Growers packed the room at a recent meeting in Yolo County, Calif. to learn how to control Botryosphaeria (‘Bot’) dieback, a yield-reducing fungal disease attacking some of the state’s walnut orchards.

The meeting held in Woodland was organized by the University of California Cooperative Extension (UCCE).

“Bot has been around for years, but has slowly and persistently developed in walnuts,” said Themis Michailides, University of California Cooperative Extension plant pathologist based at the Kearney Agricultural Research and Extension Center in Parlier.

“When growers realized they were losing yields, it was a point for the industry to do some research and try to control it.”

Bot in commercial walnut orchards is found from Glenn County in the north to Ventura County in the south, he says.

The pathogen also infects pistachios, almonds, and pecans, plus 35-plus native trees and shrubs, including sequoias and blackberries.

Michailides points out that the disease was not introduced by agricultural crops since it has been present in the native species.

“These fungi were in California before the crops were introduced,” he said.