190 Silver- Leaf Disease, III 



Few papers on silver-leaf disease have appeared since the publica- 

 tion of the paper by one of us (2) in 1913, the most noteworthy beinj; 

 that by Smolak (9), to which it is necessary to devote some attention 

 before our own results are considered. 



As pointed out in the Annals of Applied Biology (4), he has inade- 

 quately represented the views on silver-leaf expressed by one of us in 

 earlier papers. For instance. Smolak states that "according to the 

 investigations of Brooks the basidiomycete Stereuni ■jnir]iureum is the 

 cause of silver-leaf disease," whereas the view put forward was that 

 "silver-leaf disease is a general pathological phenomenon, one cause of 

 which is the fungus Stereum purpureum,'' a view which, it may be said 

 at once, we see no reason to change. 



Smolak's observations on the cytology of silvered leaves are inter- 

 esting, but there is no reason why the changes in the cell contents 

 described by him should not be attributable either directly or indirectly 

 to Stereum purpureum present in woody tissues below, even though the 

 influence of a specific enzyme carried up in the transpiration current 

 as suggested by Percival (7) be excluded. Smolak apparently finds it 

 difl&cult to conceive of action at a distance in the association of Steretmi 

 piirpurevm with silver-leaf disease, and suggests {loc. cil. p. 154) that 

 silvering is an autolytic action. The presence of the fungus in the woody 

 tissues of the stem may, however, cause some chemical or physical 

 disturbance in the transpiration current, which, upon being felt in the 

 leaves above, causes silvering. Such a disturbance in the transpiration 

 current may either directly bring about the change or indirectly do so 

 by liberating some other substance in the cells of the leaves to which 

 the change is immediately due. Where silvering is not due to StereiDii 

 purpnrexi}! the phenomenon may be caused by the liberation of the 

 same substance in response to other, at present unknown conditions, or 

 may be otherwise brought about by these particular conditions. Even 

 when the primary agent in silvering is Stereum purpureum, the last 

 links in the chain of cause and effect are not yet known. 



The cytological peculiarities of silvered leaves described by Smolak 

 are possibly incipient stages in the death of the cells slowly induced by 

 a disturbance in the normal physiology of the tissues. No statement is 

 made as to the age of the silvered leaves examined by him. 



Smolak also adds that "perhaps the inoculation experiments hither- 

 to carried on have been too few or possibly they are not yet complete 

 enough to decide the relation of Stereum. to this disease," but in view of 

 the very numerous successful inoculation experiments carried out by 



