Galls are abnormal growths that can appear on roots, stems, trunks or leaves. There are many woody plants that get galls and there are several different gall makers (causes for the gall formation); insects, mites, bacteria, fungi, or nematodes. The galls formed are unique to the gall maker and the genus or species it feeds on. For crown gall the disease is caused by a soil-inhabiting bacterium (Agrobacterium tumefaciens) The bacteria enter the plant through a wound and genetically modifies the DNA of the plant causing uncontrolled cell division, resulting in the tumor like growths called galls. The galls typically first appear on stems and roots near the soil line, thus the name crown gall, but twigs and branches can also become affected. The galls slow down the transmission of water and nutrients to the plant, which can cause stunted growth, yellowing of leaves or tip die back. There is no cure and little the gardener can do to treat an affected plant.
Identification and Damage
Tumor-like growths called galls are first found on stems and roots typically near the soil line. Later appearing on twigs and branches. They range in size from a few cm up to 30 cm (1″) in diameter. Young galls are tan in colour, fairly round, with a soft, pulpy texture. As the galls age they grow larger and become woody and hard. The outer layer turns brown and corky.
As the galls begin girdling the roots and/or stems the infected plants may be weakened and stunted, leaves may yellow, and branches may begin dying back. Plants can remain in decline for many years.
To differentiate between galls caused by insects and crown galls cut into the gall “The galls from crown gall disease appear as a mass of undifferentiated tissues, whereas insect galls have galleries or pockets with or without insects present” (UOI Extension 2005).
Life Cycle and Spread
The crown gall bacterium (Agrobacterium tumefaciens) enters the plant through a wound, usually in the root zone or near the base of the plant. Once inside the plant the bacteria insert a portion of their DNA (ISU Extension and Outreach, rev. 2018) which genetically modifies the DNA of the plant, causing uncontrolled cell division in the wounded area. Once the cells in this wounded area begin over multiplying the plants tissue begins to expand in that area and soft, pulpy growth begins to develop. As the growth continues the tumor like growth enlarges and completely encapsulates the branch. It’s texture begins to dry and harden becoming woody and corky. It’s colour gradually changes to a dark brown. Secondary galls later form contributing to the asymmetrical shape of the galls and spreading to other branches higher up on the plant. Warm weather favours it’s growth.
Over time, galls begin to decay and breakdown and the bacteria return to the soil, where they can survive for several years before infecting another susceptible plant.
Crown gall bacteria are most commonly moved to new locations on the roots of infected plants but can also be moved on contaminated soil. (Grabowski M. & Koetter R. rev. 2019).
Plants Susceptible to Grown Gall caused by the Bacterium Agrobacterium tumefaciens
Euonymus sp. are highly susceptible, but it may also occur on rose, lilac, willow, honeysuckle, weeping fig, poplar, apple, cherry, plum, apricot, chrysanthemums, asters and daisies.
Two other species of Agrobacterium cause woody galls to form on susceptible plants.
- Agrobacterium vitis causes woody galls to form on grape vines.
- Brambles like raspberry may have gall formation along their canes when infected with Agrobacterium rubi (Grabowski M. & Koetter R. rev. 2019).
The galls on forsythia, viburnum, highbush blueberry, American elm, hickory, maple, oak, and privet are believed to be caused by a fungus, Phomopsis sp. (Missouri Botanical Garden n.d.).
Prevention is the best method of control because once established in an area, the crown gall bacteria can be very difficult to eliminate.
- When introducing a new tree or shrub, especially euonymus, roses, fruit trees, poplar or willows, be sure to examine the roots and stem for any signs of cankers. If they are infected DO NOT plant them.
- Remove infected trees and shrubs that are young. Burn or black bag them. The soil will also have to be removed if you plan to plant another susceptible plant as the bacteria remain viable in the soil for years.
- Plant resistant varieties when possible like barberry, hornbeam, true cedars, ginkgo, golden-raintree, tuliptree, mahonia, spruce, linden, boxwood, catalpa, beech, holly, larch, magnolia, black gum, pine, Douglas-fir, bald cypress, hemlock, birch, firethorn, redbud, smoke tree, sweetgum, deutzia, serviceberry, yellowwood, yew, and Zelkova (UOI Extension 2005).
- Large established trees and shrubs can usually survive infection, leave them in the landscape but be mindful not to introduce new plants to your landscape that are susceptible to crown gall.
- Prune off galls sterilizing your pruners between cuts.
- Practice good sanitation.
- When working around susceptible trees and shrubs avoid unnecessary wounding.
- Agrobacterium radiobacter K-84. The entire plant and root are immersed in a solution of water and this biological control bacteria. These bacteria protect roots by producing an antibiotic (Grabowski M. & Koetter R. rev. 2019). Not available to homeowners in Ontario.
Photo Credits: all photos taken by the author.
Missouri Botanical Garden n.d. , Crown Galls, Retrieved on Feb. 7, 2021 from: https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/diseases/bacterial-galls/crown-galls.aspx
Pacific Northwest Extension n.d., Euonymus-Crown Gall, Retrieved on Feb. 7, 2021 from: Euonymus-Crown Gall | Pacific Northwest Pest Management Handbooks (pnwhandbooks.org)
ISU Extension and Outreach, rev. 2018, Crown Gall, Retrieved on Feb. 7, 2021 from: Crown Gall | Horticulture and Home Pest News (iastate.edu)
UOI Extension 2005, Home yard and garden pest, Crown Gall, Retrieved on Feb. 7, 2021 from: Crown Gall (illinois.edu)
Grabowski M. & Koetter R. rev. 2019, Crown Gall, Retrieved on Feb. 7, 2021 from: Crown gall | UMN Extension
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