UC Delivers

Outbreak of new disease affects almond orchards statewide

The Issue

During wet years in the 1990s a new and unknown plant disease, later identified as an anthracnose fungus, occurred throughout most of California's almond growing region. It destroyed flowers and developing nuts, producing toxins that killed almond tree branches up to two inches in diameter. Losses continued throughout the season whenever rains occurred. Growers were at a complete loss for control of this disease and believed they might have to remove the affected orchards. Processors also were concerned because nuts infected near harvest could have internal discoloration that was difficult to detect and reduced product quality.

What Has ANR Done?

A UCCE farm advisor recognized the problem as a new disease on almond trees and a plant pathologist at UC Riverside succeeded in identifying the pathogen as the fungus Colletotrichum acutatum. UCCE, UCR researchers and the almond industry then established a partnership to determine the disease life cycle and develop control strategies. Since most registered fungicides had no effect on the disease, initial field work focused on identifying materials that could begin to control anthracnose on almonds. These trials led to the eventual registration of four new fungicides that were effective and also more selective and environmentally friendly. Potential resistance to the fungicides was managed by rotating use of materials. Meanwhile, research to identify susceptibility of almond varieties indicated where efforts were most needed to help control the problem, and which varieties growers might choose to plant in new orchards. Other field trials confirmed that sprinkler irrigation contacting the tree canopy increased disease pressure. The irrigation industry developed new sprinklers with lower trajectories and growers modified their sprinkler systems. UCCE also determined that the fungus over-winters in dead wood where infected nuts had been the previous year. Field trials confirmed that meticulous pruning to remove the dead infected wood is essential. As these results unfolded, the UC research pathologists and UCCE farm advisors have communicated their findings at numerous local meetings, in newsletters, in trade journals and at the statewide Almond Industry Research Conferences. Work continues to investigate additional fungicides and to develop a disease forecasting model that will result in still more effective control with fewer sprays.

The Payoff

Identifying the disease, developing controls led to successful disease management..

This comprehensive program involving cultural controls and fungicide rotation has led to adoption of effective management strategies by almond growers. Through UCCE's work, the almond industry now knows how to handle this problem and substantial crop loss has been prevented. Returns to growers have improved and production of top quality almonds for the consumer is the end result.

Contact

Supporting Unit: Butte County & UC Riverside Plant Pathology Department

Joe Connell, Farm Advisor, UC Cooperative Extension, 2279-B Del Oro Avenue, Oroville, California 95965, (530)538-7201, jhconnell@ucdavis.edu

Dr. Jim Adaskaveg, Associate Professor, Department of Plant Pathology, University of California, Riverside, CA 92521, (909)787-7577, jim.adaskaveg@ucr.edu