294 Studies in Forestry [CHAP. xin. 



Dry warm weather retards and checks the development of 

 this disease ; but when, for several seasons, the spring and 

 early summer months are wet, it often occasions damage whose 

 traces never become obliterated. 



Prevention of this disease is confined solely to the selection 

 of suitable soil and situation for the different species of Pine ; 

 whilst remedial measures include the removal and burning 

 of all shoots infected, and the cutting out of all Aspen on 

 the foliage of which the change of generation as Melampsora 

 tremulae may be carried out (see also note to p. 282). 



4. Aeddium elatinum, Link., the Silver Fir Fungus. To 

 this fungus are due not only the peculiar cankerous, diseased 

 swellings occurring on the stems at a greater or less height, 

 but also the deformities or diseased twig-clusters, called ''witches- 

 brooms ' in Germany, often formed in the crowns of Silver Firs. 



The canker makes its appearance on the stem at all heights 

 as well as on the branches. It is most frequent on loamy 

 soils and low-lying localities, where the progress of the disease 

 is more rapid than on sandy soils and upland tracts. It forms 

 spindle-shaped excrescences usually extending all round the 

 stem, though sometimes only on one side, on which the 

 bark fissures longitudinally, so that the wood is laid bare and 

 begins to rot. This process is hastened by insects becom- 

 ing attracted, and by other fungoid diseases (Polyporus] 

 effecting an entrance at the wounds; whilst the mycelium 

 asserts itself throughout the bark-parenchym and the neigh- 

 bouring sap-wood, the same kind of mycelium as is found 

 occasioning the twig-deformities. Infected stems become more 

 or less unsuited for technical purposes according to the part 

 attacked and the number of cankers, of which two or three 

 may sometimes be seen on one tree ; such stems also often 

 break at the diseased parts during stormy winds. The canker 

 may continue active in trees for fifty years or more, although 

 during very hot summers it is apt to become fatal to the tree. 



Another form of the disease occasioned by this fungus is 



