This apple is the product of the Nether's Darkwood Tree's leaves - similar to regular apples dropping from Oak Trees. However, Potash Apples are visible on the leaf blocks. They heal 4 points of hunger () but inflict 1 second of poison on the player ( of damage total).
Trivia
- There is a bug that allows you get access to faster access to the infinite quality of any plant - on this case, unlimited amounts of potash apples. If you shear a potash apple leaf block, it will drop both the leaf block and a potash apple.