POPLAR DISEASES 311 



by this rot is considerable in some regions, but is in no degree 

 related to the general prevalence of the fungus. While found 

 everywhere throughout the United States, only occasional 

 living trees are attacked over most of its range. Outside of the 

 forest, oak and poplar seem to be the most often attacked. 

 The heartwood of the roots and the base of the trunk is decayed. 



Symptoms. 



The heartwood is decayed rapidly by the fungus and some- 

 times the sapwood is invaded when conditions are favorable 

 for its growth. The decayed wood appears only a little lighter 

 in color than the normal and retains its woody character. 

 It resists cutting with a knife more than its appearance and 

 light weight would indicate. Bordering the decayed .wood 

 at the center of the tree, in beech at least, is a broad dark 

 colored zone, a quarter of an inch or more wide. In this dis- 

 colored zone the process of decay seems to progress rapidly, 

 for just inside this area the decayed wood is of the same char- 

 acter as further toward the center of the tree. When split 

 lengthwise and smoothed with a knife, the decayed wood shows 

 peculiar horizontal markings lighter in color, which at first 

 may be mistaken for the tunnels of wood-boring insects, filled 

 with mycelium (Fig. 12, page 109). They are, however, simply 

 long definite channels in which the wood is reduced to a stage 

 at which the fibers are only loosely held together and mixed with 

 an abundance of mycelium. This loose completely decayed 

 wood can easily be picked out, leaving empty crooked tunnels 

 about an eighth of an inch wide and an inch or two in length. 



The sporophores are perennial and are typically almost flat 

 to convex. The upper surface is smooth, horny, light to dark 

 gray and marked with rather distinct concentric folds represent- 

 ing as many years' growth. The under surface is almost white 

 and covered with minute pores which are the open ends of long 

 perpendicular tubes which bear the spores over their inner 



