UC Delivers

Thousand cankers disease attacks commercial walnut in California

The Issue

A new disease affecting walnut trees, "thousand cankers disease," poses an environmental threat to natural ecosystems containing native black walnut species and an economic threat to commercial English walnut growers in California. If introduced to the Midwest, it may also affect commercial production of black walnut. Thousand cankers disease was first associated with widespread mortality of black walnut in Colorado, but is now known to occur in nine western and three eastern states. The disease is caused by a fungal pathogen transmitted by the walnut twig beetle, which carries spores on its back as it bores into the tree. Because numerous beetles attack a single tree, the pathogen is introduced at the many points of beetle entry. Consequently, many cankers form as the fungus colonizes and kills plant tissue.

What Has ANR Done?

UCCE farm advisors, a UC Davis faculty member and a U.S. Forest Service scientist have teamed up to document the geographic distribution of the disease, identify susceptible walnut types and monitor the flight patterns of the beetle in commercial orchards. Farm advisors work directly with growers and pest control advisers to maintain a network of beetle traps containing a male-produced pheromone. The known geographic distribution of the disease in California is better understood, in part due to extension efforts to provide images of the dramatic symptoms to the news media. Serving as liaisons between grower and PCA communities and university and government researchers, UCCE farm advisors continue to contribute samples of the insect and the pathogen to specimen collections that will help scientists understand the history of disease spread in North America.

The Payoff

California efforts pioneer nationwide detection of new disease outbreaks

The year-round monitoring of walnut twig beetle in California orchards provides valuable insight into the epidemiology of the disease. The monitoring program has provided the framework for guidelines now used nationwide for early detection of new forest infestations. The research and extension activities have raised awareness of growers and PCAs, prompting them to quickly remove infected trees from their orchards. Before 2011, all the diagnoses of thousand cankers disease in Tulare County were made on standing trees. Now, the majority of recent positive diagnoses are made on felled trees, with growers seeking disease confirmation rather than identification.

Contact

Supporting Unit: Tulare County

Elizabeth Fichtner, (559) 684-3310,
ejfichtner@ucdavis.edu