A Demon Worm lives at the root of Gaia's tree, eating away its roots and trunk.
I don't know what it looks like or what it does. There might even be more than one. But I do know that it's the source of the Gaia's tree's disease.
Borg took a packet of seeds from his pocket.
Take these seeds of Gaia and place them near the root. That should lure out the parasite.