Unusual is one of the best way to describe galls on trees. These abnormal plant growths are found on leaves, twigs, and branches. They range in size from tiny lumps to large, complex protuberances. Galls may present as bumps on leaves or as tumorous outgrowths high in the treetop. Their appearance is varied and so are the causes. Typically these plant growths are formed by insect feeding or egg-laying activity, though they sometimes result from the presence of fungi, bacteria, and viruses.
There are thousands of insects, mites and other pests that produce galls. Presence of certain insects can stimulate growth of the plant tissue where the eggs or larvae reside. The gall can then shelter, protect, and feed the insects. Some of the most common gall-making insects are gall wasps, gall midges, psyllids, and adelgids.
Many galls cause no serious harm to the tree, though they are often unsightly. A heavy infestation could result in branch dieback and decline in health of the plant. However, there are galls that indicate a more serious issue is present. This includes spruce gall aphids, an insect that causes the formation of cone-like galls and can disfigure and weaken trees. Crown gall, a disease vectored by bacteria, is another common problem that can significantly stunt plant growth. You can learn more about galls and the effect they have on different tree species by browsing our online tree resource center.
Depending on the cause of the gall and the level of severity, treatment may or may not be needed. Pruning out of the galls can help from both an aesthetic and tree health standpoint.