Maybe it has chestnut blight. Otherwise, when the top of a plant goes bad very often it is due to the situation at the roots.

My understaning was that blight was not found in Washington/Oregon...there are numerous healthy trees of American and European chestnut here. Tree was from a grower in Onalaska WA

The largest American chestnut known in the state died quickly some years ago. Chestnut blight? Undetermined. The Spanish chestnut with the largest trunk measured at the time is also gone. Yesterday I participated in the measurement of a contender for a replacement. It has numerous dead side branches throughout the canopy, of undetermined cause. Although sporting a trunk about 6' in diameter it is clear that it was grown as a sort of pollard for some years, and has numerous large, rotten stubs sticking out of the existing structure showing where it was viciously re-topped in the past (the current crown is over 80' wide, so this brutality ended some time ago). A book on local properties shows the house (dating from 1902) on the site, said the original owner brought the tree from back East (presumably at the same time).

Remaining American chestnuts of any size are greatly outnumbered by Spanish chestnuts in this state. There was also a hybrid between the two named 'Paragon' and circulated in pioneer times, that produced a tree of similar appearance to pure American chestnut trees.

Thinking about it again today I realize 'Paragon' would be the hybrid walnut that is also seen making landmark specimens here. Otherwise what I said about the hybrid chestnut trees is right.