Walnut Twig Beetles (Pityophthorous juglandis)
Walnut Twig Beetles were found to be killing trees in Colorado as early as 2003. Walnut Twig Beetles vector the disease Thousand Cankers disease caused by a fungus, Geosmithia morbida. This is a newly recognized disease 2008 of primarily Black walnut trees Juglans nigra. Thousand Cankers disease has produced widespread death of black walnut trees in many western states during the past decade. Other species of walnut such as Arizona walnut, English walnut, and California walnut have all shown varying degrees of susceptibility to this fungus.
The affected area of the walnut tree is shallow and confined to thephloem so that it can easily be missed if inspection cuts are made too deeply into the sapwood. Minor weeping may occur at points where walnut twig beetles enter the bark but often no symptoms are associated with the beetle attacks aside from minute entry wounds or star shaped cracks. Early symptoms are yellowing of leaves and foliage thinning of the upper crown of the tree. As the disease progresses larger limbs are killed which may have dead, flagging leaves associated with them. In end stages the fungus may be introduced into the trunk and large cankered areas develop in the trunk. In susceptible hosts, such as black walnut (Julgans nigra), trees usually die within three years after initial symptoms are observed in the crown of the tree.
A newly recognized fungus, Geosmithia, kills a localized area in the phloem just under the bark in >2cm wood after introduction by the walnut twig beetle. These dead areas often overlap or coalesce from numerous strikes (35 insects per square inch of wood) causing nutrient disruption to foliage and thus leading to branch dieback.
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