﻿BRANCH CANKERS OF RHODODENDRON. 



BY HERMANN VON SCHRENK. 



For a number of years the writer has observed a curious 

 canker-like formation on branches of Rhododendron maxi- 

 mum throughout the Appalachian Mountains. These cankers 

 appear in the form of large swellings of irregular shape. 

 A number of these are reproduced on plate 5. In their 

 simplest form, the swellings have a diameter in one direction 

 two or three times the diameter of the branch on which they 

 occur and anywhere from once to twice the thickness of the 

 branch upon which they occur. In other words, they appear 

 like flattened, more or less round, swellings. On one of the 

 flattened sides, the swelUngs are covered with normal bark, 

 and on the other side they usually show more or less well 

 developed dead tissue, (figure 1, plate 5), and from this all 

 the way to well marked holes, (figures 2, 3, 4 and 5, plate 5). 

 These holes are sometimes so deep that the canker appears 

 like a hollow cup. Not infrequently the cankerous formation 

 takes place where two branches have diverged, (figures 3 and 

 4, plate 5), and in this case the dead, central tissue extends 

 from one side to the other of the canker. Surrounding the 

 dead portion a more or less vigorously formed callus covered 

 with normal bark is found. 



A large rhododendron shrub may have anywhere from one 

 to forty or fifty of these cankers, sometimes two or three on 

 one branch. The parts of the branch above and below the 

 canker appear normal in every respect and usually are about 

 the same size; in other words, the canker itself does not seem 

 to materially influence the growth and development of the 

 branch upon which it is situated. In this respect, these 

 cankers differ very materially from cankers such as are caused 

 by the various species of Nectria, or the various forms of fruit 

 tree cankers. The rhododendron canker appears simply as 

 a localized swelling, which does not seem to exert any influ- 

 ence upon the branch upon which it occurs. 



