68 THE SCHOOL BOOK OF FORESTRY 



eases prevent the trees from producing more of 

 their kind. 



Most of the tree parasites can gain entrance 

 to the trees only through knots and wounds. 

 Infection usually occurs through wounds in the 

 tree trunk or branches caused by lightning, fire, 

 or by men or animals. The cone-bearing trees 

 give off pitch to cover such wounds. In this way 

 they protect the injuries against disease infection. 

 The hardwood trees are unable to protect their 

 wounds as effectively as the evergreens. Where 

 the wound is large, the exposed sapwood dies, 

 dries out, and cracks. The fungi enter these 

 cracks and work their way to the heartwood. 

 Many of the fungi cannot live unless they reach 

 the heartwood of the tree. Fires wound the base 

 and trunks of forest trees severely so that they 

 are exposed to serious destruction by heartrot. 



Foresters try to locate and dispose of all the 

 diseased trees in the State and Government for- 

 ests. They strive to remove all the sources of 

 tree disease from the woods. They can grow 

 healthy trees if all disease germs are kept away 

 from the timberlands. Some tree diseases have 

 become established so strongly in forest regions 

 that it is almost impossible to drive them out. 



