CHAPTER IV 
THE LARCH CANKER (concluded) 
On the mode of infection in nature.1_ 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 will 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 killed 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 
1 T have already published a preliminary note on this subject in the 
Quarterly Journal of Forestry, 1915. 
