Geograph - photograph every grid square

These galls, turning red at the tips, are created by the mite Aceria macrorhyncha (syn. Aceria macrorhynchus and Artacris macrorhynchus): NS3977 : Leaf galls on Sycamore.

The females overwinter in crevices in the bark, emerging in spring to feed on the new leaves. Where they feed, galls develop (each mite can be responsible for many galls). In contrast to the conspicuous structures seen here on the upper surface of the leaf, the corresponding locations on the underside of the leaf are just tiny holes, fringed with hairs; the mites use these holes to gain access to the galls, and lay their eggs in them in May. The developing larvae feed on the tissues that line the galls. This process does not appear to be harmful to the tree.

The black patches are unrelated to the galls; they are caused by Tar Spot fungus, Rhytisma acerinum. Though it can appear unsightly, it also does no real harm to the tree in moderation; it only reduces the amount of leaf surface available for photosynthesis.