8 ILLINOIS NATURAL HISTORY SURVEY CIRCULAR 46 



Mechanical Injuries. — Mechanical injuries to trees are 

 caused by man (Fig. 6), birds (Fig. 7), or other animals, and 

 by insects (Fig. 8), as well as by forces or conditions not in- 

 fluenced or controlled by man. Many of these injuries occur to 

 the exterior of the tree, especially to the trunk. Most of the in- 

 juries caused by girdling roots (Fig. 9), axes or similar tools, 

 ground fires, girdling wires (Fig. 10), nails driven into trunks, 

 lawn mowers, and motor vehicles can be avoided. Most of the 

 injuries caused by water, hail, ice (Fig. 11), snow, wind (Fig. 

 12), and lightning cannot be prevented by man. Damaged trees 

 should be treated as soon as possible to reduce the amount of 

 ultimate injury that might occur. Broken branches should be cut 

 back to sound lateral branches or to where they are attached to 

 the trunk. Injured bark should be cut back to uninjured bark. 

 Exposed wood in wounds should be protected with wound dress- 

 ing to prevent decay, as described under "Wound Treatment." 



TYPES OF TREE DISEASES 



Tree diseases are frequently classified according to the part 

 of the tree which they attack or affect. On this basis they may be 

 divided into leaf diseases, stem diseases, vascular diseases, and 

 root diseases. 



Leaf Diseases 



Leaf diseases may be infectious or noninfectious. Most in- 

 fectious leaf diseases are caused by fungi. Some are caused by 

 viruses or bacteria. The infectious diseases may produce mold- 

 like growths on the leaf surfaces, local lesions in the tissues, 

 yellowing of the tissues between the veins, or death of entire 

 leaves. Some of them may affect flowers, fruits, twigs, or young 

 branches. They may cause very little permanent damage to de- 

 ciduous trees unless infection occurs year after year. Severe leaf 

 injury and defoliation for several successive years may weaken 

 affected trees and make them more susceptible to attack by other 

 diseases and by insects. Also, they may lower the resistance of 

 trees to unfavorable weather conditions. Defoliation in late sum- 

 mer is less injurious than defoliation in early summer, since the 

 annual tree growth is usually completed by late July. Defoliation 

 of evergreens during a single growing season may cause severe 

 dieback of branches or death of whole plants. 



The names of many leaf diseases are descriptive of the 

 types of injury produced, such as powdery mildew, spot, blotch, 



