Plant galls: which are harmful, which aren't

Galls are abnormal growths of plant tissue triggered by insects, mites, nematodes, fungi, or bacteria. When a plant's growth-regulating hormones are hijacked by a foreign organism, cells divide and enlarge in patterns that produce distinctive structures. The result can look alarming -- balls.

—- title: "Plant galls: which are harmful, which aren't" slug: how-to-identify-gall-types hub: problems category: "Identification guide" description: "Identify plant galls by shape, location, and host plant. Most galls are harmless. Learn which ones warrant concern, and which are normal parts of a healthy tree ecosystem." date: 2026-06-10 updated: 2026-06-10 author: "Thomas A." reading_time: 8 —-

Galls are abnormal growths of plant tissue triggered by insects, mites, nematodes, fungi, or bacteria. When a plant's growth-regulating hormones are hijacked by a foreign organism, cells divide and enlarge in patterns that produce distinctive structures. The result can look alarming — balls hanging from oak twigs, red spindles on maple leaves, spongy masses on stems — but in most cases, galls cause minimal harm to established trees and shrubs.

The most important thing most gardeners don't know: the majority of common galls on trees require no treatment.

How galls form

Per Penn State Extension, gall formation is initiated when a small insect, mite, or pathogen stimulates plant cells to divide and differentiate abnormally. The gall provides shelter and food for the insect larva or mite colony inside. The gall's structure — its shape, texture, and location — is largely genetically determined by the insect or pathogen that caused it, not by the plant. This is why the same gall form appears repeatedly on the same host plant species.

Common oak galls

Oaks support more gall-making species than any other tree genus in North America. Per Penn State Extension, the Cynipid wasp family causes the vast majority of oak galls. Nearly all are harmless.

**Oak apple gall (Amphibolips spp.):** Hollow, apple-like spheres 1–2 inches diameter, hanging from twigs. The interior is spongy, with a central hard seed-like structure housing the larva. Per Penn State Extension, oak apple galls are conspicuous but completely harmless to the tree.

**Oak bullet gall (Disholcaspis spp.):** Hard, round, brown, 0.5–0.75 inch balls in clusters along twigs. Per Penn State Extension, heavy infestations can cause minor twig dieback but are not a health threat to mature trees.

**Oak hedgehog gall (Acraspis erinacei):** Small, spiny, ball-like structures on leaf undersides.

**Wool sower gall (Callirhytis seminator):** Pink and white spongy masses on stems of white oak in spring. Distinctive and alarming-looking but harmless. Per Missouri Botanical Garden, wool sower gall has no impact on tree health.

Oak leaf blister (Taphrina caerulescens): Not a gall in the insect sense — this is a fungal disease causing raised, blister-like areas on oak leaves. Per NC State Extension, oak leaf blister is common after cool, wet springs and causes no significant harm to trees.

Maple galls

**Maple bladder gall (Vasates quadripedes):** Small, pimple-like, red to green-red bumps on maple leaf upper surfaces. These are mite galls. Per Penn State Extension, they look alarming — thousands of tiny red bumps — but cause no significant damage and require no treatment.

**Maple spindle gall (Vasates aceriscrumena):** Similar mite galls but elongated, pointed, and red — like tiny spindles standing upright from the leaf. Same harmlessness and same management recommendation (none).

Hackberry nipple gall (Pachypsylla celtidismamma)

Distinctive brown, nipple-like projections on hackberry leaf undersides. Per Penn State Extension, this is caused by a psyllid (jumping louse) and is the most common gall on hackberry. Completely harmless to the tree.

Goldenrod stem gall (Eurosta solidaginis)

Round, ball-like enlargement on goldenrod stems in late summer — a single swelling 0.5–1 inch in diameter containing a fly larva. Per Penn State Extension, this goldenrod gall fly is one of the most studied insect-plant interactions in ecology. Woodpeckers often excavate galls for the larva in winter — look for a small hole in the side of the gall as evidence.

Crown gall (Agrobacterium tumefaciens)

This is the exception to the "most galls are harmless" rule. Crown gall is a soil-borne bacterial disease caused by Agrobacterium tumefaciens (recently reclassified as Rhizobium radiobacter by some sources). Per NC State Extension, it causes rough, irregular, corky tumors at the root crown (soil line) and on roots and lower stems.

Affected plants: Roses, fruit trees (apple, pear, cherry), euonymus, forsythia, grapes, raspberries, and many others. Per Penn State Extension, crown gall does not always kill the plant but can weaken it significantly — especially young plants — by blocking water and nutrient flow at the crown.

ID: Rough, irregular, light brown to dark brown, corky or woody tumor at or below the soil line. Unlike smooth oak or maple galls, crown gall is rough, cracked, and irregular. Multiple galls may coalesce.

Management: Per NC State Extension, there is no chemical cure. Options:

• Remove and dispose of infected plants (do not compost)
• Do not replant susceptible species in the same location for several years
• Biocontrol using Agrobacterium radiobacter K84 (sold as Galltrol) applied as a root dip at planting shows preventive efficacy per Penn State Extension

Gall type comparison table

Management philosophy

Per Penn State Extension, for virtually all insect and mite galls on trees:

• Do not spray. The larva or mite is inside the plant tissue and is not contacted by surface sprays. Preventive treatments timed before the inducing stage (egg hatch) might theoretically reduce gall formation, but this requires knowing the exact timing of the inducing adult flight, which is impractical for homeowners.
• Tolerate cosmetic damage. A large red oak covered in oak apple galls in June is not in danger.
• Remove heavily galled shoots on high-value ornamentals if aesthetics are important. Pruning out galled twigs in winter, when the adult has long since emerged, causes no harm and removes the disfiguring structure.

Recommended gear: Best disease-resistant rose cultivars (Knock Out, Drift, Earth-Kind) — our buyer's guide covering picks for every budget, ranked by Extension publication consensus and personal use.

Frequently asked questions

My roses have lumpy growths at the base. Is this normal? No. Lumpy, rough, irregular growths at the soil line on roses are crown gall (Agrobacterium tumefaciens) until proven otherwise. Per Penn State Extension, this is one of the most common and serious rose diseases. Inspect new plants before purchase — infected plants should be returned. Established plants with galls can persist but often weaken over time.

Can I pop or cut open oak galls? Yes, and it's interesting — you'll find the wasp larva in a central chamber surrounded by spongy tissue. Per Penn State Extension, opening galls educates on the biology and does not spread the pest (the wasps emerge naturally from the gall at maturity, not from human opening).

Are gall wasps the same as stinging wasps? No. Per Penn State Extension, gall-forming cynipid wasps are tiny (1–5 mm), solitary, and do not sting humans. They are not related to yellowjackets, hornets, or paper wasps. Many have complex alternating-generation life cycles involving sexual and asexual generations on different parts of the same or different plants.

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Sources:

1. Penn State Extension — Plant galls
2. Penn State Extension — Crown gall
3. NC State Extension — Crown gall disease
4. Missouri Botanical Garden — Wool sower gall