CARTER: ILLINOIS TREES: THEIR DISEASES 



67 



Fig'. 62. — Maples affected with Verticillium wilt have green discolora- 

 tion in the young sapwood, especially in the current-season wood. This 

 green discoloration frequently appears as fine streaks. Occasionally it may 

 appear as solid bands of green in individual wood rings. In trees of other 

 kinds, the discoloration is brown. 



MOUNTAIN ASH 



Leaves of mountain ash are occasionally affected with a 

 fungus disease called scab. This disease is described in the sec- 

 tion on crab apple. The most destructive disease of mountain ash 

 in Illinois is fire blight. 



Fire Blight. — This bacterial disease, caused by Erwinia 

 amylovora, produces branch and trunk cankers (Fig. 63), twig 

 blight or dieback, and blight of leaves, blossoms, and fruit of 

 mountain ash. It affects other ornamental plants, also : coto- 

 neaster, crab apple, fire thorn, Japanese flowering quince, rose, 

 serviceberry, and spirea. 



Leaves and blossoms of a mountain ash affected with fire 

 blight suddenly wilt and turn brown to black, as if scorched by 

 fire. Affected twigs die. The disease may spread from twigs to 

 branches and produce extensive cankers. Cankers usually are 

 bounded by cracks between the dead and living bark. The dead 

 bark usually turns reddish brown. Cankers that girdle the trunk 

 may cause sudden leaf blight, followed by death of the tree. 



Fig. 63. — I'ire blight cankers are usually conspicuous because of the 

 rough, scaly, depressed appearance of the diseased bark and the formation 

 of fissures or cracks between the living and diseased bark. The canker 

 shown in this picture is on crab apple. 



