99 



it is doubtful if the results pay for the labor of the operation. 

 In cases of severe infection it is worth while to clean up the 

 cankers on the limbs. Cases of collar-blight should be treated 

 as cankers. (See general treatment under "Cankers," page 

 100.) It is seldom, however, that such treatment is entirely 

 successful, especially if the canker has gained much headway. 

 Sometimes bridge grafting is resorted to, but in most instances 

 replacement of the affected tree is advisable. 



Cankers. 



The term "canker" is applied to a diseased area or lesion 

 in the bark. Cankers are usually common in old and neglected 

 orchards. In young and well-managed orchards they are rela- 

 tively unimportant as a rule. When renovation of an old or- 

 chard is undertaken, cankers are likely to present one of the 

 chief problems, for often, when long neglected, they may in- 

 volve considerable areas on main limbs and trunks, and in 

 such cases heroic measures often are necessary to save a tree. 



Several kinds of cankers occur on apple trees in Massachu- 

 setts, and these are caused by different organisms. 



Black-Rot Canker. 

 This is caused by the same fungus that produces black-rot 

 of the fruit and spotting of leaves. Cankers commonly form 

 on older limbs, but this is by no means always the case, and 

 younger branches and even new twigs may be attacked. On 

 limbs, the cankers usually start on the upper side. A dis- 

 colored, sunken patch forms in the bark and this later becomes 

 dark brown or nearly black. A distinct crack or crevice soon 

 divides the diseased from' the healthy bark, and late in the 

 season small black fruiting bodies of the causal fungus may 

 appear scattered over the diseased surface. A roll of corky 

 tissue develops along the limiting crevice, usually on one side. 

 This is evidence of the effort of the tree to heal the injury by 

 the formation of new tissue. The following year, however, the 

 canker may spread through and beyond this corky barrier and 

 finally girdle the limb. At first the diseased bark remains 

 closely pressed against the underlying wood (Fig. 15), but 



