Crown gall causes rough woody tumor like galls to form on roots, trunks and occasionally branches of many different trees and shrubs. Galls can interrupt the flow of nutrients and water within the tree, reducing overall plant growth and vigor. Young plants with many galls and plants with a gall completely encircling the main stem are the most severely affected and can be killed by the disease. Mature trees often tolerate many galls with few negative effects. Plants with crown gall are more susceptible to drought stress, winter injury and secondary diseases that enter the plant through cracks in the gall.
Pathogen and susceptible plants
Crown gall is caused by the bacterium, Agrobacterium tumefaciens. This pathogen is found on more than 600 different species of plants worldwide including many common vegetables, weeds, deciduous and evergreen trees and shrubs. In Minnesota, the most commonly affected woody plants are roses (Rosa spp.), willow (Salix spp.), poplar (Populus spp.) and fruit trees like apple (Malus spp.), cherry, plum or apricot (Prunus spp.). All of these plants are highly susceptible to crown gall.
Agrobacterium vitis is another crown gall species that can infect grape vines. Agrobacterium rubi infects brambles like raspberry, causing gall formation along canes. These species of Agrobacterium have a narrower host range compared to A. tumefaciens but the identification, biology and management of the three pathogens are very similar.
- Irregular tumor-like growths called galls are found on stems and roots ranging in size from 1/10th inch up to 1 foot in diameter.
- New galls are round, rough textured, 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.
- Galls are commonly found on the main stem at the point where the stem enters the soil.
- Galls can also form on below-ground roots.
- In some plant species, rows of galls form along stems or branches.
Crown gall bacteria enter plant roots through wounds created by planting, grafting, soil insect feeding, root damage from excavation, or other forms of physical damage. The wounded roots release chemicals that the bacteria sense and move towards. Cells that are newly wounded remain susceptible to bacterial infection for a few days during the growing season to several months if the plant is dormant.
Once the bacteria have entered the plant, they cause the cell to create unusually large amounts of plant hormones which then leads to gall or tumor formation. More specifically, infected cells divide uncontrollably and grow to unusually large sizes. Galls may be large enough to be detected 2-4 weeks after initial infection during the growing season. If inoculation occurs during the dormant season, tumor formation will be delayed until growth resumes. Over time the bacteria create secondary galls which contribute to the asymmetrical shape of the galls.
In highly susceptible plants like rose, willow and poplar, the bacteria may move internally up into stems and branches, initiating galls above ground. Galls may also form on pruning cuts or wounds in stems or branches if the bacteria are introduced into the wound through rain splashed soil or contaminated pruning tools.
Over time, galls begin to decay and breakdown. The bacteria return to the soil where they can be further dispersed by water or equipment. Crown gall bacteria survive for many years in soil and by colonizing roots of many different plants in the landscape. Root colonization by the crown gall bacteria may not result in gall formation on some plants but the pathogenic bacteria are still present and will infect a new susceptible host that is planted nearby. Crown gall bacteria are most commonly moved to new locations on the roots of infected plants.
- Prevention is the best method of control because once established in an area, the crown gall bacteria can be very difficult to eliminate. Carefully examine all new plants. Do not plant any tree or shrub with galls on the roots or stems. Extra close attention should be made when planting roses, fruit trees, poplar or willows.
- 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 it. Do not add infected plant material to compost piles! It is best to dispose of infected woody plants by burning.
- Established trees and shrubs often tolerate infection with crown gall and can be left in the landscape. Care should be taken to sterilize pruning tools if pruning is performed on infected trees to prevent spread of the pathogen to other trees and shrubs.
- If infected plants exist on your property, avoid planting highly susceptible species like rose, willow, poplar and fruit trees.
- Trees and shrubs can be protected from crown gall infection during planting by dipping roots of bare root plants or drenching potted plants with a solution of water and biological control bacteria, Agrobacterium radiobacter K-84. These bacteria protect roots by producing an antibiotic and occupying wound sites that might be infected by the crown gall bacteria. Galltrol is a commercial formulation of A. radiobacter K-84 that is registered for use in Minnesota.