CARTER: ILLINOIS TREES: THEIR DISEASES 



59 



Fig. 55. — In dry weather, the spore-horns of the mature gall of the 

 cedar-apple rust contract and appear as wrinkled threads. 



discoloration of the tissue, these spots become visible on the 

 under surfaces of leaves. At this time, black dots appear on the 

 upper surfaces of the spots. Mature spots, about three-eighths 

 inch in diameter on the under surfaces of leaves, are yellowish- 

 orange and frequently bordered by a red band or a chlorotic (yel- 

 low) halo. The tissues in these spots become thickened, and in 

 July and August a number of raised areas or protuberances ap- 

 pear. Later these raised areas develop into cylindrical tubes, 

 which split into segments at their distal ends. These split ends 

 curl outward and backward to expose light brown spores, which 

 are the spores that infect junipers. 



Since cedar-apple rust requires juniper and apple or crab 

 apple as alternate hosts, these hosts usually should not be grown 

 close to each other. Junipers planted within a mile of apple or 

 crab apple should, if feasible, be species or varieties resistant to 

 rust. A list of resistant and nonresistant junipers may be ob- 

 tained from the Illinois Natural History Survey. 



In ornamental plantings, where alternate hosts are planted 

 close to each other, protection from rust infection can be ob- 



