, A. H. C. 1994. Weather variables associated with infection of lettuce by downy mildew (Bremia lactucae) in coastal California. Phytopathology 84:860-865.

Scherm, H. and van Bruggen, A. H. C. 1993. Response surface models for germination and infection of Bremia lactucae, the fungus causing downy mildew of lettuce. Ecol. Model. 65:281-296.

Scherm, H. and van Bruggen, A. H. C. 1993. Sensitivity of simulated dew duration to meteorological variations in different climatic regions of California. Agric. For. Meteorol. 66:229-245.

Sensor location

Between plants, at a height of 10 cm above the soil.

Input variables

Environmental: Morning leaf wetness duration.

Calculated: Dew periods from dew simulation model.

Description of model

The model is based on morning leaf wetness duration. Morning leaf wetness duration is being determined by in-canopy measurement, by forecasts of weather variables from weather companies, or by the dew simulation model in conjunction with weather forecasts. The algorithm for the dew simulation model is in progress.

Action threshold

According to the model, treat when the leaf wetness period ends at 10:00 a.m. or later (11:00 a.m. during Daylight Savings Time), after a minimum of a ten day spray interval.

Model validation

Validation trials have been conducted in Monterey Co. and Santa Barbara Co.

Scherm, H, Koike, S. T., Laemmlen, F. F., and van Bruggen, A. H. C. 1995. Field evaluation of fungicide spray advisories against lettuce downy mildew (Bremia lactucae) based on measured or forecasted morning leaf wetness. Plant Dis. 79:511-516.

Model implementation

The model is still in the validation stage.

Current limitations

The dew simulation model needs further validation. There are constraints in the accuracy of dew forecasts produced by weather forecasting companies.

Future directions

The role of spore survival in disease development is under evaluation. Fog drizzle as a source of leaf wetness will be incorporated into the dew simulation model.

Related work

Scherm, H., and van Bruggen, A. H. C. 1995. Concurrent spore release and infection of lettuce by Bremia lactucae during mornings with prolonged leaf wetness. Phytopathology 85:552-555.

Scherm, H., and van Bruggen, A. H. C. 1995. Comparative study of microclimate and downy mildew development in subsurface drip - and furrow - irrigated lettuce fields in California. Plant Dis. 79:620-62.

Scherm, H., and van Bruggen, A. H. C. 1994. Effects of fluctuating temperatures on the latent period of lettuce downy mildew (Bremia lactucae). Phytopathology 84:853-859.

Model developer and citation

Modifications by University of California, the Iceberg Lettuce Advisory Board and Adcon Telemetry.

Scherm, H., and van Bruggen, A. H. C. 1994. Weather variables associated with infection of lettuce by downy mildew (Bremia lactucae) in coastal California. Phytopathology 84: 860-865.

Scherm, H., and van Bruggen, A. H. C. 1993. Response surface models for germination and infection of Bremia lactucae, the fungus causing downy mildew of Lettuce. Ecol. Model. 65:281-296.

Scherm, H., and van Bruggen, A. H. C. 1993. Sensitivity of simulated dew duration to meteorological variations in different climatic regions of California. Agric. For. Meteorol. 66:229-245.

Sensor location

In adjacent fields.

Input variables

Environmental: Morning leaf wetness duration.

Forecasted: Air temperature and humidity, cloud cover, rainfall and wind speed.

Description of model

The model is based on morning leaf wetness duration. The algorithm of the model is under development.

Action threshold

According to the model, treat when leaf wetness period ends at 10:00 a.m. or later (11:00 a.m. during Daylight Savings Time), after a minimum of a 8- or 9-day spray interval, depending on the number of leaf wetness events during this period.

Model validation

Validation trials are currently being conducted by University of California, the Iceberg Lettuce Pest Management Research Group, and Adcon Telemetry.

Model implementation

The model is still in the validation stage.

Related work

Scherm, H., and van Bruggen, A. H. C. 1995. Concurrent spore release and infection of lettuce by Bremia lactucae during mornings with prolonged leaf wetness. Phytopathology 85:552-555.

Scherm, H., and van Bruggen, A. H. C. 1995. Comparative study of microclimate and downy mildew development in subsurface drip - and furrow - irrigated lettuce fields in California. Plant Dis. 79:620-62.

Scherm, H., and van Bruggen, A. H. C. 1994. Effects of fluctuating temperatures on the latent period of lettuce downy mildew (Bremia lactucae). Phytopathology 84:853-859.

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