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 Fic. 21. — White heart-rot; cross section 
vase hatic ‘ght. of an affected limb showing the rotted heart- 
heart is w hitish or light wood, the black lines and at the right a young 
yellow (Fig. 21). Bor-  fruit-body of the pathogene. 
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- 
