The UC Cooperative Extension in the Central San Joaquin Valley is testing a new risk model for the prediction and treatment timing of powdery mildew on processing and fresh market tomatoes.

Farm Advisor Michelle Le Strange in Tulare and Kings Counties is working with vegetable crops Farm Advisor Brenna Aegerter in San Joaquin County and UC Graduate Student Mike Davis to evaluate a powdery mildew risk forecasting model for a number of stations established throughout San Joaquin Valley tomato regions.

The trial compares fields treated according to alerts from the risk model to fields treated based on calendar sprays and untreated checks. The forecasting model uses weather stations to measure humidity and temperature and determine when conditions are right to treat for powdery mildew, based on the growth stage of the tomato plant, canopy cover, and climate situation in the field.

“PCAs and growers are very interested in this, and we’re getting great cooperation from growers,” Le Strange says.

“Growers spray for powdery mildew pretty much on a calendar -type basis. “The model’s goal is to minimize the number of sprays while minimizing the risk of powdery mildew.”

A similar risk assessment has been successfully used by grape growers to predict when powdery mildew control is needed and intervals for powdery mildew control measures.