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 Myxosporium 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 



FIG. 34. Superficial 

 bark-canker. 



