CARTER: ILLINOIS TREES: THEIR DISEASES 



57 



Cedar-Apple Rust. — This disease is caused by the fungus 

 Gymnosporangium juniperi-viyginianae. It has juniper and apple 

 or crab apple as alternate hosts. On juniper it affects twigs and 

 leaves, especially awl-shaped leaves. It stimulates the infected 

 tissue to form galls, which become visible on a juniper first as 

 small, deep red, smooth, globular growths in the axils of leaves 

 in June of the first spring after infection. These galls enlarge 

 until they become m.ature the second spring after infection, when 

 they may measure up to 2 inches in diameter. 



The mature galls, called cedar apples, are greenish-brown, 

 globular to irregular-shaped, and corky (Fig. 53). As the galls 

 grow, a series of small, circular, pitlike depressions, each with a 

 small pimple-like protuberance in the center, forms on their sur- 

 faces. The gelatinous, finger-like, orange spore-horns which pro- 

 trude from the surfaces of the galls arise from the circular de- 

 pressions. These spore-horns, one to over a hundred per gall, are 



Fig. 53. — Cedar-apple rust on juniper is conspicuous because of the 

 corky, chocolate-brown galls it produces on twigs. A mature gall is called 

 a cedar apple. Spore-horns develop in the depressions of the gall. 



