33 Crown Gall 
Agrobacterium tumefaciens is a bacterium that causes a disease known as 
crown gall in many species of dicotyledonous plants. The disease is 
characterized by production of nondifferentiated growth that is not self- 
limited. Crown gall disease causes crop losses, most marked in 
horticultural stone fruits (e.g. plums, peaches) and in certain ornamental 
crops. Galls may occur on the crowns, leaves, and stems. The galls may 
range from a millimeter to about 20 centimeters in diameter and from less 
than one ounce up to 100 pounds in weight. Losses occur primarily at tree 
nurseries due to injury caused by grafting, pruning, and harvesting. 
Infected plants can become stunted and subject to blow over during 
windstorms. 
Wounds are required for penetration of the host plant by the pathogen and 
for altering the host cells to a susceptible state. Wounds may occur by 
insects and from cultural practices such as root pruning of nursery stock. 
A. tumefaciens, introduced into tissues by vacuum infiltration in the 
absence of wounding, does not produce galls. Host susceptibility is 
dependent on the type of the plant, age of the plant, and the site of 
infection. Younger tissues generally are more susceptible than older 
tissues. 
One hundred thousand to one million cells of the A_. tumefaciens pathogen 
are required to cause a gall in a single plant. A_. tumefaciens has been 
isolated from pasture lands that had never been tilled or cultivated; 
therefore, the organism is expected to be found in most soils. Soils with 
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