![]() The walnut twig beetle is about the size of half a grain of rice. “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." They carry spores of the fungus into their galleries, and the resulting fungal infection causes formation of cankers, which coalesce and girdle branches and stems.īetween 20, 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, Seybold said. ![]() Walnut twig beetles (WTB) tunnel into branches and trunks of walnut ( Juglans) where they create galleries for mating and reproduction. Native to southwestern United States and northern Mexico, the bark beetle, about half the size of a grain of rice, “has invaded urban, orchard and native forest habitats throughout the United States, as well as Italy,” 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. 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. ![]() ![]() (Photo by Kathy Keatley Garvey)The deadly thousand cankers disease, an emerging insect-fungus complex, has caused profound damage to black walnut trees not only in urban areas of California and other western states, but in Pennsylvania, Tennessee and Virginia, according to a newly published review by UC Davis-affiliated scientists and their colleagues. Thousand cankers disease is killing a giant black walnut tree, estimated to be 150 years old, on the 100 block of E Street, Davis. ![]()
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