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, 

 but ascospores may be found on the discs at any time. It is 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. 



