66 THE LARCH CANKER 



for many years, and probably snow accounts for the fall of 

 most of them. At any rate the dense screen of small lateral 

 dry branches so characteristic of our own woods is generally 

 absent in the Alps ; and since, as has been shown above, 

 dead branches are one of the nlost important sources of 

 canker, the absence of such branches in the Alps is a note- 

 worthy factor in the suppression of the fungus. 



In the case of British plantations it is questionable 

 whether the altitude at which larch is planted is ever great 

 enough materially to lessen the attack of the canker fungus. 

 Trees may be as badly attacked at a height of 1,000 ft. as 

 at sea-level, and where a mountain-side is immune from the 

 disease this may be accounted for by the favourable soil 

 conditions that such a slope provides. 



Though altitude has little effect in restricting canker, low 

 hollows seem to encourage it very markedly. Such hollows 

 are peculiarly liable to late frosts, which have an adverse 

 effect on the general health of the trees and render them 

 more liable to infection, and frost-wounds may admit the 

 fungus in some cases. Also the atmosphere is there more 

 humid, and conditions are favourable for the formation of 

 fructifications of the fungus. 



No direct relation can be traced between the frequency 

 of canker and the rainfall or the site exposure. 



Soil. Though it is very difficult to determine exactly the 

 soil conditions which favour canker and the reverse, never- 

 theless the edaphic factor seems to be of more importance 

 than the climatic. Even in flat country neighbouring plots 

 of larch may be very variously affected, and since this cannot 

 be attributed to climatic differences, it seems a just inference 

 that soil is responsible. On the whole we may say that 

 soil conditions which favour larch-growing in other respects 

 are generally least conducive to canker ; a good, deep, well- 

 aerated soil is better than one which is peaty or rendered 

 shallow by a pan near the surface. Forbes (1904) lays 

 great stress on the fact that young trees are often notched 

 or slitted into the upper six inches of soil quite unsuited to 

 their requirements. If it is composed of dry peat or debris 



