Symptoms and Signs
As the name suggests, symptoms of thousand cankers disease include numerous cankers resulting from fungal infection at multiple points of pathogen introduction. The pathogen is introduced to the inner bark by the walnut twig beetle, Pityophthorus juglandis, a phloem-feeding insect that completes its life cycle in the tree. A small, push pin-sized hole is associated with each canker, usually denoting the entrance hole for the walnut twig beetle vector. Cankers often bleed, leaving dark ooze and staining on the outer bark surface. These cankers may coalesce over time and girdle individual branches and main scaffolds, leading to decline of the tree crown. The outer bark can be peeled away to see walnut twig beetle galleries in the phloem, a key feature for diagnosing the thousand cankers disease complex.
Comments on the Disease
The pathogen responsible for thousand cankers disease has not been found on walnut trees in the western USA in the absence of the walnut twig beetle vector. Walnut twig beetle attacks all species of walnut and wingnut, and the disease has been observed on Northern California black and Paradox rootstocks as well as on English walnut. The disease has been recorded throughout the commercial walnut-growing regions in California.
Currently, good cultural practices and sanitation of infested materials are the primary strategies for disease management within orchards and also for prevention of spread of the disease and vector to regions with low rates of infection.
- Prevent tree stress through proper irrigation and fertilization.
- Remove trees with less than 50% live crown to reduce the buildup of walnut twig beetles and inoculum in the trunk and larger scaffold branches.
- Remove infested woodpiles and prunings during the winter months in advance of the primary period of walnut twig beetle emergence and flight activity in the spring (April–June).
- When possible, burn infested material to reduce the presence of primary inoculum in the vicinity of orchards.
Chemical control with either fungicides or insecticides is not recommended for management of thousand cankers disease.