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 hyphe 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 
