370 



Diseases of Econo?nic Plants 



ROOT-ROTS 



Fungi similar to those that cause wood-rot may also at- 

 tack the roots of trees and thus cause death either with or 

 without any previous rotting of the trunk wood. Prominent 

 among such diseases are the following: 



Shoe-string root-rot ^^^ (Armillaria mellea Vahl). — The 

 fungus usually enters the root through wounds, and grows in 



Fig. 194. — Armillaria mellea attackmg a tree. After Freeman. 



the cambium, through which it spreads until it encircles 

 the tree. As its growth proceeds the layers adjacent to the 

 cambium become dry, and the top of the tree is killed by 

 stoppage of its water supply. A characteristic accompani- 

 ment is the profuse development of string-like, hard, black 

 mycelial strands which permeate the soil near the base of 

 the affected tree. From the mycelium around the base 



