12 FOREST TREE DISEASES. 



Diseases of the root system have the same disastrous 

 effect on the. water supply of the tree, and destroy Un- 

 reserve materials stored in the roots. 



Diseases of the flowers and seeds destroy (lie faculty 

 of reproduction. 



CAUSES OF DISEASE. 



Whenever a tree, growing under the same conditions 

 as its thrifty neighbors, appears in any way unhealthy, 

 there is reason to assume some kind of attack by para- 

 sitic fungi, mistletoes, or insects. Yet there are quite a 

 number of other things which bring about disease or 

 death in trees, and before we decide finally that one of 

 the three factors named is responsible, except of course 

 where there can be no possible doubt, we should first 

 make sure that there is not some other cause for the 

 abnormal condition of the tree in question. Parasites 

 are plants or animals which live off the living tissues of 

 other plants or animals, not merely by mechanically 

 destroying them, but by partly or totally penetrating 

 the tissues of the so-called " host " that is, the plant 

 or animal they live on. Certain parasites are able to 

 enter the youngest parts of trees, twigs, or leaves 

 directly; the majority of fungi causing decay of the 

 wood can get into the interior of the living tree only by 

 way of a pin knot or a wound. For this reason every 

 wound caused by lightning, by fire, by man, or by ani- 

 mals constitutes a menace of infection, unless it receives 

 an aseptic dressing such as the coniferous trees to a 

 certain extent give in covering the exposed surface with 

 resin. 



