126 MANUAL OF FRUIT DISEASES 



In America, its range and destructiveness have not reached 

 proportions which even approach those in Europe. Yet in 

 certain sections, already referred to, the canker is becoming 

 troublesome ; and it seems to be on the increase. 



So far as varietal susceptibility is concerned, little information 

 is at hand for American conditions. But German authorities 

 say that growers have always spoken of cankerous varieties. 

 It is held by some that thick-barked varieties are more resistant 

 than others. Again, the matter of whether a variety is sus- 

 ceptible or resistant is said to depend directly upon its suscepti- 

 bility or resistance to frost-injury. Certain writers say that 

 varieties of apples that grow erect and open, remain free from 

 this canker, whereas others near by suffer severely. In con- 

 tradiction to these observations and opinions, it is held that all 

 varieties suffer alike, and that so-called resistant sorts, when 

 planted among diseased trees, soon exhibit a susceptibility equal 

 to that of their neighbors. Any variety, it is said, which is 

 exposed is just as liable to infection as any other. It is admitted 

 by some that weak-growing varieties show larger cankers, be- 

 cause of their poor vigor, yet this is in most cases not worthy of 

 practical notice. In Germany, susceptible varieties include the 

 Canada Russet, Rheinischen Bohnapfels, Harbert's Russet, 

 Norman Bitter-Sweet, Gray French Russet, Hubbardston, 

 Red Cardinal, Canada Reinette, Red Fall Calvill, White Winter 

 Calvill, Winter Goldpearmain and others. Those resistant are : 

 Fursten, Schaffelder, Boiken, Gravenstein, Carpentin, Eiser and 

 Purple Red Cousinot. In England, Warner's King is said to 

 be notably less susceptible than other varieties. 



Symptoms. 



In later stages it is not at all difficult to detect the presence 

 of the European apple-tree canker, nor to distinguish it from 

 other kinds of cankers (Fig. 33) . In the younger stages, how- 

 ever, it is likely to be overlooked by the casual observer ; and 

 when noticed it is found to be not markedly different in appear- 



