Most leaves are great for adding to compost. But some trees, like black walnuts, have toxins in their fallen leaves called hydrojuglone that can cause other plants to turn yellow, wilt and die. English walnut, hickory and pecan trees also produce small amounts of hydrojuglone. These leaves, and any other tree leaves that are infected with fungus or disease, should be disposed of with rubbish and not included in your compost.