Goblins are natural allies of the forces of Angmar. The encampment of Grothum far to the north-east, houses such goblins. Go there and ask what you can do to assist.
'We have our methods for corrupting and the goblins have theirs. Let us ask their leader at Grothum for assistance. Go to Bok, and ask him how they would befoul a creature of nature. I am certain that there is some nefarious means that the goblin leader will devise.'
Make your way to Grothum and speak with the leader Bok to learn how the goblins would go about perverting the natural order that would grow this sapling.
Go into the Ettenmoors and hunt down any of the Free Folk guards there. They will likely carry oil that can be used to poison the roots of the sapling. Replace water with oil and watch the sapling become rancid.