Larch and Spruce Canker. 



181 



Professor Marshall Ward (4) considers that late frosts are to 

 a great extent responsible for the prevalence of canker in larch. 

 I have once observed in the middle of May, a time when 

 seedlings and young larches are replete with sap, that after a 

 sharp frost, followed by bright sunshine, the branches were 

 studded with minute drops of sap, which eventually hardened 

 into resin. Ripe ascospores were not forthcoming at the time 

 when this observation was made, but I can readily conceive that 

 canker would have resulted if spores had been deposited on the 

 freshly extruded beads of sap. 



Owing to the comparative paucity of sap in the branches of 

 old trees, a May frost would not be so likely to result in injury. 



In the case of young trees, infection often takes place just 

 above the ground line, and is thus explained by Dr. Somervilie 

 (8). " Not only is infection specially liable to occur low down on 

 a stem on account of the abundance of moisture, but the chances 

 of an outbreak of disease at such a place are also favoured by 

 the frequent presence of wounds near the surface of the ground. 

 These wounds may be caused by lifting the plants from the 

 nursery beds, or they may be formed by the feet or the tools of 

 the workmen during planting. Where trees are planted by 

 * notching ' the turf is firmed round the base by the repeated 

 application of the heel of the planter, and in doing so it not 

 unfrequently happens that the boot comes in contact with 

 the stem and abrades the bark. And should the plant 

 escape injury during planting, it is still very likely to be 

 injured near the ground by rabbits, hares, voles and other 

 animals." 



For the reasons already stated, the trunks of trees over 

 10 years of age are practically safe against attack from canker, 

 except near the apex, and there the drier air does not favour the 

 earliest stage of inoculation so much as the damper conditions 

 that prevail near the ground. If the trunk escapes the disease, 

 canker attacking the branches at a later stage does not, as a 

 rule, interfere with the continued growth and development of 

 the tree. 



The cup or ascophore varies considerably in size, ranging from 

 two to five millimeters, externally snow-white, and minutely 

 hairy under a pocket-lens ; the disc varies from deep orange to 



