CHAPTER IV 

 THE LARCH CANKER {condnded) 



On the mode of infection in nature.^ Importance of wounds as a source 

 of canker. Contributory causes of canker. Methods of prevention. The 

 synonymy of Dasyscypha calycina. 



On the mode of infection in nature. After the examination 

 of a large number of cankers I have arrived at the conclusion 

 that the ordinarily accepted methods of infection by the 

 disease are insufficient to account for the numerous cankers 

 which occur on larch stems. I shall consequently give 

 a summary of the theories that have been advanced, and 

 then proceed to the consideration of a new one which may 

 explain the occurrence of canker on main axes. 



For the sake of clearness it \\ill be well to divide infec- 

 tions into two classes : (i) those on young stems and small 

 lateral shoots ; (ii) those on main trunks which occur when 

 the latter are more than two years old. 



The first class of canker is comparatively unimportant, 

 since side branches die quickly whether they are attacked 

 or not ; and when a main shoot is affected early, it is gener- 

 ally killed and its place taken by a lateral. 



(i) It is a matter of observation that a shoot does not 

 become cankered till the end of its first year's growth. 

 This is a necessary corollary of the view propounded by 

 Berkeley and accepted by all subsequent writers that the 

 annual ring is always complete at the base of a canker, so 

 that the cambium must be Idlled in the winter. This does 

 not, however, preclude the possibility of infection having 

 occurred in the outer tissues during the summer, and it is 

 only the failure to find any sign of fungal growth in shoots 



' I have already published a preliminary note on this subject in the 

 Quarterly Journal of Forestry, 1915. 



