Most master gardeners and landscapers can recognize crown gall on plants; or at least they consider it when the galls are spotted. It is most often confused with insect galls but resembles the rust galls described on pine and a few fungal galls. Galls from crown gall disease appear as a mass of undifferentiated tissues; insect galls have galleries, or pockets, with or without insects. Abnormal growths on plants, called burls, can resemble crown galls. The bark of the host usually remains on the burls but is not present on crown galls.
Crown gall is a disease caused by a bacterial patho-gen, Agrobacterium tumefaciens. The bacterium en-ters the plant through a wound, and the plant forms a gall. 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 becomes incorporated into the hostís coding. As you can imagine, this system has been studied and used extensively in research on genetic manipulation. Consult Report on Plant Disease (RPD) no. 1006 for details. Contact your local Extension office or view it at http://www.ag. uiuc.edu/~vista/horticul.htm.
The bacterium survives for long periods (many years) in the soil. Once you have an infection, you live with weakened plants or you replace them. Obvi-ously, you must use a species that is not a host. Unfor-tunately, many plant species are susceptible. At the clinic, the most common host is creeping euonymus, but crown gall may occur on rose, lilac, willow, honeysuckle, and other common landscape plants. The conifers are resistant to this disease.
Plants reported not to host crown gall include andromeda, baldcypress, barberry, beech, birch, black gum, boxwood, catalpa, cedars (true), deutzia, Douglas fir, firethorn, ginkgo, golden-raintree, hemlock, holly, hornbeam, larch, linden, magnolia, mahonia, pine, redbud, serviceberry, smoke tree, spruce, sweet-gum, tuliptree, yellowwood, yew, and Zelkova. As conifers are not hosts, replace that infected euonymus ground cover with a recumbent juniper. Do not buy plants with galls. As plants may have the disease and be symptom-free, do not move seemingly healthy ones from an infected bed to a new site.