Jeffrey Hahn, UMN Extension Entomologist
Photo 1: Birch erineum gall. Jeff Hahn.
Some birch trees are exhibiting red, fuzzy patches on some of their leaves. It is common to assume that this is a type of disease but it is actually a type of gall. Galls are abnormal plant growths due to a variety of different organisms. In this case is caused by a species of eriophyid mite. Eriophyid mites are very tiny and nearly microscopic. This gall is similar to ones found on maple, viburnum, and linden. Like other insect and mite leaf galls, birch erineum galls have very little, if any, impact on tree health. You do not have to do anything if you find this gall in your tree, just ignore it.