The Hidden Forest

Description says that it increases disease damage by 80 percent.

After buying and testing it is not increasing disease but instead is activating on poison spells.

Disease and poison damage augments are reversed: Disease focii affects poison spells, poison focii affects disease spells. It's a long-standing bug that annoys your average Necromancer to no end, and irks SKs, Shamans, and Druids too.

Coupled with spell availability, it's a big reason why your average necro sticks with fire and magic DoTs.