282 DISEASES OF CULTIVATED PLANTS 



attempts at inoculation by means of the conidia of the fungus 

 also resulted in failure. On the other hand, inoculation by 

 means of inserting ascospores or minute portions of wood or 

 of resin containing mycelium, into small incisions made in 

 the bark, resulted in the production of canker, on which the 

 fruit of the fungus eventually appeared. I have not succeeded 

 in producing the disease when conidia of the fungus were used 

 as infecting material. The wounds occurring under ordinary 

 conditions, through which infection may take place may be 

 grouped under four headings : 



1. Wounds caused by wind, or by snow resting on branches 

 which in consequence are partly torn away from the trunk. 



2. Extrusion of sap caused by late spring frosts. 



3. Nibbling of the cortex by insects, rabbits, etc., and more 

 especially punctures made by the larch aphis (Chermes /aria's, 

 Hartig). 



4. Wounds made near the base of the stem, when planting 

 young trees. 



As a rule when trees under ten years of age are attacked by 

 canker, they are either killed outright, or so far injured as to 

 be of inferior value if they continue to grow. The reason for 

 this is, in young trees the main stem is usually attacked, 

 whereas in older trees the bark of the trunk is too thick to be 

 punctured by insects or injured by frost. In such cases, as a 

 rule, the branches only are attacked, and the wounds are not 

 serious. The larch aphis is undoubtedly responsible for 

 much of the canker on seedlings and young trees, and perhaps 

 it is not too much to state that if larch aphis was absent, 

 there would be no larch canker to the extent of an injurious 

 epidemic. I have proved by experiment that if a young 

 branch is punctured by inserting the point of a needle, a 

 minute drop of sap oozes out ; now if spores of the canker 

 fungus are placed on this drop, infection follows. Similar 

 extrusion of minute portions of sap follow late spring frosts, 

 and as the fungus spores are mature at this season, it is 

 highly probable that infection occurs from this cause on a 

 large scale. 



In the case of young trees, canker is often present in 

 abundance on the stem just above the ground-line. This is 

 accounted for as follows by Somerville : ' 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 



