My neighbor has a huge 35 year old ash dominating the front yard. It has 4 upward-reaching branches (each 2 ft. diameter) from the trunk. A well had developed in the crotch of the tree which is beginning to rot from water accumulation. Is there any substance that could be filled into this well cavity to seal it off and slow the deterioration of this tree?

How deep is the well?

No jokes, folks.

Personally I'd just clean it out myself if it's not deep and easily accessible. Keeping leaf litter out might help too.

I've heard of someone taking care of one like this by dremmeling a hole in part of it, putting in a piece of PVP that fit VERY snug, so as not to leave room for bad stuff in between it and the wood, and they let it drain that way. Not sure if that's a good option though- I'd defer to Howard on that one.

This is why it is recommended you get straight single trunks where possible and prune branches that form that kind of crotch when they appear, so they don't become a problem later.

In that last bad windstorm, our neighbor's hackberry tree fell on us because it had a large crotch like that that had rotten. You could see the nice white wood give way to brown stained wood and rot where it used to be. We got some pics, I'll have to see if I can host some.