CARTER: ILLINOIS TREES: THEIR DISEASES 55 



enlarged. Shoots from the affected buds usually become dwarfed 

 and grow in clusters (Fig. 51). The twigs in each cluster are 

 short and stubby, and the diseased leader fails to develop fully. 

 An open type of broom is formed when a healthy leader has nu- 

 merous short, stubby twigs produced around its base. 



Witches'-broom is more common and widespread on Ameri- 

 can hackberry than on Chinese or Mississippi hackberry. 



There is no effective control for this disease ; however, if the 

 brooms are objectionable to the owner of an affected tree, they 

 can be removed by pruning, as described in the section "Prun- 

 ing." Feeding the tree to stimulate vigorous growth may be de- 

 sirable; this is described under "Feeding." 



HAWTHORN 



The three most destructive diseases of hawthorn in Illinois 

 are leaf blight, rust, and fire blight. (Fig. 52, 57, 59, 63). 



Rust. — Three fungi, Gymnosporangium globosum (cedar- 

 hawthorn rust), G. juniperi-virginianae (cedar-apple rust), and 

 G. clavipes (cedar-quince rust), cause rust on hawthorns. Of the 

 three rusts, cedar-hawthorn rust is most destructive to haw- 

 thorn. These rust diseases are described under juniper. Haw- 

 thorns can be protected from these rusts by ferbam or thiram 

 sprays (Table 1). The first spray should be applied in the spring 

 as soon as spore-horns begin to protrude on the cedar-apple galls. 



Fire Blight. — This bacterial stem disease, caused by Erwinia 

 amylovora, is described in the section on mountain ash. 



Leaf Blight. — The fungus Fabraea maculata causes a seri- 

 ous leaf blight of hawthorn, especially English hawthorn and 

 Paul's scarlet thorn. It also affects apple, chokeberry, cotoneas- 

 ter, Japanese quince, medlar, mountain ash, Photinia, and ser- 

 viceberry. Affected leaves (Fig. 52) first show small, angular, 

 reddish-brown spots with irregular or radiating margins on their 

 upper surfaces during the early part of the growing season. 

 These spots enlarge and some of them may coalesce to form large, 

 irregular diseased areas. Severely affected leaves fall prema- 

 turely; extensive defoliation occurs when the disease is serious. 

 A small, black, raised, pimple-like spot, fruiting body of the fun- 

 gus, develops in the center of each diseased area. Spores produced 

 in the fruiting bodies cause additional infection during the grow- 

 ing season. 



The raking and burning of fallen leaves will help to control 

 leaf blight. Spotting of leaves can be prevented by spraying with 



