When most people see a gall on a plant, they think of insects as the cause. Crown gall may resemble insect galls but is a disease caused by a bacterial pathogen, Agrobacterium tumefaciens. The bacterium enters the plant through a wound, and the plant forms a gall in response to this infection. Actually, crown gall is an odd plant disease. The bacteria cause uncontrolled cell division in the host plant, resulting in gall formation. Genetic coding from the bacterium actually becomes incorporated into the host genetic coding. As you can imagine, this disease system has been studied and used extensively in research on genetic manipulation with plants.
Many plant species are susceptible to crown gall. We usually see the galls on creeping euonymus; but it may occur on rose, lilac, willow, honeysuckle, and other common landscape plants. I have seen it on stems of raspberry and on weeping fig. The conifers are resistant to this disease.
The galls usually appear first as swellings on the stem near the soil line–thus the name, crown gall. Eventually, galls may appear on the trunk, crown, roots, and outer stems of the host plant. Young galls are white or tan, usually round, and are quite soft and spongy. As the gall ages, it develops an irregular, convoluted, rough, corky surface and a dark brown, hard, woody interior. These galls might be mistaken for insect galls. Cut into the gall to make the distinction. 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. Abnormal growths on plants, sometimes called burls, can also resemble crown gall infection. The bark of the host usually remains on the burls but is not present on galls of crown gall.
There are also some fungal galls that may be confused with crown gall. For instance, there is a Phomopsis gall on forsythia, a pine-oak gall rust on pine, and even a smut gall on corn. Diagnostic laboratories can help determine the cause of various galls.
Crown gall is quite persistent because Agrobacterium can survive in the soil more than 5 years without a host. It is easily spread in soil water or rain splash but can penetrate plants only through fresh wounds. Such wounds might be made during pruning, cultivating, transplanting, budding or grafting, or feeding by insects or other pests. If you let your dog run through the planting, enough wounding occurs to let the pathogen enter.
Control of this disease is very difficult. If you decide to remove plants and start over, you need to use plants that cannot host this disease. For instance, if you had a bed of creeping euonymus with crown gall, do not put healthy creeping euonymus back in that bed. With time, they will become infected. If you are moving to a new site, inspect new plants for galls. Do not buy plants with galls. Because plants may have the crown gall disease and remain symptomless, do not try moving seemingly healthy plants from your infected bed to the new site.
Some plants that are not reported to host crown gall include 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. As stated, the conifers do not host crown gall, so you can replace that infected euonymus ground cover with one of the recumbent junipers. For more information on crown gall, consult Report on Plant Disease (RPD), no. 1006, at http://www.ag.uiuc.edu/%7Evista/horticul.htm.