Black walnut hull is the green outer casing of the nut, and it contains juglone — a compound the tree produces to suppress competing plants around it. That same suppressive chemistry works against fungi, and black walnut has been the standard traditional treatment for ringworm and nail fungus for generations.
It is one of the few things in this index with a real claim to matching tea tree on stubborn toenail infections, and it works differently enough that people who have plateaued with tea tree sometimes make progress with it.
It stains ferociously. Juglone was historically used as a dye, and it will darken skin, nails, cloth and porcelain.
