Beecrofter wrote:

Sounds more like a canker disease

I assume that this web site describes what you are referring to:

http://www.pfc.forestry.ca/diseases/...canker6_e.html

That doesn't look like what I have.

Basically, I had a complete dissappearance of the wood (sap wood?) in
a uniform manner all the way to the wood core, where there are quite
prominent worm trails etched in the surface of the core
(heart-wood?). This gash or dissappearence was completely hidden
under the outer surface of bark (which was 1/8 inch to 1/4 inch
thick). The outer bark surface was COMPLETELY NORMAL LOOKING and you
wouldn't know that something really wrong was going on underneath.
There were no holes in this over-covering section of bark nor anywhere
else I've looked. No fruiting bodies in the 5 years I've been the
owner of this tree.

The cavity formed by this gash was moist and in places seemed to
retain fluid. The sides of the gash resembled a thin layer of dry rot
(probably due to water retention and lack of air circulation). I
scraped away the thin layer of rot and exposed what looked like
healthy sap wood. I expect the exposed sap wood will harden
relatively quickly.