OAK DISEASES 261 



base of the trunk. The form on oak is usually thick, with a 

 dark gray, rough upper surface, an acute margin and a slightly 

 roughened, white under surface. This heartwood-rot is more 

 fully described under poplar diseases, on page 310. 



White Root-Rot 



Caused by Polyporus dryadeus Fries 



Many species of oaks are affected by a white root-rot which 

 occurs apparently throughout the range of the oaks in the 

 United States and Europe. Although not as important as 

 many of the other wood- and root-rotting fungi, oaks growing 

 under adverse conditions are often found affected. The ulti- 

 mate result of the attack, is the death of the tree or it may be 

 uprooted during wind-storms. 



Symptoms. 



The first indication of the rot is a reddish or brownish colora- 

 tion of the inner bark-tissues. The adjacent sapwood then be- 

 comes reddish brown and watery. The discoloration advances 

 into the wood and the color of the decayed areas changes to 

 white. The bark becomes loosened and shreds into strips. 

 The rot finally involves the larger roots and extends into the 

 butt of the tree but does riot progress above the surface of the 

 ground. The smaller roots are completely decayed and look 

 like pith. They are light in weight and when twisted break 

 into concentric layers. Older partially decayed roots, in 

 longitudinal section, show the white or cream-colored rotted 

 bark and wood bounded by a dark brownish zone one to three 

 inches wide which marks the progressing area of change from 

 normal wood to white punk. The radial and longitudinal 

 whitish bands appearing in the affected wood are due to the 

 mycelium of the fungus which is aggregated in the porous 

 regions of the annual rings. White patches of mycelium ap- 



