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Extension > Garden > Yard and Garden > Diseases > Crown gall

Crown gall

Rebecca Koetter and Michelle Grabowski

E. Barnard, FL Dept. of Ag. and Consumer Services, Bugwood.org

Crown gall on a young juniper

Importance

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.

G. Felt, UMN Master Gardener

Aerial gall on willow

Identification

Biology

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.

M. Grabowski, UMN Extension

Crown gall on a landscape rose

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.

University of Georgia Plant Pathology Archives, Bugwood.org

Crown gall on bare root transplants

Management

 

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