Trees that are severely infected by apple scab will become defoliated and infected fruit are not edible. Apple scab also attacks flowering crab apples, mountain ash and firethorn. Infection and subsequent lesion development occur on both sides of leaves, usually developing first on the lower side. On leaves, young lesions are velvety brown to olive green with indistinct margins and are often not noticeable until after fall. With time, olive green lesions turn dark brown to black. Lesions on older leaves are typically raised, dark green to gray brown with distinct margins, and cause cupping on the underside of the leaf. Leaves that are heavily infected with scab will curl, shrivel and fall from the tree.