What is causing our maple trees to have a small “thorn” growing upright on the leaf?
The “thorn” you are describing is probably maple spindle gall, which occurs on sugar maple. The galls are thin, elongate, greenish, upright, and about a half inch long or slightly longer. These galls rarely distort the leaf but if numerous, can make the leaves unsightly. Though galls may be numerous, they seldom, if ever, cause permanent injury to a tree. Spindle galls are caused by mites. The mites overwinter under loosened bark and near callous tissue that forms around wounds and pruned branches. In early spring when maple leaves first appear, the mites move to expanding buds and begin to feed on the underside. This causes a blister to form that turns into a hollow spindle as the leaf expands. The mites enter the gall and continue to feed inside. These mites reproduce inside the galls and the new generation of mites matures by late June to mid?July. Then, the galls dry out and tiny holes open to allow the mites to escape. These mites then seek overwintering sites on the trunk and bra
