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Thousand Cankers Disease

Whitney Cranshaw, Colorado State University, Bugwood.org
Geosmithia sp. Dieback and mortality of eastern black walnut (Juglans nigra) in several Western States have become more common and severe during the last decade. A tiny bark beetle is creating numerous galleries beneath the bark of affected branches, resulting in fungal infection and canker formation. The large numbers of cankers associated with dead branches suggest the disease’s name—thousand cankers disease.

The principal agents involved in this disease are a newly identified fungus (Geosmithia sp. with a proposed name of Geosmithia morbida) and the walnut twig beetle (Pityophthorus juglandis). Both the fungus and the beetle only occur on walnut species. An infested tree usually dies within 3 years of initial symptoms.
Thousand cankers disease has been found in many Western States  and is making it's way to this area. The first confirmation of the beetle and fungus within the native range of black walnut was in Tennessee (July 2010). The potential damage of this disease to eastern forests could be great because of the widespread distribution of eastern black walnut, the susceptibility of this tree species to the disease, and the capacity of the fungus and beetle to invade new areas and survive under a wide range of climatic conditions in the west.

The three major symptoms of this disease are branch mortality, numerous small cankers on branches and the bole, and evidence of tiny bark beetles. The earliest symptom is yellowing foliage that progresses rapidly to brown wilted foliage, then finally branch mortality. The fungus causes distinctive circular to oblong cankers in the phloem under the bark, which eventually kill the cambium. The bark surface may have no symptoms, or a dark amber stain or cracking of the bark may occur directly above a canker. Numerous tiny bark beetle entrance and exit holes are visible on dead and dying branches, and bark beetle galleries are often found within the cankers. In the final stages of disease, even the main stem has beetle attacks and cankers. 
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