Q: What on Earth is going on with this maple leaf? I saw it on a wild tree while taking a walk down a neighborhood path, but wonder if it’s something that can spread to nearby gardens.
A: This is a great example of a gall, which is a tissue deformity on a plant caused by either insects, mites, fungi, bacteria, or nematodes. Usually galls cause swelling or weird projections on leaves or plant stems, but sometimes the more obvious feature is a color change like this.
The activities of the organism responsible creates chemical changes in the leaf tissue, redirecting tissue formation to suit its needs. For instance, insect-made galls give the larvae their own little house to feed in while being protected from most predators or harsh weather. (Impressively, tiny parasitoid wasps, little bigger than a dash on this page, still find their prey inside these structures and interrupt their life cycle. Isn’t that amazing?)
Despite how drastic galls may look to us, they don’t cause much harm to their host plants, which can be trees, shrubs, or perennials. Oak trees are renowned for harboring many kinds of eye-catching galls, some of which become most noticeable when they fall out of the canopy onto our lawns or gardens. See if you can find anything living inside those swollen red or brown lumps or balloon-like pockets on leaves. A wise bird or other insect may have beaten you to it, though, or the culprit is long gone and already flew away as an adult before the plant jettisoned the injured leaf.
If an eyesore, you can clip off heaviest infestations of leaf galls on witchhazel (caused by insects), azaleas (fungus), oak saplings (usually insects), and any other easy-to-reach plant. Keep in mind that the unaffected portions of those leaves are still functioning to feed the plant, so don’t remove too much growth. Otherwise, I suggest you leave them alone and just marvel at the intricacies of the natural world. Gall-forming insects can feed songbirds and don’t risk the health of the plant. As with any organism, populations wax and wane over time and galls might be prevalent one year and nearly absent the next.
At the Home and Garden Information Center, we have several web pages with more information about galls, including Shade Tree Galls and Eyespot Galls on Trees.
By Miri Talabac, Horticulturist, University of Maryland Extension Home & Garden Information Center. Miri writes the Garden Q&A for The Baltimore Sun. Read more posts by Miri.
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Very interesting insight on gall and how our trees support other live and in turn other life forms. Leaves host gall that may feed birds and wasps. The black spots on maple trees, that is disease, right?
Lynn – Yes, there are several types of leaf spot diseases that affect maples (e.g., anthracnose, tar spot, and others). We welcome the submission of photos of leaf spot issues on maples or any other plants, if you would like to find out what you are seeing. You can send questions and photos to our horticulture team at Ask Extension using this form on the University of Maryland Extension website: https://extension.umd.edu/programs/environment-natural-resources/program-areas/home-and-garden-information-center/ask-extension
Thanks for your question!