The non-native walnut twig beetle (Pityophthorus juglandis) carries a fungus (Geosmithia morbida) to trees in the Juglans species, including black walnut (Juglans nigra) causing Thousand cankers disease (TCD).
Thousand cankers disease kills black walnut, and Minnesota has approximately six million black walnut trees at risk. Black walnut is highly valued for lumber and veneer, with one to two million board feet of walnut harvested annually.
Trees are slow to show symptoms, and the disease can be present in a tree for seven years or more before flagging branches signal an infection. Although Minnesota has an exterior quarantine that prohibits bringing any black walnut (except processed lumber or wood products) originating in areas known to have TCD into Minnesota, monitoring black walnut for any symptoms is an important part of preventing the spread of TCD.
Black walnut with Thousand cankers disease
Photo: Whitney Cranshaw, Colorado State University, Bugwood.org
Walnut twig beetle
Photo: Steven Valley, Oregon Department of Agriculture, Bugwood.org
Black walnut branch with Thousand cankers disease
Photo: Mary Ann Hansen, Virginia Polytechnic Institute and State University, Bugwood.org