APPLE DISEASES 



73 



external evidence of the disease is the appearance of the char- 

 acteristic sporophores of the parasite (Fig. 20) which grow out 

 through knot-holes where branches have broken off (Fig. 21, 

 right). These fruiting bodies are more or less hoof-shaped, 

 hard, black, and checked on the upper surface (Fig. 20), and 

 dark-brown and porous on the lower surface. The pores are 

 extremely small, their diameters being not more than one-six- 

 teenth of an inch. The size of the sporophores themselves 

 varies greatly. The in- 

 ternal symptoms of 

 disease are evident when 

 the tree is cut or blown 

 over. Cross sections of 

 the diseased portion 

 show symptoms quite 

 distinctive of the trouble 

 (Fig. 21). Affected trees 

 never become hollow, 

 but the rotten wood re- 

 mains in place with a 

 few cracks. The cen- 

 tral area of the diseased 

 heart is whitish or light- 

 yellow (Fig. 21). Bor- 

 dering this area is a 



narrow black line; sometimes there are several of these 

 black lines arranged concentrically with white areas between 

 them, and a yellowish to reddish brown zone, with an in- 

 definite border, just outside the outermost black ring (Fig. 

 21). The character of the wood in these areas is as follows : 

 (1) in the white central area the wood is soft and crumbly when 

 rubbed between the thumb and finger; (2) between the con- 

 centric black lines it is similar to that in the center, except that 

 decomposition has not progressed so far ; (3) outside the outer- 



FIG. 21. White heart-rot ; cross section 

 of an affected limb showing the rotted heart- 

 wood, the black lines and at the right a young 

 fruit-body of the pathogene. 



