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Matching Causes with Symptoms: Research Improves Diagnosis of Declining Eastern Black Walnut

Eastern black walnut branch sample collected for dissection, observation of damage types present, and isolation for fungi associated with each damage type.

Visual diagnosis of thousand cankers disease (TCD) in a declining black walnut is difficult in the Eastern United States because the general dieback symptoms are typical of several different disease and insect problems. Work by a Northern Research Station scientist and her partners will aid plant health specialists and laboratory diagnosticians in determining causal agents of branch dieback in this valuable hardwood species

Diagnosis of thousand canker disease in eastern black walnut is complicated by “look-alike” symptoms of other walnut diseases and insect pests. Inconsistent isolation of the TCD fungus from branches and stems of walnut also hampers laboratory confirmation of the disease. Eastern black walnut is one of the most commercially valuable hardwood tree species in the United States. This species is highly susceptible to TCD, which leads to branch dieback and tree mortality. TCD results from an interaction of a canker-causing fungus (Geosmithia morbida) and attacks by the walnut twig beetle (WTB) that transmits the fungus. However, other canker fungi and insect pests of black walnut can cause similar-looking branch and tree crown symptoms. Examination of branch samples for galleries of the WTB and for Geosmithia cankers and isolation of the pathogen are critical for TCD diagnosis. Visual observations of damage on dissected branches from TCD-affected black walnut in eastern Tennessee were documented photographically and in writing, while isolation of fungi was attempted from representatives of each damage type observed. Three known pathogens of black walnut were found associated with two or more damage types: G. morbida, members of the Fusarium solani species complex, and Botryosphaeria dothidia. The latter two pathogens may exacerbate branch dieback and mortality of diagnosed TCD trees in eastern states. This research enhances walnut field surveys and allows laboratory scientists processing samples to more accurately diagnose the cause or causes of declining black walnut.

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External Partners

  • Margaret E. McDermott-Kubeczko, University of Minnesota – St. Paul
  • Sharon E. Reed, University of Missouri
  • William E. Klingeman, University of Tennessee