CARTER: ILLINOIS TREES: THEIR DISEASES 



47 



cause abundant premature defoliation (Fig. 44) . Two to four ap- 

 plications of organic mercury or dodine (Table 1) at 2-week in- 

 tervals should control scab. The first application should be made 

 as the buds are opening. Apple scab spreads most rapidly during 

 cool, wet weather. 



Rust. — Cedar-apple rust (Fig. 45), caused by the fungus 

 Gymnosporangimn juniperi-virginianae, and other rust diseases 



Fig. 45. — On crab apple, cedar-apple rust produces raised, orange spots, 

 which may have dark brown centers. Severely affected leaves are distorted. 



of crab apple are described in the section on juniper. Spraying 

 for control of these leaf diseases on valuable specimen plants 

 may be desirable. The sprays recommended are ferbam or thiram 

 (Table 1). Four or five sprays applied at 7- to 10-day intervals 

 are recommended. The first spray should be applied as the buds 

 are opening. 



Fire Blight. — This stem disease (Fig. 63), caused by the bac- 

 terium Erwinia amylovora, is described in the section on moun- 

 tain ash. 



ELM 



Elms are subject to many diseases, some of which are among 

 the most destructive of the tree diseases in the United States. 



Dutch Elm Disease. — This vascular disease, caused by the 

 fungus Ceratocystis ulmi, in 1962 was known to occur in 29 

 states, the District of Columbia, and three provinces of Canada. 



