Minnesota Plant Diseases. 



53 



ual, it is living parasitically on the remainder of the host plant. 

 That the broom itself is not injured, but rather stimulated, in 

 its growth is seen by the production of such numerous and 

 large-sized branches. But the ultimate effect upon the whole 



FIG. 23. Witches'-broom on balsam fir, caused by a rust fungus (Aecidium elatinum). 

 The branches of the broom are vertical instead of horizontal, as are the normal, un- 

 diseased branches in the right of the picture. Original. 



plant is injurious because the normal balance of nutrition and 

 work has been interfered with for the rapid production of a 

 group of larger but worthless branches. In a word, therefore, 

 the witches'-broom may be described as a bush- or broom-like 



