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 

 rapidly in the direction of 

 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 



FIG. 22. Blister-canker of apple. 

 Old canker, showing "nail-heads," or 

 "blisters." 



