i8o 



Larch and Spruce Canker 



I have constantly observed that in the month of May — when 

 practically all infection occurs — the proportion of sap is much 

 greater in seedlings and quite young trees than in old ones. 

 What I mean exactly is this, that if branches of equal age of a 

 young and an old tree are wounded by inserting a needle or a 

 lancet, sap oozes to the surface in the case of the young tree, but 

 not in the old one. 



It is this sap that retains floating spores, and also serves as a 

 nutrient solution in which they readily germinate. 



Commencing with the causes which lead to the development 

 of canker in seedlings and young trees, the larch aphis {Chermes 

 laricis, Hartig) is most responsible ; in fact, I consider it to be 

 no exaggeration to state that if the aphis could be obliterated, 

 larch canker, as a destructive epidemic, would cease to exist in 

 this country. As to whether the interdependence between aphis 

 and canker holds good in other countries, I am not in a position 

 to state. 



During the early part of May, it is not at all unusual to find 

 the axils of many dwarf branches on the stems of seedling and 

 young larches each occupied by an aphis " foundress " and her 

 stalked eggs. Now, as to whether the foundress actually 

 punctures the bark at this point or not, I have not been able to 

 determine, but there is always a certain amount of moisture on 

 the bark where a foundress is located, and I have repeatedly 

 produced canker by placing ascospores underneath the 

 foundress. 



Situated on the bark quite near to each foundress is a small 

 patch of white flocculent matter, the centre of which is occupied 

 by a fairly large drop of sap. As evaporation proceeds after 

 two or three days, the resinous portion of the sap alone remains 

 and becomes solid (Fig. 5, PI. I.). If spores are placed in 

 this drop when first extruded, and in a liquid condition, canker 

 follows. 



The significance of this white fleck with its central drop of 

 plant sap I have not been able to determine. Its constant 

 presence is alluded to by Buckton (5), who also figures it, with- 

 out comment or explanation. The one important feature in 

 this connection is to remember that it is a channel through 

 which inoculation can take place. 



