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Walnut twig beetle and a fungus killing black walnut trees

7th June 2019

The deadly thousand cankers disease, an emerging insect-fungus complex, is causing profound damage to black walnut trees not only in urban areas of California and other western states of USA, but in Pennsylvania, Tennessee and Virginia, according to a newly published review by University of California (UC) Davis-affiliated scientists and their colleagues. The article, “Status and impact of walnut twig beetle in urban forest, orchard and native forest ecosystems,” published in the Journal of Forestry, updates the spread of the disease, and chronicles the role of the bark beetle, Pityophthorus juglandis, and the canker-producing fungus, Geosmithia morbida, in killing walnut trees, especially black walnuts.

Native to southwestern United States and northern Mexico, the walnut twik beetle (WTB), about half the size of a grain of rice, tunnel into branches and trunks of walnut (Juglans) where they create galleries for mating and reproduction. They carry spores of the fungus, G. morbida, into their galleries, and the resulting fungal infection causes formation of cankers, which coalesce and girdle branches and stems.

Between 2005 and 2016, the disease killed nearly 60 percent of the 210 specimens of southern California black walnut mature trees in the USDA Agricultural Research Service’s National Clonal Germplasm Repository Juglans Collection near Winters. “This is only an estimate and the true proportion of the mortality is likely much higher, as only six of the 210 trees were rated as having healthy crowns in August 2016,” said lead author and forest entomologist Steven Seybold of the Pacific Southwest Research Station, USDA Forest Service, Davis, and a lecturer and researcher with the UC Davis Department of Entomology and Nematology.

Seybold noted that the disease is “unique because of its multifaceted negative impact on walnut trees involved in landscaping, food production, and forestry. Walnut trees are valuable ecologically and for food and timber, so the walnut twig beetle is a good model in which to study the impact of a bark beetle on forest and agro-ecosystem services.”

Taken from the excellent ISPP Monthly Newsletter – click here to check it out.