TKEE DISEASES 155 



ailment has gone too far, it may not be wise to try to save 

 the tree. A timely removal of a tree badly infested with 

 insects or fungi may often be the best procedure and may 

 save many neighboring trees from contagious infection. 

 For this, however, no rules can be laid down and much will 

 depend on the local conditions and the judgment and 

 knowledge of the person concerned. 



Fungi as factors of disease: The trees, the shrubs and 

 the flowers with which we are familiar are rooted in the 

 ground and derive their food both from the soil and from 

 the air. There is, however, another group of plants,^ — 

 the fungi,— the roots of which grow in trees and other plants 

 and which obtain their food entirely from the trees or 

 plants upon which they grow. The fungi cannot manu- 

 facture their own food as other plants do and consequently 

 absorb the food of their host, eventually reducing it to 

 dust. The fungi are thus disease-producing factors and the 

 source of most of the diseases of trees. 



When we can see fungi growing on a tree we may safely 

 assume that they are already in an advanced state of 

 development. We generally discover their presence when 

 their fruiting bodies appear on the surface of the tree as 

 shown in Fig 109. These fruiting bodies are the familiar 

 mushrooms, puffballs, toadstools or shelf-like brackets 

 that one often sees on trees. In some cases they spread 

 over the surface of the wood in thin patches. They vary 

 in size from large bodies to mere pustules barely visible 

 to the naked eye. Their variation in color is also significant, 

 ranging from colorless to black and red but never green. 

 They often emulate the color of the bark, Fig. 110. 



Eadiating from these fruiting bodies into the tissues 

 of the tree are a large number of minute fibers, comprising 

 the mycelium of the fungus. These fibers penetrate the 



