114 



MANUAL OF TREE DISEASES 



Symptoms. 



The decayed wood is yellowish and cracks radially and tangen- 

 tially. The rot is uniform and in the final stages it is very 

 light in weight and easily crushed to a powder. 



The sporophores of the causal fungus are very common on 

 birch. They are corky annual bodies and are quickly destroyed 

 by insects. From the point of attachment to the trimk, they 



Fig. 13. — Fruiting-body of Polyporus betuUnus. 



hang as bell-shaped bodies varying from three to ten or more 

 inches across. The outer surface of the sporophore is smooth 

 and light to dark mottled gray in color (Fig. 13). The margin 

 is incurved and projects below the under surface. The lower 

 siu-face is white or yellowish and roughened by ragged projec- 

 tions. The pores are small and the entu-e layer of tubes 

 separates easily from the fruiting-body. 



