78 MANUAL OF FRUIT DISEASES 



the healthy tissues, sometimes a crevice marking the bound- 

 ary of the canker. The dead area is usually a little de- 

 pressed, due to a dying and shrinking of the bark. Fre- 

 quently branches are girdled, hence the parts above wither 

 and die. It is no uncommon sight to see dead fruits and 

 leaves clinging to limbs which have just been girdled by the 

 blister-canker pathogene. In late summer and autumn small 

 tan-colored cushions, fruiting structures of the pathogene, 

 appear under the bark at or near the margin of the canker for 

 that season. Later the cankers become blackened and appear 

 burned. The bark becomes very dry and brittle, and irregular 

 patches fall, exposing the dead wood (Fig. 22, at the top). 

 Sometimes whole trees are barked ; and frequently one side of 

 a tree is thus devoid of its bark, the other side remaining normal. 

 Cankers sometimes extend from the roots to the limbs ten feet 

 above ground. Ordinarily, however, the cankers extend three 

 fee.t or less, and are confined to the larger limbs. Older can- 

 kers show, in place of the cushions mentioned above, dark 

 fruiting cushions, called "nail-heads" by growers (Pig. 22). 

 The presence of these cushions gives a blistered appearance to 

 the canker, and they form the most pronounced distinguishing 

 feature of this disease (Fig. 22). 



Cause. 



The pathogene, Nummularia discreta, was described from 

 eastern America in 1834, but at that time it was not recog- 

 nized as the causal factor in blister-canker. Now there is no 

 doubt as to the ability of this fungus to cause this disease. 



The fungus is in no strange habitat wlien found on dead 

 wood. And it appears that it is unable to establish itself 

 without the presence of an injury of some kind in the bark. It 

 may live about the orchard indefinitely on dead limbs of the 

 apple ; it also easily lives on Sorbus, Cercis, magnolia and elm. 

 Hence these trees may be regarded as possible sources of 

 trouble wherever blister-canker prevails. 



