APPLE DISEASES 131 
critical notice be taken, however, it will be seen that the pustules 
on the superficial bark-canker are of an open, saucer-shaped 
type, while on the black-rot canker they are closed, flask-shaped 
bodies. The dead bark clings tightly to the limb for some time. 
Later, bits of bark fall from the tree. 
Old cankers usually show considerable 
checking of the bark (Fig. 34); these 
crevices are short and sometimes extend 
at right angles to the long axis of the 
affected limb. 
Cause. 
The fungus Myzosporium corticolum is 
responsible for the superficial bark-canker 
of the apple and the pear. The known 
facts connected with its life-history and 
habits are few. The mycelium of the 
fungus grows in the outermost bark- 
tissues only, never reaching the cambium. 
Scarcely before it penetrates to an ap- 
preciable depth its progress is halted by 
a cork-layer developed by the tree in 
response to the stimulation induced by 
the invader. This plate of cork is ap- 
parently never penetrated by the fungus, 
therefore its attacks are confined to the 
surface cells. The affected tissue is killed 
and eventually it sloughs away. In this 
process the cork-layer marks the line of 
cleavage. During its course of develop- 
ment the fungus forms fruiting bodies, acervuli, just beneath 
the surface of the bark. At maturity these break through the 
bark and expose a saucer-shaped interior. Conidia arise from 
the inner wall of this cavity; and in the moist weather of 
spring the spores ooze forth in white masses. These spores 
Fic. 34. — Superficial 
bark-canker. 
