4 ILLINOIS NATURAL HISTORY SURVEY CIRCULAR 46 



are bare of leaves, may appear unsightly. However, mistletoe is 

 famous for its sentimental and decorative values at yuletide. 



Noninfectious Diseases 



The common causes of noninfectious diseases in trees are 

 unfavorable environmental conditions, chemical injuries, and 

 mechanical injuries (Fig. 6-12). 



Unfavorable Environmental Conditions. — Certain environ- 

 mental conditions unfavorable to trees are associated with de- 

 ficiencies or excesses of mineral elements in the soil. They cause 

 physiological disturbances or disorders within trees. These dis- 



m- 







- 



^fe-'.^i 





■vJi'?***.. ■ ' 



•x "^ ^^ 





Kit ' • 



if^: 





^, ' 



' /f -^r l^.> *. 



/ ... 



,v 





Fig. 6. — Yellowing and wilting of foliage on trees may be caused by 

 root injury from excavation of soil. Such injury to elms may produce 

 foliage symptoms similar to those of phloem necrosis. Mechanical injury 

 of this kind may cause dieback of branches or death of trees. 



