POPLAR DISEASES 301 



tissue, but the parasitic effects of the fungus exert a stimulus 

 which results in an increase in number and size of the mesophyll 

 cells of the leaf, causing the blister. The mycelium is not peren- 

 nial and new infections are caused by the spores which are 

 produced on the surface of the blister and winter-over. 



Catkin-Deformation 

 Caused by Taphrina Johansonii Sadebeek 



The fertile catkins of different species of poplar are affected 

 in similar manner as those of alders (see page 87). The 

 elements of the catkin become larger than normal and at a 

 certain stage are covered by the yellow fruiting-structures of 

 the causal fimgus. 



Canker 



Caused by Dothichiza populea Saoo. and Briard 



This canker of black or Lombardy poplar and the common 

 Cottonwood has recently been found destructive in nurseries 

 and on large trees planted for shade and ornament. The 

 disease was first reported and studied in Europe, where it is 

 known to be especially destructive to the cottonwood in 

 France and Italy. It has now been found in the United 

 States in New Hampshire, Massachusetts, New Jersey, New 

 York, Pennsylvania, Delaware, Maryland, Ohio and New 

 Mexico. 



Apparently it originated in nurseries in this country which 

 have imported the ornamental Lombardy poplar, and from these 

 centers diseased trees have been shipped to various regions, 

 thus distributing the fungus. Since the disease has only re- 

 cently been discovered and because of its destructiveness could 

 hardly have been overlooked previously, it seems certain that 

 the causal pathogene has been newly imported and is just 

 beginning to show its potentialities in this country. Its 



