Leafgalls

Leafgalls are localized growths on leaves that arise when a plant tissue responds to the presence of an invading organism or pathogen. They appear as swellings, blisters, or distortions that vary in size, color, and texture, and are typically confined to the leaf or its surface. Galls result from biological interactions rather than simple weather-related injury, and they often persist through the growing season.

The primary causes are insects and mites that induce gall formation, along with some fungi, bacteria, and rarely nematodes. Gall-inducing arthropods include gall wasps, gall midges, aphids, and eriophyoid mites. These organisms deposit eggs or secretion compounds in leaf tissue, manipulating development to form a protective enclosure around the developing immature. Fungi and bacteria can also produce gall-like leaf deformities, reflecting a broader spectrum of plant-pathogen interactions.

Appearance and examples are highly species-specific. On oaks, many cynipid wasps create conspicuous oak leaf galls, sometimes called apples or pouches. Maple leaves may exhibit bladder or blister galls caused by mites. Other deciduous trees can host a variety of leaf galls as well. In many cases the galls remain on the leaf for the season and do not spread to other leaves.

Life cycle and ecology: the gall provides food and protection for the inducer’s larvae. The plant tissue is hormonally manipulated, and the gall interior often houses the developing organism. Galls can locally affect photosynthesis and growth but are generally not lethal to the host plant.

Management: most leaf galls cause minimal harm and do not require treatment. If cosmetic impact is a concern, removing heavily infected leaves or pruning can help, though insecticides and fungicides are often ineffective once galls are formed.