United Kingdom 
An apple tree in our garden has a large cavity where a branch was removed close to the base of the trunk at least 30 years ago. It is approx 5" diameter and approx 4" deep. A few years ago I cleaned it out and coated the interior with a resin, but this has all disappeared and the cavity is now deeper after the removal of some damp wood. The cavity attracts soil blown in the wind, so is permanently damp in the winter. Is there anything I can fill the cavity with before further deterioration kills the tree? I have attached a photo.
11 Nov, 2015
Answers
The problem is whoever cut that branch off did not do it correctly; they cut it flush and removed the branch collar. The tree is having a difficult time healing itself as a result - leaving the living tissue exposed to disease and rot. Don't use resin or any sort of dressing. That will make it worse - destroying the living tissue. Leave it be as stated above.
12 Nov, 2015
after 30 yrs I'd leave it alone too. the soil would be seeded with plants that will then take up some of the moisture you are worried about.
12 Nov, 2015
Probably fungi in the wound not uncommon in wounds of any tree. Leave it be. The tree will take care and has been taking care of this wound itself in its own fashion for thirty years as you say. Look what happened when you decided to intervene. I have seen trees a hundred years old or more with very large wounds from lightning strikes and large limb blowdowns and they will still be basking in the sun when I am gone. Not interfering with this will give your apple tree a chance to do the same:)
12 Nov, 2015