Crown gall is a common disease of many perennial plants. It causes fleshy tumors to develop on the plant and usually results in plant death. Grapes are among the most sensitive fruit crop to crown gall.
The disease is caused by the bacterium Agrobacterium tumefaciens. This is the same bacterium that is used to genetically modify important crops. There are at least three biovars of A. tumefaciens that cause crown gall. The particular strain that infects grapes is biovar 3. This strain was renamed Agrobacterium vitis since it only infects grape vines and close relatives.
The life cycle of Agrobacterium is interesting. The bacterial cells infect the plant through wounds. These can be caused by insects, mechanical damage, or in the case of grape vines, cold injury. Once the tissue is damaged, the bacterial cells can attach to the plant cells. However, rather than causing a canker or rot by replicating inside the plant tissue, the bacteria release a tumor-inducing plasmid, or circular piece of DNA, into the plant cell. It is this TI plasmid that causes the plant to react. The DNA from the plasmid gets inserted into the plant genome and infected cells begin to divide and proliferate rapidly, producing callous tissue that forms the galls that are visible on the plant surface. The rapid proliferation of callous disrupts the vascular tissue at the site of the gall and causes a reduction in water and nutrient movement in the plant. Infected plants typically die from a type of induced water and nutrient stress.
Starting with clean plant material is the best method of avoiding crown gall. Planting on well drained soils is also important. Once infected, plants cannot be cured. However, bringing up new shoots from below the gall to establish new trunks is one method of managing crown gall disease. Often the plants must be completely replaced. Economic losses to crown gall can be significant when cold tender cultivars are grown in regions where cold injury is likely.
Crown gall on grape trunk