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Twig beetle and black walnut disease
Black walnut (Juglans nigra) is native to much of the eastern deciduous forest, is a majestic tree in our landscape, and also produces prized wood. It provides valuable hard mast (nuts) for wildlife, although most walnuts sold in the U.S. for human consumption are from orchards of English or Persian walnuts (J. regia).

In the past few years, a disease complex has been killing black walnuts planted outside the species' natural range, especially in urban areas along the eastern slope of the Rocky Mountains in Colorado. Apparently the few walnut street trees in Colorado Springs have died. In Boulder, the city removed 250 street trees during the period 2004-2007; it condemned over 400 additional trees this summer. Trees in Denver are starting to die. Outbreaks have also been confirmed in Utah, Boise, and near Sacramento; and are suspected in New Mexico and Oregon's Willamette Valley.

Dr. Ned Tisserat of Colorado State University has determined that the trees have been killed by a disease complex consisting of the walnut twig beetle (Pityophthorus juglandis) and a fungus in the genus Geosmithia. This beetle is native to the Southwestern United States and neighboring Mexico; it is apparently a minor pest of Arizona walnut (J. major). The origin of the fungus is unknown, although visits to Arizona appear to have recovered the fungus. Arizona walnut does not appear to be harmed by the complex. Thus, the disease complex might be native to that region, although it is too early to know for sure.

Preliminary inoculation of butternut (J. cinera) indicates that this species might not be as vulnerable as black walnut.

Studies continue on both host range and possible origin of the fungus. Geosmithia spp. are associates of bark beetles of hardwood trees but have not previously been reported as a pathogen of Juglans or an associate of P. juglandis. Dr. Tisserat and colleagues are collaborating with entomologists in California to determine the flora associated with P. juglandis in its native range and working with a researcher in the Czech Republic on fungal taxonomy.

Information provided by:
Ned Tisserat, Professor of Plant Pathology
Bioagricultural Sciences and Pest Management
Plant Sciences 1177
Ft. Collins CO 80523
970-491-6527
FAX 970-491-3862
ned.tisserat(at)colostate.edu