Potential Impacts of Thousand Cankers Disease on Commercial Walnut Production and Walnut Germplasm Conservation

Thousand cankers disease (TCD) is a recently identified insect/fungal disease complex responsible for mortality of eastern black walnut trees, Juglans nigra, planted in non-native areas of the western United States. The presence in California of both the insect vector, the walnut twig beetle, Pityophthorus juglandis, and the fungal pathogen, a species of Geosmithia, raises concerns regarding the potential impact of the disease on commercial walnut production and the long term health of other walnut species native to California and American Southwest. Movement of the disease eastward could threaten native J. nigra populations in hardwood forests of the eastern United States. Identification of both the vector and the fungal pathogen of TCD from trees in the USDA Germplasm Collection near Davis, CA may impact ex situ germplasm management.