294 



THE MONTHLY BULLETIN. 



will often seem to be healing up nicely in the fall and winter but will 

 start again on one side when the active growing season begins the next 

 year. Even in severe cases it may take sometimes three or four years 

 "for a canker to completely girdle a large limb and cause its death. 



Tlie first investigation of the disease was made in the early part of 

 1913, by W. H. Nixon and C. A. Hollister, students of the University 

 of California, who proved that the disease was infectious b}^ trans- 

 mitting it from diseased cankers to perfectly healthy limbs, by means 

 of infected axe or chisel cuts, and also by inserting pieces of diseased 

 bark from active cankers into healthy limbs. In the latter part of 

 June, 1913, further inoculation experiments by the writer with material 

 from diseased cankers shoAved the same results as those obtained by 

 Nixon and Hollister. In these inocidation experiments diseased bark 

 and wood were taken from large cankers on the trunk oi' larger limbs 

 ami put into smaller limbs, and diseased bark was also taken from 



^}^- 62.— Mehixiima cankers on walniiL tree. The arrows show the location of 

 canker.s. Tlie stain on the bark below the cankers is produced by "black sap" oozing 

 out from tlie diseased areas. (Original.) 



smaller limbs and put into the larger limbs with the result that the 

 withering of the smaller limbs and the production of the characteristic 

 black cankers were proved to be due to the same cause. At this time 

 a fungus, a species of Dothiorella, was discovered in these diseased 

 cankers and pure cultures were obtained. In the latter part of July 

 nioculatious were made by placing some of this fungus from pure cul- 

 tures into cuts in healthy limbs. It was found that when this fungus 

 was placed in small cuts in healthy large limbs it produced black 

 cankers, and when placed in cuts in small limbs it produced a girdling 



