Within the past decade an unusual decline of black walnut (Juglans nigra) has been observed in several western states. Initial symptoms involve a yellowing and thinning of the upper crown, which progresses to include death of progressively larger branches. During the final stages large areas of foliage may rapidly wilt. Trees often are killed within three years after initial symptoms are noted. Tree mortality is the result of attack by the walnut twig beetle (Pityophthorus juglandis) and subsequent canker development around beetle galleries caused by a fungal associate (Geosmithia sp.) of the beetle. The outbreak of thousand cankers disease in Boulder County may result in a total loss of black walnuts in that area.
Below is a quote from a recent Pest Alert- Walnut Twig Beetle and Thousand Cankers Disease of Black Walnut.
"Because of the extended period when adult beetles are active and the extensive areas of the trees that are colonized, foliar insecticide spray applications likely have limited effectiveness. Furthermore, colonization of the bark and cambium by Geosmithia may continue even if adult beetles or larvae are killed by the insecticide. The involvement of the fungus also will likely limit the ability of systemic insecticides to effectively move in plants in a manner to kill beetles. Rapid detection and removal of infected trees currently remains the primary means of managing thousand cankers disease. Drenching insecticides applied to the trunk in late summer may also be useful in killing beetles as they seek winter hibernation quarters."
