LARCH CANKER 147 



above the area occupied by the mycehum of the fungus ; 

 and, if conditions are favourable, the ascophores of the 

 fungus appear on those sunk and diseased areas. During 

 the summer the mycehum does not extend to any great 

 extent owing to the formation of cork between the healthy 

 and diseased portions of bark; but in the autumn the 

 mycelium succeeds in entering the living bast, and 

 consequently the canker-spot continues to enlarge year 

 by year. 



In those cases where the growth of the parasite is rapid, 

 compared to the growth of the tree, the canker soon 

 completely girdles the trunk or branches, the part beyond 

 the region of disease dying. On the other hand, where the 

 disease is confined to one side of the trunk or branch, the 

 struggle between host and parasite may continue undecided 

 for a long time. Hartig states that in the Tyrol he has 

 seen larches still alive bearing canker wounds of a hundred 

 years' standing. 



In very damp localities larches soon die when attacked, 

 and without any large canker wounds making their 

 appearance. Under such circumstances the mycelium 

 appears to spread quickly through the entire plant, as 

 proved by the development of myriads of ascophores over 

 every part of the tree, and not, as is usually the case in 

 dry districts, confined to the canker-spots. 



Preventive Means. — The larch is truly an inhabitant 

 of Alpine regions ; and although larch canker exists on 

 trees growing in the higher Alps, nevertheless it never 

 extends its ravages to the same extent as when the tree is 

 planted in damp, stuffy lowland localities, hence such 

 should not be selected. 



The disease can be held in check by keeping a sharp 



