APPLE DISEASES 79 



On the surface of the old fruiting pustules the stromata 

 ascospores can be seen in the spring exuding from the sunken 

 perithecia. These spores are washed and blown to infection- 

 courts, chiefly wounds, where they germinate and their germ- 

 tubes establish a food relationship with the bark. It has been 

 observed that one of the most common seats of blister-canker 

 injury is in old stubs which have been left by careless pruning. 

 The fungus now in the bark extends into the wood where it 

 grows even more rapidly than in the bark. Some limbs, if care- 

 fully examined, show brown, discolored wood at points several 

 feet away from the canker. It is probable that the fungus can 

 gain an exit where the bark is injured, at which places new 

 cankers would be formed. Wherever the mycelium penetrates 

 the bark the tissue is killed, and finally is disintegrated. For 

 some reason the spread of the fungus within a given area is not 

 complete at first, but here and there sound bits of bark are left 

 unaffected for a time. This explains the mottled appearance, of 

 the younger cankers. Finally, however, these spots are also 

 killed. The growth and spread of the hyphse within the bark 

 is so rapid that no callus is laid down at the advancing margin 

 as in many other canker diseases. The only attempt made on 

 the part of the tree to inhibit the advance of the fungus is the 

 formation in the summer of a layer of cork. It seems that 

 growth of the pathogene in the tissues continues as long as 

 there is sufficient oxygen and moisture present. Its attack on 

 heart-wood and sapwood appears to be conditioned by these 

 factors. The supply of air and water must be in proper pro- 

 portions. 



In July and August the fungus develops its stromata by a 

 massing of the mycelium in certain portions of the diseased 

 bark. As they increase in size the bark is ruptured, exposing 

 their light, tan-colored surfaces. On these disc-shaped stro- 

 mata are developed, on erect conidiophores, honey-colored 

 conidia. It is believed that these conidia may be carried to 



