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 
