Thousand Cankers Disease

In walnut trees across Western Washington, you may notice the telltale signs of thousand cankers disease: numerous small cankers appearing on trunks and branches, often with amber-colored ooze at their centers. These cankers start as tiny lesions but accumulate over time, giving the disease its name. The small bark beetles that vector the disease work year-round in our region, though activity peaks during warmer months. An infected tree might look deceptively healthy at first, with full foliage despite the cankers forming beneath the bark.

If you spot these symptoms on a walnut, the disease is already progressing and will eventually kill your tree; there is no cure once infection takes hold. What matters most is preventing spread to other walnuts in your area. Do not move firewood, chips, or branches from infected trees to other locations, and if you must remove an infected tree, chip the material promptly as the beetles cannot survive in properly dried chips. The key to managing this disease is prevention: avoid planting susceptible walnut species in areas where thousand cankers disease is already established.