Black Walnut Mortality in Colorado Caused by the Walnut Twig Beetle and Thousand Cankers Disease

Since 2001, widespread mortality of black walnut (Juglans nigra) has been reported in Colorado, USA. Affected trees initially show a yellowing and thinning of leaves in the upper crown, followed by twig and branch dieback and ultimately tree death. We report that this mortality is the result of a combination of an expanded geographic range of the walnut twig beetle (Pityophthorus juglandis), its aggressive feeding behavior on black walnut, and extensive cankering caused by an unnamed Geosmithia fungus associated with the beetle. Geosmithia was consistently recovered from the bodies of P. juglandis and this insect introduces the fungus into healthy trees during gallery formation. This is the first report of Geosmithia as a pathogen of black walnut. We propose the name Thousand Cankers to describe this disease because mortality is the result of bark necrosis caused by an enormous number of coalescing branch and trunk cankers. A second pathogen, Fusarium solani, was isolated from the margins of elongate trunk cankers during the final stages of decline, but not from cankers surrounding beetle galleries. Thousand Cankers Disease is eliminating black walnut along the Front Range of Colorado and poses a grave risk to this species in its native range in eastern North America should the insect/Geosmithia complex be introduced. Introduction Black walnut (Juglans nigra) is one of the most highly valued timber species in North America (8). The wood is prized for use in cabinetry, gunstocks and other finished wood products. The nuts are also an important nutritional source for wildlife. Black walnut is native to eastern North America, and is widely distributed on deep alluvial soils from New England and the Appalachian Mountains west to the Great Plains and from the Canadian border south into Texas and into the Florida panhandle (8). It has been widely planted outside its native range in the western United States as an ornamental and timber tree, and for nut production. As early as 2001 arborists and foresters in Boulder and Colorado Springs, Colorado noted decline and mortality of black walnut. Affected trees initially showed a yellowing and thinning of leaves in the upper crown, followed by twig and branch dieback (Fig. 1). Over a period of several years, progressively larger branches were killed and affected trees eventually died. Trees typically were killed within two years after initial symptoms developed although smaller trees (< 10-cm diameter at breast height), or those growing on sites prone to drought stress, declined more rapidly. Basal sprouts often developed on trees in advanced stages of decline, or from stumps of removed trees, but these sprouts also wilted and died within one or two years after emergence. 11 August 2009 Plant Health Progress By fall 2008, over 700 trees were killed and removed in Boulder alone, representing the majority of black walnut in that municipality. Similar mortality rates occurred in Colorado Springs, which now has few surviving black walnuts. Tree decline is now occurring in several communities in the Metro Denver area. The disease was also observed on black walnut in the city of Delta approximately 400 km west of Denver. Black walnut mortality was initially attributed to a drought that persisted from 2000-2003. However, tree deaths commonly occurred at sites receiving supplemental irrigation, and mortality rates accelerated after annual precipitation returned to normal in 2004. This suggested an alternate cause for the unusual tree mortality. We report here on the association of the walnut twig beetle and the canker fungi Geosmithia and Fusarium solani with mortality of black walnut in Colorado, and the potential threat this disease complex could have on black walnut if it were introduced into the native range of this tree