80 MANUAL OF FRUIT DISEASES 
the limbs of the apple and other hosts where infection results, 
Just when these conidia function as inoculum is not clear, 
although possibly as soon as developed. Whether they are 
capable of overwintering is not known. 
As the stromata grow older a ring of black stromatic tissue 
is formed beneath the disc. The ring, or cylinder, extends into 
the wood of the limb. The connection of the stromata with 
the wood allows them to persist for as long as ten years, even 
after the bark has long since fallen away. 
In the upper portion of the stromata there are formed, from 
April to June of the following year, flask-shaped cavities, with 
long necks opening at the surface; these are the perithecia. 
These contain asci and ascospores; the ascospores are dis- 
charged and come to lie in small, black heaps on the surfaces 
of the stromata. This expulsion is said to occur in the spring, 
- butascosporesmay be found on the discs at any time. Itis prob- 
able that they cling there for a year or more; at least their 
vitality is retained for several months. There is some indi- 
cation that the woolly aphis carries the spores. : 
Control. 
The blister-canker is one of the most difficult diseases to 
combat. The rapid growth of the fungus in the heart-wood 
renders a cure practically impossible. If the canker is found 
in its early stages, however, it may be profitable to cut away 
the injured bark and wood and cover the wound with coal- 
tar or some good dressing. (For methods of cutting-out 
cankers, see page 54.) Limbs showing older and more ex- 
tended cankered areas should be removed entirely. Since the 
fungus enters the tree through stubs left by careless pruning, 
a step toward the control of this disease consists in avoiding 
such indifferent procedure. It should always be remembered 
that the fungus can get into the bark only through injured - 
tissue. In those regions where blister-canker is especially 
troublesome, resistant varieties may be sought with profit. . 
