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. Ina 
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 Fic. 33. — European 
% . apple canker. 
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 
