Michelle Grabowski, UMN Extension Educator
Photo 1: Young crown gall on a rose Photo by FL dept. Ag and CS Bugwood.org.
As you purchase new plants for your garden or landscape this spring, one plant disease to look out for is crown gall. Crown gall is caused by the bacteria Agrobacterium tumefaciens
. As its name implies, the crown bacteria causes a tumor like growth on the stems and roots of infected plants. Galls are round, rough textured growths. New galls are often light colored and may be smooth and slightly spongy. Older galls become hard and dry; often dark in color with many rough cracks and fissures. The most common place for galls to form is on the main stem, at the point where it enters the soil. Galls can also form on below ground roots. In some plant species, the bacteria travel through the plants vascular system and initiates rows of galls along branches.
The presence of galls on roots or the main stem of a plant may or may not affect the overall growth and productivity of the plant. Young plants with many galls and plants with a gall completely encircling the main stem are the most severely affected. Galls can restrict the flow of water and nutrients through the plant, resulting in reduced growth, low flower and fruit production, and in some cases wilting and death of leaves and stems. Plants with crown gall are more susceptible to drought stress, winter injury and secondary diseases like Armillaria root rot, that enter the plant through cracks in the gall. That being said many plants tolerate a few galls without showing any obvious above ground symptoms. Mature trees of some species have been found with many galls that appear to have little effect on the trees overall growth and productivity.
Photo 2: Crown gall on the trunk of a peach tree Photo by G.Felton UMN Extension.
Over 600 species of plants from over 90 different families can be infected with crown gall. This includes a wide variety of ornamental trees and shrubs, fruit trees, as well as several perennial flowers and vegetables. In Minnesota, the most commonly affected plants are roses, willow or poplar trees and fruit trees like apple, plum, cherry or apricot. All of these plants are especially susceptible to crown gall.
The crown gall bacteria are often brought into a yard or garden on infected plants or soil. A wound is necessary for the crown gall bacteria to infect a plant. Nursery activities like grafting, pruning and transplanting provide ample opportunity for the bacteria to enter and infect susceptible plant tissue. If the plant is actively growing at the time of infection, a gall can be seen in 2-4 weeks and hopefully the plant will be culled before sale. If the plant is dormant however, it may be a much longer time before the gall is visible and of course below ground galls may go completely undetected. If an infected plant is placed in the landscape, the crown gall bacteria can move into the soil and spread to other plants.
Photo 3: Older crown galls on rose stem and roots Photo by M. Grabowski UMN Extension.
As galls age, they begin to break down. Layers of the gall slough off, releasing the crown gall bacteria into the soil. Crown gall bacteria can survive for a long time by living freely in the soil or in association with the roots of a wide variety of plants. Once established in an area, it can be difficult to get rid of the crown gall bacteria, so prevention is the best method of control. Be sure to inspect all new plants prior to planting them in the yard or garden. Pay especially close attention to roses, fruit trees, and poplars or willows as these are known to be highly susceptible to crown gall. Do not plant any tree or shrub with galls on the roots or stem. If crown gall is found on a recently planted tree or shrub, dig up the plant along with the soil immediately around the roots and dispose of them. Large established trees have been shown to tolerate infection with crown gall. If an established tree or shrub is found in the garden, it can be left but care should be taken to sterilize pruning tools after use on these plants. In addition presence of an infected tree indicates presence of the crown gall bacteria. Gardeners with infected trees or shrubs should avoid planting the highly susceptible trees and shrubs mentioned above.