THE DISEASES OF TIMBER TREES. 57 



CHAPTER VI. 



THE DISEASES OF TIMBER TREES. 



34. Definition, Causes. 



Under diseases of trees are comprised all disturbances of the 

 organism, in consequence of which the whole plant or any portion 

 of it dies prematurely. In the former case, premature interruption 

 of the canopy formed by the crop, and loss of increment will be 

 the result ; whilst in the latter, the individual stem is frequently 

 totally, or partially, useless for the more valuable technical 

 purposes. 



Disturbances in the normal development will always precede 

 or accompany any total or partial death ; but not every such 

 disturbance in growth or development, as so often happens in 

 consequence of want of nourishment, light, or moisture, can be 

 termed a diseased condition. 



The predisposition of a tree to disease is any condition, even 

 if only temporary, which, although not in itself involving 

 any injurious consequences, and perhaps even quite consistent 

 with the normal appearance of all plants at certain times, may 

 ultimately develop into a disease by the addition of any second 

 external factor. Such predisposition to disease may be occa- 

 | sioned by either extreme of age, (old age in particular rendering 

 ! many species liable to disease), by early or late flushing of 

 foliage in spring, by smoothness of the bark (scorching or sun- 

 burn), by growth in shade or on moist soil, or else through 

 wounds or injuries which open a door to infection with fungoid 

 disease, &c. 



Diseases of trees may be occasioned as follows : 



1. By external injuries. 



2. By influences of the soil. 



3. By atmospheric influence. 



4. By plants, phanerogamous or cryptogamous. 



