OAK DISEASES 263 



black partially rotted central mass may remain for years at- 

 tached to the root or trunk. 



Cause. 



The white root-rot of oaks is caused by the fungus Polyporus 

 dryadeus. The sporophores are rarely found but the rot is not 

 uncommon. The method of infection has not been described. 

 No fungous strands are found in the soil around the rotted 

 roots, as in the shoe-string root-rot, and the trees apparently 

 are not attacked in groups. The spores produced in the tubes 

 of the sporophore probably find lodgment on exposed roots 

 and thus initiate infection. 



The mycelium grows first in the bark and then into the sap- 

 wood and heartwood. The brownish watery zone of the first 

 stage of decay is due to the production of a brownish liquid 

 which fills the cells. Later this disappears and the cell con- 

 tents and a portion of the cell- walls are dissolved. There is 

 little delignification, although the wood appears white. The 

 ease with which the wood splits into concentric rings and frac- 

 tures crosswise is due to the very thin walls left in the porous 

 part of the annual ring and medullary-rays. The tree suffers 

 general decline because of the destruction of the conducting 

 tissue of the roots and they may be killed outright when the 

 larger roots are attacked first. 



Rbfbbencb 



Long, W. H. Polyporus dryadeus, a root parasite on the oak. Jour. 

 Agr. Res. 1 : 239-250, pis. 21-22. 1913. 



