Minnesota Plant Diseases. 



47 



is thus slowly killed and the fungus continues to live on in its 

 saprophytic way. The wounds, through which these fungi 

 penetrate, may be caused in many ways: the breaking of 

 branches in storms, or the wounds by falling trees, the rubbing 

 together of trees swayed by the wind ; injury by cattle and 

 deer ; lightning strokes ; frost cracks ; hailstones, sun scalds, 

 holes of woodpeckers, squirrels, boring insects ; injury to roots 

 by burrowing animals. Moreover, man himself is responsible 

 for many such opportunities for wound parasites, chief among 

 which are the wounds of shade and orchard trees in pruning. 

 Such wounds should be covered with tar or some other sub- 

 stance which will prevent the development of the fungus myce- 

 lium. Of the wood parasites by far the greater number are allies 

 of the mushroom group. Gill fungi and pore fungi and a few 

 burnt-wood fungi are also found in this class. The best known 

 is the common fall- or honey-mushroom which occurs in clus- 

 ters at the bases of stumps and trees in the autumn. Many of 

 the very common "shelf fungi" are also in this class. Not -all 

 half-saprophytes are dependent upon wounds for their entrance 

 to the tree trunk. A few such, as the honey-mushroom, may 

 gain entrance by attacking the smaller roots with a shoestring- 

 like strand composed of thousands of threads. The latter pene- 

 trate the bark to the growing layer just beneath. They then 

 enter upon a parasitic life and make their way up through the 

 roots to the stem. In this way the fungus can proceed from 

 one living tree to another and cause epidemics. 



