The physical appearance of Leucostoma canker depends on the part of the tree infected. Infections of small twigs appear as sunken, discolored areas, often with alternating zonation lines, usually around winter-killed buds or leaf scars from the previous year's foliage. Nodal infections are easily observed 2 to 4 weeks after bud break. The infected tissues become darker with time and an amber gum may ooze from the infected tissue unless the twig is killed entirely. Cankers that form on the main trunk, branch crotches, scaffold limbs, and older branches are the most conspicuous expression of fungal infections.
Cankers appear elliptical along the length of the stem.
Usually the first external symptom of such cankers is the copious quantity of amber-colored gum. Gum production is a natural response of the tree to irritation, but that due to infection by Leucostoma spp. is excessive. As the canker ages, the gum becomes dark brown, the infected bark dries and cracks open, exposing blackened tissue beneath. Leucostoma canker can occur on weakened branches without the production of copious gum, especially if winter injury precedes infection. Beginning in late spring and continuing through summer, the tree grows rapidly and resists further penetration of the fungus into healthy tissues.