CARTER: ILLINOIS TREES: THEIR DISEASES 21 



galls interferes with sap flow and may cause stunting or death 

 of trees. Wounds should be treated promptly to prevent infection. 



Dieback. — Dieback is the gradual dying of a twig or branch 

 from tip to base (Fig. 24). Frequently it develops on trees that 

 are severely affected by canker diseases, especially on those 

 branches girdled by cankers. Dieback may develop also on trees 

 affected by conditions that reduce the amount of food and water 

 below the minimum required for growth. Treatment for dieback 

 includes removing all dead wood and supplying food and water 

 to stimulate growth. 



Witches'-Broom. — This disease produces broomlike growths, 

 composed of clusters of shoots, on branches of affected trees 

 (Fig. 25). Usually the shoots in these clusters are dwarfed; they 

 grow from swollen or enlarged areas on the branches. The broom- 

 like appearance is most conspicuous when the trees are without 

 leaves. Witches'-brooms occur on both deciduous and evergreen 

 trees; they are caused by fungi, viruses, mistletoes, insects, or 

 mites. In Illinois, witches'-broom of hackberry is common. 



Fig. 26. — Fruiting bodies or conks of the bracket-type, wood-rotting 

 fungi frequently develop at wounds on the trunks or branches of trees. In- 

 fection by fungi occurs through such wounds. This picture shows conks 

 of the white mottled rot fungus on white ash. 



