Walnut Twig Beetle and Fusarium
The walnut twig beetle (Pityophthorus juglandis) is causing major decline of Juglans nigra in Boulder, Westminster and Arvada and is spreading to other areas in Colorado. In addition, there have been multiple trunk and branch cankers caused by the fungus Fusarium solani on trees. This fungus has caused long vertical cankers on J. nigra during periods of stress.
These cankers may not be immediately evident because the bark on the trunk doesn't slough off during early stages of colonization. However, the darkly stained cambium in the trunk or large branches can be seen by removing the outer bark. These cankers may be very long and extensive.
Whether the insect or canker comes first is still debatable, but we suspect the canker follows the insect. However, we have no evidence yet that the beetle transmits the fungus.
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Updated Tuesday, October 13, 2015