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Project description

Forecasting of Downy Mildew on Lettuce. (92DS001)
Program UC IPM competitive research grants program
A.H.C. van Bruggen, Plant Pathology, UC Davis
Host/habitat Lettuce
Pest Downy Mildew Bremia lactucae
Discipline Plant Pathology
Decision Support
Start year (duration)  1992 (Three Years)
Objectives Evaluate and update a preliminary forecasting system for predicting infections by Bremia lactucae in California lettuce fields.

Analyze regional weather patterns in the Pajaro, Salinas, and Santa Maria Valleys as a basis for determining the required density of weather stations for downy mildew forecasting.

Develop a meteorological forecast for the duration of leaf wetness in the morning, the most important microclimatic variable for downy mildew infections in California lettuce fields.

Evaluate the updated forecasting system for accuracy of prediction, effectiveness of downy mildew control, reduction in fungicide use, and practical feasibility in growers' fields.

Implement a network of electronic weather stations for downy mildew forecasting in the Pajaro, Salinas, and Santa Maria Valleys.

Final report Downy mildew of lettuce is usually controlled with calendar-based sprays of protective fungicides (maneb or fosetyl-Al). Despite regular fungicide use, downy mildew sometimes causes substantial losses in commercial lettuce production. At other times, disease pressure is low and most or all fungicide applications are unnecessary. In a two-year field study in the coastal valleys of California, we found that infection of lettuce by Bremia lactucae occurred primarily on days when leaf wetness ended later than 10:00 am. Weather variables other than leaf wetness were not consistently associated with infection. Environmental conditions were generally conducive for sporulation of the pathogen; thus, a warning system based on sporulation would not be useful. Since the infection stage was limited by morning leaf wetness, our downy mildew warning system will be based on predictions of leaf wetness in the morning. We conducted 7 field trials to test whether fungicide sprays (maneb and fosetyl-Al) timed according to predicted morning leaf wetness would give improved downy mildew control and/or lead to a reduction in the number of applications compared to a calendar-based spray schedule with three sprays per crop. Leaf wetness forecasts were generated using a dew simulation model with numerical weather forecasts from the National Meteorological Center as input. More than 90% of all days were correctly classified as days with or without a prolonged morning wet period. All wet periods that were missed in the forecasts were due to fog drizzle which was not included in the simulation model. The total number of sprays in the seven trials was reduced by more than 50% relative to the calendar-based schedule. There was no significant difference between the two fungicides and spray schedules in disease incidence or severity (except for one trial in Santa Maria, see below). Final disease levels were too low to generate differences in yield. In a study comparing different irrigation practices, we showed that the daily leaf wetness periods were longer and disease severity higher in fields under furrow than under drip irrigation, indicating that the microclimate may influence disease development when the mesoclimate is marginally conducive for infection. Thus, the network of weather stations and/or forecast locations needs to be fairly high for an accurate disease warning system. Research is currently underway to improve our forecasting system by using high-resolution weather forecasts as input for the dew model and considering leaf wetness caused by drizzle. In addition, the risk of infection will also be related to the availability of inoculum.

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