The agent responsible for decline appears to be Hypoxylon canker, a fairly common pathogen that can kill trees that have become stressed by some other issue, like drought, over-watering or poor soil drainage during rainy periods, root injury from digging/construction, or other environmental factors. If a certified arborist can confirm Hypoxylon, there is no treatment. Dead wood can and should be removed promptly (at the very least for safety), but when an active infection is present in still-living wood, especially a trunk, the tree might need to be removed instead. This type of wood decay or infection cannot be pruned out in the sense of carving-out affected trunk wood; such a wound would worsen the risk or pace of decay and the tree cannot heal such an injury.

If the location of this tree does not risk harming people or damaging property when it eventually falls, keeping it is a viable and cost-saving option, since dead and dying wood can be of great value to wildlife, and the fungus responsible for this canker won't necessarily be a threat to nearby trees if they remain healthy. Otherwise, an arborist will likely suggest that a diseased tree be removed, both because it cannot be cured, and because of safety concerns. Arborists can help make a diagnosis by inspecting a tree in person and looking for sources of tree stress that might be able to be alleviated, but they can't treat every condition, and sometimes tree decline has been occurring far longer than any obvious symptoms have been detectable, which makes tracing-back the origin of a disease or pest issue harder to do.

Miri