Plant diseases may be caused by fungal, bacterial, or viral pathogens. They are transmitted from plant to plant by wind, water, soil, or insect vectors, often entering into the plant by way of natural openings or wounds. One of the most prevalent soil borne diseases affecting walnut production in California is crown gall, caused by the bacteria Agrobacterium tumefaciens. Crown gall manifests itself as large irregular growths at the base of the tree and reduces the flow of water and nutrients to the tree. All rootstocks are susceptible, but Paradox seedling is more susceptible than Northern California black walnut (UC IPM website, 2012). Other important soil borne fungal diseases causing crown and root rot are Phytophthora and Armillaria mellea (oak root fungus).
Walnut blight (Xanthomonas juglandis) is the most common nut and foliage disease. It is most common in wet and humid spring conditions. Cultivars with early leaf dates tend to be more susceptible to walnut blight, although fungicide treatments are effective in managing the diseases in most years. Other diseases that may affect the trunk, scaffolds and branches include deep bark canker (Brennaria rubrifaciens), shallow bark canker (Brennaria nigrifluens), and branch wilt (Hendersonula Toruloidea) (Teviotdale and Schroth 1998).
The most important viral disease of walnut is blackline, caused by the cherry leafroll virus. Blackline disease is prevalent in California coastal regions and San Francisco bay area counties. The virus is transmitted through pollen. Viral infections are asymptomatic in the English walnut scion but the rootstock has a hypersensitive reaction killing the infected cells and girdling the tree at the graft union. This disease can be avoided by growing trees on their own roots.
UC Statewide IPM Program: How to Manage Pests: Walnut
These photos of some of the major pests are accessed from the UC ANR Repository, are available courtesy of University of California research and extension personnel and programs, including the UC Statewide IPM Program. Photo information, including the photographer, is displayed when the larger image is viewed.