ASCOMYCETES 



283 



destructive in that state. Under ordinary circumstances the fungus 

 is doubtless a mere saprophyte, and it is not restricted to the apple 

 or to other members of the apple family. It has in fact been found 

 upon such trees as the following : American elm (Ulmus americana\ 

 Magnolia sp., Judas tree (Cercis canadensis), and Sorbus sp. 



Symptoms. The disease is usually found upon the larger limbs 

 or upon the trunk, and the appearance of the canker areas is so 

 characteristic that it cannot be 

 mistaken, at least in late stages 

 of the disease, for any of the 

 cankers thus far discussed. 

 At first the infested areas are 

 brown, slightly sunken, and con- 

 sist of small spots of healthy 

 tissue intermingled within the 

 general diseased area. These die 

 later, but there is an irregular 

 and mottled effect which per- 

 sists and is readily observed. 

 The infected .area may cover 

 many square inches of surface, 

 and it is sharply delimited from 

 the healthy tissue, due to the 

 drying and cracking. Occa- 

 sionally there is a slight de- 

 velopment of slime during the 

 early part of the season, but 

 this has not been associated with the action of the parasite. 



Hasselbring describes the external appearance during the late 

 season as follows : 



The bark of the older parts becomes much roughened and blackened as if 

 it had been charred. Numerous rifts and cracks appear over the surface of the 

 dead bark, which is very dry and brittle, and falls off in irregular patches, ex- 

 posing the dead wood. The circular stromata are firmly attached to the wood 

 by means of a ring of hard fungous tissue, so that they remain seated on the 

 wood even after the bark has fallen away. The entire blackened area is dotted 

 over with the circular stromata, which form the most pronounced distinguishing 

 feature of this canker. The disease is always easily recognized by these stromata 

 (Fig. 1 25), which distinguish it clearly from the New York apple tree canker. 



FIG. 125. BLISTER CANKER OF APPLE 



