54 



Minnesota Plant Diseases, 



individual, formed by the partnership of a fungus and certain 

 branches of the host, and living at least partially as a parasite 

 upon the remainder of the host plant. It behaves as does a par- 

 asitic mistletoe plant and is not unlike it in appearance. Most 

 of the witches'-brooms of Minnesota trees are due to rust fungi. 

 One of the most common is the birds-nest broom upon red 



FIG. 24. Witches'-broom on white spruce, caused by a mistletoe (Razoumofskya pusilla). 

 The spruce is badly affected. Numerous brooms are seen below and the whole upper 

 part of the tree is broomed. (See also Fig. 25 and Fig. 101.) Photograph by author. 



cedars. These brooms occur in great numbers in many parts 

 of the state and look like crows' nests in the distance. The 

 branches are very numerous and the broom stands on a lateral 

 branch like an independent plant. The leaves are not like the 



