Is there something weird growing in your landscape? I recently spotted this horned oak gall and knew I needed to find out what it was! Galls are something gardeners often notice—usually with alarm. What are they and do you need to do anything about them?
What are galls?
Unusual plant growths are often—but not always—galls.
- Causes for gall growth include:
- Critters—from adelgids and aphids to beetles, midges, mites, sawflies, and wasps
- Bacteria and fungi
- They affect many plants:
- And many plant parts:
- Buds, flowers, and fruit
- Stems, twigs and branches
- And even roots!
- Galls may appear in different seasons, as well as change across seasons.
This spring you may have seen signs of azalea/camellia leaf galls (caused by Exobasidiumvaccinii/
Exobasidium camelliae fungi) or maple eyespot galls (caused by Acericecis ocellaris midges) on your maple tree leaves.
- Come summer you may have seen signs of crown gall (caused by soil bacterium Agrobacterium tumefaciens) on your roses, or oak apple galls (caused by wasps Amphibolips confluenta or quercusinanis) on your oaks.
- As summer progresses, you may see evidence of life stages of horned oak galls (caused by Callirhytis cornigera wasps).
- Come fall and winter, goldenrod galls (caused by Eurosta solidagnis flies) are easy to spot.
Which need attention?
Aesthetics. Although galls may be fascinating, if you find them unsightly, you may want to remove them even if they will do no lasting harm to your plants.
- Removal may improve your plant’s appearance
- Cleanup may help prevent further damage or disease.
- Removal may be impossible—think leaf galls that affect many leaves, such as Witch hazel cone galls.
- Cleanup may remove the benefits to wildlife of some galls—birds feed on the larvae in goldenrod galls; birds, and some mammals, feast on the wasp larvae in oak apple galls.
Plant decline. Some galls left untreated will harm plants and may eventually kill them! Do not ignore Azalea/Camellia galls, crown galls, and horned oak galls.
What to do?
Prevention. When bacteria cause galls—crown galls, for example—there must be an opening to cause an infection. Careful handling to avoid wounding your plants as well as removing insect damage can help. Take care to clean and sanitize tools that might spread infection. Sanitation is also important for preventing fungal galls, such as Azalea/Camellia galls—dispose of diseased tissue in the trash, as well as clean any tools used.
Pruning is the answer to many aesthetic as well as more serious problems. Removing affected leaves or other plant parts may be all that is needed. In other cases, it may help, but it isn’t a cure-all. In the case of bacterial disease, for example, the cause of the gall is systemic and may or may not have a cure. Removal of early stages of horned oak gall wasp infestation can save trees.
Article by Buncombe County Extension Master GardenerSM Volunteers
For more information:
Azalea/Camellia leaf galls: https://www.buncombemastergardener.org/time-maintain-remove-leaf-gall-azaleas-camellias/
Bacterial crown gall: https://extension.umd.edu/resource/bacterial-crown-gall-flowers
Horned oak gall wasp:
Goldenrod gall fly:
Oak apple gall: https://bygl.osu.edu/node/344
Sampling of gall photos:
Series of American Nurseryman articles on galls: