APPLE DISEASES 77 
sists in the destruction of both large and small limbs. Not infre- 
quently large trunks are affected, and trees are killed. The 
only available estimate upon the actual importance of this 
disease comes from Ohio. In the southern part of that state 
from 1 to 90 per cent of the trees are affected, amounting to 
an annual loss of thousands of dollars. In the summer of 1912 
thousands of trees, and in 
many cases entire orchards, 
were practically ruined by 
blister-canker in Iowa. The 
Ben Davis appears to suffer 
more than other varieties. 
Symptoms. 
The later stages of the 
blister-canker are very char- 
acteristic (Fig. 22). How- 
ever, in the earlier stages 
the lesions are distinguished 
from those of other cankers 
with some difficulty. At 
first the spots are not con- 
spicuous, and would be easily 
overlooked by the casual 
observer. The bark takes 
on a dingy, brown appear- 
ance, the size of the cank- 
ers varying at this early age. 
The lesion enlarges most Fig. 22. — Blister-canker of apple. 
rapidly in the direction of Old canker, showing ‘‘nail-heads,” or 
; ; “blisters.” 
the long axis of the limb. 
The interior of the generally diseased area shows healthy 
areas of bark scattered through the whole affected portion. 
This gives a peculiar mottled appearance to the canker. 
The margin of the diseased bark is sharply delimited from 
