40 FOREST TREE DISEASES. 



DISEASES OF BOOTS, TRUNKS, AND LIMBS. 

 EFFECT. 



Typical diseases of the stem, (heartrots) invariaMy 

 do a great deal of damage by destroying merchantable 

 timber, often, however, without impairing the health 

 and growth of the tree except in weakening it mechanic- 

 ally. Different fungi act in different ways on the 

 wood they inhabit ; in other words, every fungus causes 

 a more or less characteristic decay by which, in many 

 cases, we can determine the kind of fungus in the ab- 

 sence of fruiting bodies (sporophores). Entering the 

 tree through a wound or some other opening in the 

 bark, the mycelium of the fungus grows in the dead 

 heartwood. As a rule the living sapwood is untouched. 

 The inability of most of the fungi of this class to in- 

 vade living tissue explains why they are only found on 

 older trees and not on younger ones, which contain no 

 or very little heartwood. The age at which the trees 

 of a given species may become infected varies and is 

 closely connected with the age at which the species 

 forms heartwood ; it varies also with the species of the 

 attacking fungus. In general, diseases of the trunk 

 are rare in sugar pine, more frequent in yellow and 

 Jeffrey pine, common in Douglas fir and red fir, and 

 very common in incense cedar and white fir. 



The fungi which cause root diseases, on the other 

 hand, are able to attack living tissue, at least when 

 offered entrance by a wound caused by rodents, plow- 

 ing, etc. They invade and kill the living bark, the 



