CHERRY DISEASES 185 



again. In the event of renewed activity in the winter the 

 bacteria spread at the edge of the old cankers, thus enlarging 

 these lesions. The bacteria probably lie dormant also in the 

 buds. The effect of their action is not ordinarily observed 

 until the buds fail to open in the spring or until, after opening, 

 they suddenly die. From the cankers, bacteria may possibly 

 be carried by insects in the summer to new points where in- 

 fections result. The bacteria attack the outer bark, then the 

 phloem and cambium. These affected elements all turn brown. 

 Brown streaks are found in the bark (between the phloem and 

 outer cortex) extending above and below a canker. Gum 

 pockets are formed under the bark, which splits and allows the 

 gum to ooze forth. 



The theory of gum-formation has created no little interest. 

 It is now generally held that gum is formed through the trans- 

 formation and liquefaction of the walls of certain cells. Such 

 cells are formed abnormally by the cambium as the result of 

 stimulation by parasites or other factors already enumerated. 

 In bacterial-gummosis of the cherry this stimulating factor is 

 Bacterium Cerasi. Probably it produces an enzyme which 

 dissolves the walls of the cells, with the result that gum is 

 formed in pockets made in turn by the dissolution of the cell- 

 walls in a local area. In order that gum may be produced, an 

 abundance of water seems necessary. It also seems essential 

 that the tree be in a growing condition for gum-production. 

 It may then be understood why young trees exude gum more 

 often than do old trees. 



Control. 



Recommendations for the control of the bacterial-gummosis 

 of the cherry are made in the Northwest along three lines : 

 (1) the removal of cankers; (2) the protection of susceptible 

 trunks and limbs with coarse cloth or burlap ; (3) the use of 

 resistant seedlings and the growing of resistant varieties. 



Cankers on old trees are rare, so that surgical methods apply 



