I have read here that moth balls around the base of the trunk will deter borers on peach/nectarine trees. Don't know why it wouldn't help with apple trees. That's what I'm going to try on my apple trees.

I've never seen borers go after the base of an apple- not that they never do, but I look at an awful lot of apple trees here in southeastern NY and the borers have always struck further up the tree for apples and are seldom lethal in home orchards IME.

From your description it would seem to be borers but maybe there's something else going on as well. What rootstock were these trees on. Were they growing vigorously before they died?

Fedco has a recipe for a white wash with joint compound in it. I think it helps on smooth barked, young trees. Good for SW injury also.
Bberry

You can also use nylon screen taped to the trunks with the rubber electric tape (not vynyl which can girdle). If you pull the soil away a bit to get this pretty low it should completely eliminate crown type girdling by borers.