APPLE DISEASES 



127 



ance from young cankers of other kinds. The first intimation 

 of the disease consists of a slight browning of the bark in a more 

 or less circular area. This is quickly followed by an intense dark- 

 ening of the affected tissue. The surface of the canker sinks, 

 leaving the edges prominently raised. In a 

 short time a crevice is evident along the 

 margin of the healthy and diseased bark. 

 The affected tissues are killed, become dry, 

 and slough off, exposing the wood, and at 

 the margin a callus develops. The enlarge- 

 ment of these cankers is slow; within a 

 year the length of the lesions may not in- 

 crease more than one-half of an inch. But 

 they increase in size the second year, and 

 even for as many as a score of years after, 

 thus resulting in large cankers. 



Older cankers are recognized as of two 

 sorts : open cankers and closed cankers ; 



and there are various intervening types. 



These names are very descriptive. In the 



first kind, the wound is open, the wood is 



exposed, and a series of dead callus-layers 



center about a common point (Fig. 33). 



While the edge of this type of canker is 



raised, there is no distinct swelling, and 



no gall-like appearance. In the closed 



canker, the whole presents the appearance 



of a rough gall. The edges are swollen to 



the extent that the wound is practically 



closed and the wood almost covered. It is said that open 



cankers prevail on those erect and more vigorous growing 



branches and twigs in the crown of the tree. On the other 



hand, the closed type predominates on the horizontal or 



hanging limbs. In the case of open cankers the margin 



FIG. 33. European 

 apple canker. 



