Leaf Galls on Maple and Oak: Those Bumps, Explained

By Linda Marsh, Pests & Diseases. Last updated: July 9, 2026.

Here is what tends to send Rochester homeowners looking for answers in early summer:

• Rows of tiny red or green pimples covering the top of maple leaves
• Round, apple-like balls hanging from oak twigs and leaves
• Fuzzy or felt-like patches on the underside of foliage
• Horn-shaped or spindle-shaped projections standing up off the leaf

What are leaf galls on my maple or oak?

A gall is abnormal plant tissue that a tree grows in response to a tiny insect or mite feeding on it. When the organism starts feeding, usually on brand-new spring foliage, the leaf responds by rapidly building tissue around the feeding site, effectively enclosing the pest inside a custom-made shelter. The bump you see is the tree's reaction, not the pest itself.

Galls are extremely common because there are so many organisms that make them. The University of Missouri Extension reports that there are "more than 1,500 species of gall producers, the majority being insects and mites" (University of Missouri Extension). Oaks in particular are gall magnets: Ohio State University's Buckeye Yard and Garden Line notes there are "somewhere around 800 different types of arthropod galls found on oaks in the U.S.," of which "about 700 are produced by cynipid wasps" (Ohio State University BYGL).

The usual local suspects are easy to recognize once you know the pattern. Maple bladder gall shows up as round, wart-like beads on the upper leaf surface that start green and turn pink, then red, then black. Penn State Extension describes them as "globular growths about 2.5-3 mm in diameter on the upper leaf surface" (Penn State Extension). Maple spindle gall makes narrow, upright projections on the same leaves. On oaks, the showstopper is the oak apple gall, a light, papery ball up to golf-ball size produced by cynipid wasps.

Are leaf galls harmful to my tree?

Almost never. On an established, otherwise healthy tree, leaf galls are a cosmetic problem, not a health threat. The leaf keeps doing its job. According to the University of Missouri, galled leaves "are usually able to carry out photosynthesis at near-normal levels," which is why even a tree covered in galls typically shrugs them off.

Bruce A. Barrett, an entomology state specialist at the University of Missouri, puts it plainly: "Despite their unsightly appearance on the foliage, which detracts from the normal beauty of a tree or shrub, galls generally cause little real damage" (University of Missouri Extension). The Morton Arboretum reaches the same conclusion, stating that "in the majority of cases, plant gall damage is an aesthetic problem" (The Morton Arboretum). For oaks specifically, Ohio State's Joe Boggs writes that the common leaf and twig galls "cause no appreciable harm to the overall health of their hijacked hosts; controls are not warranted" (Ohio State University BYGL).

The rare exceptions involve galls on twigs and stems rather than leaves. A heavy load of certain stem galls can girdle small branches over time, and a young or already-stressed tree has less margin to spare. That is a different situation from the leaf spotting most people are worried about, and it is worth an expert eye rather than a guess. If your tree also has yellowing or curling foliage alongside the bumps, our guide on why tree leaves curl, yellow, or die will help you separate a gall from a genuine stress signal.

What causes the different gall shapes?

The shape tells you the culprit, because most gall-makers are host-specific and each species produces its own signature structure. Two groups account for the vast majority of what Rochester homeowners see.

Eriophyid mites cause the small stuff: the bead-like bladder galls, the spindle galls, and the fuzzy or felt-like patches (often called erineum) on the undersides of maple leaves. These mites are microscopic, well under a millimeter long, and you will never spot the animal itself, only the growth it triggers. They ride the wind and overwinter in bark crevices, then move to swelling buds in spring.

Cynipid wasps cause the more dramatic oak galls: oak apples, and various horned, spiny, and bullet-shaped galls on leaves, twigs, and acorns. These are tiny, stingless wasps, not the yellowjackets people fear. Their life cycles can be strange, alternating between generations that produce completely different-looking galls on the same oak. Aphids, psyllids, and midges round out the cast on other hosts. Because the relationship is so specific, an oak gall will not jump to your maple and a maple gall will not appear on your oak.

Should you treat leaf galls with a spray?

Generally, no. Spraying is both unnecessary and, for practical purposes, ineffective. The timing problem is the reason: by the time you notice galls, the pest is already sealed inside where a spray cannot reach it. The Morton Arboretum explains that "by the time the galls become noticeable, the insect or mite causing the injury is protected from chemical sprays" (The Morton Arboretum). The University of Missouri is blunter still: "nonsystemic insecticides are virtually useless once the gall has been formed."

Penn State Extension likewise advises that "leaf galls do not seriously affect the health of a tree and chemical control is seldom warranted" (Penn State Extension). The far better strategy is to keep the tree vigorous so it easily outgrows the cosmetic damage: proper watering during Upstate New York dry spells, a ring of mulch instead of a mower-scarred trunk, and no unnecessary stress. If a small, heavily galled ornamental truly bothers you, picking or pruning off the affected leaves is a reasonable cosmetic fix, and raking up and removing fallen galled leaves in autumn can modestly reduce next year's load. Chasing galls with pesticide, by contrast, mostly kills the beneficial insects that would otherwise keep the gall-makers in check.

When is a bump on a leaf actually a problem?

The bump matters when it is not really a gall. This is the whole reason to look closely, because a few serious tree problems can masquerade as harmless lumps and spots to an untrained eye. Sunken, discolored dead patches on bark are cankers, not galls, and they signal a fungal or bacterial infection. Small round or D-shaped holes in the trunk point to wood-boring insects, not leaf-feeding mites; that pattern is the calling card of pests like emerald ash borer across Monroe County. Raised orange or rusty pustules, powdery coatings, or spreading dark blotches are fungal diseases such as rusts, tar spot, and apple scab, which behave nothing like a gall.

Chewed, skeletonized, or hole-riddled leaves are a different story again, usually feeding damage rather than a growth. If the "bumps" turn out to be clustered beetles or ragged edges, compare against something like Japanese beetle damage on Rochester trees before you assume the worst. The honest answer for a homeowner is that these distinctions are hard to make from a phone photo, and the cost of guessing wrong runs in both directions: treating a harmless gall wastes money, while dismissing a real canker or borer as "just a gall" can cost you the tree. When the bumps come with broader decline, our diagnostic walkthrough on why a tree looks sick is a good next step before you decide it is nothing.

FAQ

Will galls spread to my other trees?

Not in the way a contagious disease would. Gall-makers are highly host-specific, so an oak gall wasp will not colonize your maple and a maple gall mite will not move to your oak. The mites and wasps do reproduce and can be a bit heavier some years than others, but a gall on one tree is not "spreading" to unrelated trees nearby.

Do galls hurt the leaves' ability to feed the tree?

Very little. University research finds that galled leaves generally keep photosynthesizing at near-normal levels, which is why even a heavily galled tree usually shows no decline. A leaf that is completely covered may work slightly less efficiently, but the tree has plenty of other leaves and easily makes up the difference.

Can I just pick the galls off?

Yes, if it bothers you cosmetically. Picking or pruning off galled leaves does no harm and will not hurt a healthy tree, though it also does nothing for the tree's health since the galls were not damaging it. Raking and removing fallen galled leaves in fall can slightly reduce the following season's numbers.

Are spruce galls different from maple and oak leaf galls?

Yes. The pineapple-shaped or cone-like galls on spruce twigs are caused by adelgids, not by leaf mites or gall wasps, and they form on the new twig growth rather than on leaves. They are still mostly cosmetic on an established spruce, but because they sit on the twigs, heavy or repeated galling on a young tree is more worth a professional look than a maple leaf full of harmless bladder galls.

Sources

• Monster Tree Service of Rochester
• University of Missouri Extension: Insect and Mite Galls (G7272)
• Penn State Extension: Leaf Galls on Maple
• The Morton Arboretum: Plant Galls
• Ohio State University BYGL: Oak Apples, Woolly Catkins, and Leaf Blisters