50 THE LARCH CANKER 



recorded artificial infection with larch canker was per- 

 formed by a practical forester named Fischer, who caused 

 canker in trees by inserting into a wound made in healthy 

 bark a suitably shaped piece of bark and phloem from 

 a canker on another tree. This experiment proved that 

 canker was due to a transmissible cause and was not merely 

 the result of unsuitable growth conditions. That the fungus 

 Dasyscypha calycina, is actually responsible for canker was 

 first proved by Hartig when he induced the disease by 

 infection with ascospores of the fungus. An account of his 

 experiments has already been given on p. 23, and need 

 not be repeated here. 



Massee (1902) also produced canker by means of artificial 

 infection, and obtained results which are important in two 

 respects. Firstly he found that canker resulted when 

 ascospores were placed on the mucilaginous excretion of 

 Chermes abietis in the spring. This he attributed to the 

 small holes which the insect makes in forcing its proboscis 

 through the cork to the living tissues beneath ; and he 

 supposed that the fungal germ tubes grew down these holes 

 and thus gained admission to the cortex and phloem below. 



Massee attached great importance to this method of 

 infection as a source of canker in nature, and considered 

 that if the larch aphis could be destroyed, canker would 

 cease to be a serious epidemic disease. As I shall show, 

 however, the aphis can only assist in the infection of com- 

 paratively young stems, whereas the cankers which are 

 important to foresters are mostly those which have been 

 induced after the stems are three years old. 



Secondly, Massee found that Dasyscypha calycina is 

 capable of infecting other trees besides the European 

 larch. From the spores of the fungus he produced canker 

 on the Scots pine and on two other species of larch the 

 Siberian (Larix siberica) and the Japanese (Larix leptolepis). 



In my own infection experiments, inoculation was 

 obtained by inserting a small piece of agar-agar, with 

 fungus mycelium in pure culture, in a slit made with a knife 

 in the bark of young trees. Infections made in Munich in 



