For information of this sort I usually reach first for Robert Graves' The White Goddess and, sure enough, there's quite a bit of information about the wild apple tree. As a sign of authority, it may be that he's carrying it because it is one of the seven 'chieftan' trees (the felling of which carried the death penalty). It may be that it is associated with his name, Queu, as it stands for Q or Quert in the Ogham alphabet. Whether this would be common knowledge to the audience ta the time, I can't say.