CARTER: ILLINOIS TREES: THEIR DISEASES 23 



destructive wilt diseases caused by fungi are Dutch elm disease 

 and oak wilt. The only widespread and destructive wilt disease of 

 trees caused by a virus is phloem necrosis of elm. 



Root Diseases 



Root diseases, some of which produce abnormal growths or 

 tumors on the roots, may weaken or kill the roots of affected 

 trees. Infectious agents that produce root diseases include fungi, 

 bacteria, and viruses. Shoestring root rot, a fungus disease, is 

 one of the common root diseases of trees that have previously 

 been weakened or injured. Loss of roots from this disease de- 

 prives affected trees of sufficient food and water for normal 

 growth and frequently results in branch dieback and staghead 

 (frontispiece) . 



DIAGNOSIS OF TREE DISEASES 



In some tree diseases, the causes may be determined readily 

 by careful examination of the affected parts. For example, the 

 fungi that cause certain leaf diseases may produce conspicuous 

 gray to white powdery growth on the surfaces of affected leaves 

 or they may produce circular to irregular, brown, dead areas in 

 the leaves. These diseased areas may have visible bumps or 

 pustules in the affected tissues. In many other tree diseases, the 

 causes are obscure and may not be found on or in the portion of 

 a tree that shows disease. In wilt diseases, for example, the 

 leaves wither and die, but the organism that causes wilt is in 

 the sapwood of the stem. An accurate diagnosis can be made 

 only after sufficient evidence and information about the tree and 

 its surroundings have been obtained to develop a complete case 

 history of the trouble. 



TOOLS AND THEIR USES 



Tools useful for examining trees suspected of being diseased 

 include pocket knife, hand shears, leather punch, saw, hatchet 

 or ax, shovel, increment borer, reading glass or hand lens, and 

 field glasses (Fig. 28). The leaves and branches of small trees 

 may be reached from the ground and examined for diseases. On 

 trees so tall that the leaves and branches cannot be reached from 

 the ground, injury to leaves, cankers on branches, or other signs 

 of disease frequently can be detected with field glasses. 



To determine the specific agent or type of agent causing the 

 trouble, it is necessarv in most cases to obtain a sample or sam- 



