APPLE DISEASES 111 



prevail in the nursery. The latter form is the more common 

 in orchards. 



Cause. 



Extensive and reliable experiments have shown that this 

 disease in its widely different forms is bacterial. The organism, 

 Bacterium tumefaciens, has been known but a short time, and 

 all points in its life-history are not fully understood. 



It is unknown just where the bacteria hibernate, but most 

 likely in the soil. They may live independently scattered 

 through the soil, or in old galls which may persist from year to 

 year. It is thought that the bacteria are carried from tree to 

 tree : (1) by water of irrigation ; (2) by cuttings from diseased 

 plants ; (3) probably by the pruning knife ; and (4) by insects. 

 The bacteria enter the host through wounds, and as a result of 

 their rapid multiplication and stimulative action, the attacked 

 plant shows signs of crown-gall within a week or less. The plant 

 tissue is invaded and the bacteria are found in the cells, but 

 death does not result. On the other hand, the cells are stimu- 

 lated to excessive multiplication. Soon after the first or pri- 

 mary gall is produced, particularly if the plant is rapidly grow- 

 ing, affected cells push out in strands, into the normal tissue, 

 along the lines of least resistance. This effect is not visible 

 externally, and is seen internally only with the aid of the 

 microscope. The, presence of such a strand is evidenced by 

 the fact that the removal of galls is not an effective control 

 measure ; new galls arise after the excision. As the strand of 

 diseased cells proceeds, bacteria are carried to points, above 

 and below the primary gall, where secondary galls arise. This 

 manner of originating secondary galls represents a phenome- 

 non similar to that exhibited in the case of malignant animal 

 tumors or cancers; because of this similarity the disease is 

 sometimes referred to as plant-cancer. 



It is understood then that infection may arise internally 

 and externally. The bacteria may come not only from the 



