Thousand cankers disease of walnut - a new threat from afar

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

An insect-vectored fungal disease has been found responsible for widespread decline and death of black walnut trees in the western United States over the past decade. The insect vector is the walnut twig beetle (Pityophthorus juglandis), a tiny bark beetle native to the southwestern United States and northern Mexico. The disease is caused by a newly discovered species of Geosmithia fungus whose origins are unknown. Together, they create an insect-disease complex called “thousand cankers disease” for the vast number of cankers that develop on – and ultimately kill – infected trees. Significant losses of black walnut are occurring in the western United States, despite it being planted on a very scattered basis. The current easternmost range of thousand cankers disease is Colorado’s Front Range, from Boulder to Colorado Springs, 400 miles from the native range of black walnut in the eastern United States.

Agricultural officials from Michigan and other states throughout the Midwest recently agreed to band together and develop regulations to keep this complex out of the native range of black walnut. Effective May 17, the Michigan Department of Agriculture established a quarantine, which places restrictions on movement of black walnut logs, green lumber and nursery stock from the quarantined area. This includes the states of Arizona, California, Colorado, Idaho, Nevada, New Mexico, Oregon, Washington and Utah. The quarantine also regulates shipment of hardwood firewood from the nine regulated states. Exempt items include nuts, nut meats, processed lumber and finished wood products without bark including walnut furniture and instruments.

MDA asks everyone to help protect our valuable timber resources. Be on the lookout for unusual decline and mortality in black walnut trees, and contact MDA or Michigan State University Extension if you notice sudden losses of walnut trees.

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