Haunted appletrees are located in Avalon Isle (SI) just across the border from Isle of Glass at loc #55561, 50795.
Part of the "quick quests" given by the officers in Camelot. Quest: Harrowing the Haunted Appletree (Level 24). Step #1 - Locate and destroy a haunted appletree in the lands surrounding Avalon City.
Upon killing the Haunted Appletree you immediately complete the quest. You do not have to return to the Quest-giving NPC.
Although I got this quest at level 24, I completed it at level 25 and received:
216,865 XP.