Blight Canker of the Apple Tree. 



193 



blighted watersprout soon dries up and falls away, leaving often a very 

 indefinite scar in the cankered area so that the following season it is usually 

 impossible to tell with certainty the manner of infection. Observation on a 

 large number of trees during the past season convinces me that the blight- 

 ing of adventitious shoots on trunk and limb is responsible for most of the 

 cankers in such locations. A number of cankers were produced in this 

 way by artificial inoculation. (Fig. 68.) 



Another source of infection was found to be the pruning knife. Along 

 one side of an orchard of some 350 trees which was under observation 

 throughout the season, it was early noticed that the pruned stubs of 1904 

 especially, showed collars of dead bark often two or three inches in width. 

 (Fig. 75.) Instead of forming a callus and healing over the wound, as 



Fig. 65. — Canker near base of apple tree, restdltng from 

 artificial inoculation with bacteria direct from canker on 

 another apple tree. 



would normally occur, the tissue had died and shriveled up but still clung 

 to the stub. In most cases the bacteria which had caused the death of the 

 bark had died out the first season. In a few instances, however, the 

 canker was observed to be active early in the spring, extending down the 

 side of the adjoining limb. (Fig. 76.) Two badly diseased trees on this 

 side of the orchard seem to have been the source of infection. Owing to 

 their diseased condition, they had been severely pruned the previous sea- 

 son and very probably the knife or saw had carried the bacteria to the 

 healthy trees. Flies which were observed constantly to follow the pruner 

 to suck up exuding sap, may have been the direct agent in many cases in 

 transferring the bacteria. The knife itself may convey the disease, as 



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