I have found the a rot infected mob at the top of the gnome underground citadel in Solusek Eye, the item dropped was a Desiccated Dungeon Rot Spores.
Arkanium 49 Necromancer,
Rathe - Nexus server.
The "Dungeon Rot Infected" began appearing after a patch on December 8, 2010, which introduced new hot zones. See this quest entry for information.