18 



ILLINOIS NATURAL HISTORY SURVEY CIRCULAR 46 



living bark (Fig. 18). Ridges of callus may be produced by the 

 living bark surrounding the cankerous areas. On trees having 

 light-colored bark, diseased bark that does not become depressed 

 may turn reddish brown. Such cankers are conspicuous because 

 of the abnormal color of the diseased bark. Inconspicuous can- 

 kers may be revealed by removal of a thin outer layer of bark and 

 exposure of the brown to black dead bark of the diseased areas 



Fig. 18 (top). — The conspicuous, depressed canker on this branch of 

 Chinese elm is surrounded by callus. The diseased bark, in which black 

 fruiting bodies of the fungus are visible, is cracked and sloughing from 

 the wood. 



Fig. 19 (center). — Cankers may be evident only by discoloration of the 

 bark or by the presence of bumps (pustules) in the diseased bark, as shown 

 in this canker on mountain ash. In this canker, the underlying diseased bark 

 shows alternate bands of light and dark brown, in contrast to the white color 

 of living bark. 



Fig. 20 (bottom). — Fruiting bodies of a fungus frequently break 

 through the diseased bark surface and become conspicuous as black spots 

 or bumps, as shown on this branch of red oak. 



