As the known geographic range of thousand cankers disease continues to expand in California, the disease has also emerged in three East Coast states, now threatening eastern black walnut (Juglans nigra) in its native range. The disease, caused by the fungus Geosmithia morbida, is vectored by the walnut twig beetle (wtb) (Pityophthorus juglandis). The association between Geosmithia species and bark beetles is a worldwide phenomenon; however, G. morbida has only been observed in the USA, and is specific to Juglans (walnut) species.

Thousand cankers disease timeline

Over the past decade, thousand cankers disease has been associated with decline and mortality of eastern black walnut in several western states. Although eastern black walnut is native to the eastern United States, it is planted extensively in landscapes in western states and serves as a rootstock for commercial production of English walnut (Juglans regia) in California. In 2008, the disease was first observed in California on northern California black walnut (Juglans hindsii) in Yolo County. By the close of 2009, however, the disease was known to affect commercial walnut orchards throughout the state, with infections documented on both black and Paradox rootstocks and on several varieties of English walnut.

Since 2010, thousand cankers disease has been reported in three East Coast states: Tennessee, Virginia, and Pennsylvania. Although the pathogen and insect vector are native to North America, they are still exotics when introduced to eastern landscapes. In eastern landscapes, the pathogen and wtb vector are introduced, exotic species attacking eastern black walnut in its native range.

Active quarantines

In Tennessee six counties are currently under quarantine in the Knoxville area, and ten surrounding counties serve as a buffer zone. In Virginia seven counties are quarantined around the Richmond area. Bucks County, Pennsylvania is also under quarantine. In these situations, the quarantine prevents the movement of walnut wood, and associated materials (ie. stumps, roots, mulch, etc) out of quarantined counties. The success of quarantine activities in preventing the spread of thousand cankers disease is challenged by the fact that spores may be transmitted over some distance by wtb flight.

Ohio is considering implementation of an exterior quarantine for thousand cankers disease, which would restrict movement of walnut materials from infected counties in the 11 states known to have thousand cankers disease, including California. Nuts, nut meats, hulls, processed lumber and finished wood products (ie. furniture or gunstocks) would be exempt from quarantine.

The take-home-message for California walnut growers is that the pathogen and wtb are not moved in walnut meats or hulls; therefore, these commercial products do not pose a risk of disease transmission. To prevent the spread of the disease to uninfested regions of the state and country, however, infected, untreated wood should not be moved.