Crown gall is a disease caused by a soil inhabiting bacterium that infects many ornamentals and fruit trees in the garden. It is often brought to a garden on the stems or roots of an infected plant and spread with contaminated pruning tools and soils. This bacterial disease causes the formation of large corky galls up to several inches in diameter. They appear at the base of the plant and on stems and roots, and commonly on the bud union. The galls are rounded with rough, irregular surfaces and may be dark and cracked. Plants with numerous galls are weak; growth is slowed and leaves turn yellow. Branches or the entire plan may die back. Plants with only a few galls often show no other symptoms. Crown gall is not specific to roses and can affect apples, raspberries, honeysuckle, euonymus, and many vegetables. For this reason, roses should not be planted where plants susceptible to crown gall have been removed because of the disease.
Crown gall is a tumor that is caused by Agrobacterium tumefaciens entering the plant through a wound from pruning, cultivating, from frost damage or one made by a chewing insect. This bacterium changes the DNA in the plant’s cells around the wound, and a tumor starts to grow. The abnormal cells proliferate rapidly. Galls usually begin as green, pliable tissue; then develop into dark, crusty growths. The galls may disrupt the flow of water and nutrients up the roots and stems. Following gall formation, growth is stunted, foliage is sparse, and bloom production is reduced.
Most often galls grow at the soil level or just below it. Many rose plants can live and perform beautifully for many years with crown gall. Or gall can kill a plant – it is unknown why this is the case, but it’s usually a good idea to destroy galls, or even the plant once they are discovered. It can be spread by pruners than haven’t been disinfected between plants.
Weather conditions must be right for the development of crown gall – usually the kind of warm, sunny weather that roses like best. The bacteria are not active in cold weather, but can remain viable until the conditions are right – in the soil for 2 – 3 years even as much as 15-20 years in the soil even when it has no rose host. It can remain dormant on harvested plants held in cold storage for spring shipment. Because most galls grow beneath the surface of the soil, they can go undetected for long periods of time, and when you do see the, you might be surprised by their magnitude.
There are no effective controls for crown gall. Avoid buying plants with suspicious swellings or gall on lower stems or crowns. However, do not confuse crown gall with normal swellings that you see as a result of the budding process. Protect plants from injury on stems during cultivation. Maintain vigor with fertilization and watering. Remove and destroy badly infected plants and do not replant in that area for at least five years. New bareroot plants should be dipped in a dilute solution of bleach and water (˝ cup per five gallons). All parts of the plant should be immersed to destroy any bacteria that may have arrived on the plant. Remember that commercial rose growers grow plants for only two years before they are harvested and shipped to you. While most growers sterilize the soil in which the new plants are grown, evidence of the bacteria may not show up for years.
On existing plants, most galls can be easily cut off once the soil at the base of the plant has been pushed aside. Remove all pieces of the gall(s) and destroy them. Paint as much of the base of the plant as possible with the dilute bleach solution. Remove as much of the soil from the planting hole as possible, and replace it with fresh soil before replanting.
Photo from “Common Rose Diseases” by Ann Hooper and Rich Baer
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