According to the scant info I have in an out-of-print Christmas Carol book I grew up on and cherish more each year, because of its history notes of each song and the really unusual songs which are in it,"Wassail is derived from the Anglo-Saxon Waeshael(Be in health)."

It doesn't say anything about apple trees, but it does say the carol, "Here we come a'caroling" is derived from "Here we come a'wassailing". It goes on to say wassailing was a pagan Yule custom of the British Isles. "It was customary for groups of revelers to go about offering a hot drink from the wassail bowl and a song, in exchange for gifts".While at the time the book was printed it says the custom was still practised in Englad, "it has been largely displaced by caroling". From what I can gather, the drink itself was usually spiced ale.

The book is The Christmas Carolers' Book by Torstein O. Kvamme, Hall & McCreary Company, Chicago, circa 1930/40's.

I hope this was some of what you were looking for.

katlaughing