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. Accepted for publication 26 May 2009. Published 11 August 2009.

Widespread morbidity and mortality of Juglans nigra has occurred in the western USA over the past decade. Tree mortality is the result of aggressive feeding by the walnut twig beetle (Pityophthorus juglandis) and subsequent canker development around beetle galleries caused by a filamentous ascomycete in genus Geosmithia (Ascomycota: Hypocreales). Thirty-seven Geosmithia strains collected from J. californica, J. hindsii, J. major and J. nigra in seven USA states (AZ, CA, CO, ID, OR, UT, WA) were compared with morphological and molecular methods (ITS rDNA sequences). Strains had common characteristics including yellowish conidia en masse, growth at 37 C and absence of growth on Czapek-Dox agar and belonged to a single species described here as G. morbida. Whereas Geosmithia are common saprobes associated with bark beetles attacking hardwoods and conifers worldwide, G. morbida is the first species documented as a plant pathogen.

Utley, C, Nguyen, T., Roubtsova, T, Coggeshall, M., Ford, T. N. 2013. Susceptibility of walnut and hickory species to Geosmithia mórbida. Plant Dis. 97:601-607.Thousand cankers disease (TCD) of walnut is a result of feeding in the phloem by the walnut twig beetle (WTB), Pityophthorus juglandis, and subsequent canker formation caused by Geosmithia mórbida around galleries. TCD has caused extensive morbidity and mortality to Juglans nigra in the western United States and, in 2010, was discovered in the eastern United States, where the tree is a highly valuable timber resource. WTB and G. mórbida also have been found in / regia orchards throughout major production areas in California, and the numbers of damaged trees are increasing. We tested the susceptibility of walnut and hickory species to G. mórbida in greenhouse and field studies. Carya illinoinensis, C. aquatica, and C. ovata were immune. All wal-nut species tested, including / ailantifolia, J. californica, J. cinérea, J. hindsii, J. major, J. mandshurica, J. microcarpa, J. nigra, and J. regia, developed cankers following inoculation with G. mórbida. J. nigra was the most susceptible, whereas / major, a native host of the WTB and, presumably, G. mórbida, had smaller and more superficial cankers. Canker formation differed among maternal half-sibling families of / nigra and / cinérea, indicating genetic variability in resistance to G. mórbida. Our inoculation studies with G. mórbida have corroborated many of the field observations on susceptibility of walnut and hickory species to TCD, although the ability of the WTB to successfully attack and breed in walnut is also an important component in TCD resistance.

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