6 



that had not yet been girdled. In old cankers there is likely to 

 be more or less destruction of tissue at the center, so as to 

 leave a somewhat open wound exposing the decaying wood 

 (figs. 3 and 5). Old cankers are likely also to show the work 

 of woodpeckers and insect larvae. 



When a Avell-defined canker is not produced the extent of the 

 lesion may be indicated by an elliptical discolored area which is 

 generally somewhat depressed. Even when the bark of this 

 area is not destroyed it is rougher than surrounding portions 

 due to more or less shrinking and cracking. In all cankers the 

 characteristic black nodules may be noted on those portions on 

 which the bark has remained intact. 



In some infections there is a pronounced enlargement or 

 hypertrophy of the entire invaded area (fig. 6), instead of 

 atrophy or lessening of growth. In the open cankers there rnay 

 be more or less enlargement or distortion of the trunk (figs. 3, 

 5 and 7), due to the stimulating effect of the parasite. Such 

 effects appear to be more frequent on the oaks than on the 

 chestnut. 



The disease is least evident in its diffuse form. In this type 

 there is an absence of well-defined cankers and the fungus ap- 

 pears to spread so rapidly that little or no callus is developed. 

 In such cases the young tree is killed before time has been given 

 for the development of a canker. This points to the fact that 

 the larger trees showing well-defined cankers represent the few 

 individuals that have offered the most resistance to the attacks 

 of the parasite. The diffuse type of the disease has not yet been 

 found on anythmg but young trees up to 3-4 inches in.diameter. 

 It should not be understood that there are two entirely distinct 

 types of the disease, for there are all gradations between the 

 pronounced canker types and the diffuse types ; in the inter- 

 mediate types the lesion may be marked by a more or less con- 

 centric zonation (figs. 2, 4 and 7), while in the extreme diffuse 

 type no zonation is evident (fig. 8). 



The disease appears to start in a branch axil. In all young 

 infections or old cankers this relation is an invariable rule, and 

 in lesions of any age the dead stub of a branch marks the center 



