302 



Diseases of Economic Plants 



other trees. The wood turns brown, and crumbles under 

 pressure. The rot extends from 3-4 m. above and below the 

 sporophores. These are pore-bearing, triangular in section. 



Fig. 189. — Polyporus sulphurcus showing ^. 

 tree. After von Schrenk 



effect upon wood of oak 



The upper surface is very hard and bears fine, irregular 

 fissures parallel to the edge. When mature, the upper 

 surface is red-brown. The pores are barely visible without 

 a hand lens. 



Soft-rot ^^^ (Polyporus ohfusus Berk.).— The black oaks 



