DISEASES OF SPECIAL CROPS 



79 



than a pinhole, while upon the lower surface of the dis- 

 eased spot appear some weeks later the fringe-toothed 

 cluster cups. This spot is 

 sometimes so abundant 

 that its presence upon 

 orchards may be recog- 

 nized at great distances 

 by the characteristic hue 

 imparted to the foliage 

 mass as a whole. It robs 

 the tree of nourishment 

 and renders normal fruit- 

 age impossible. 



A peculiar relation ex- 

 ists, in that the causal 

 fungus spends part of its 

 life on one plant, the apple, 

 and the remainder upon a 

 totally different plant, the 

 red cedar, the Alternate 

 Host of the apple rust. 

 The fungus summers upon 

 the cultivated apple or the 

 wild crab-apple tree, at 

 the approach of winter its 

 spores are borne by the 

 wind from the apple trees 

 to an adjacent juniper or 

 red cedar tree. There the 

 fungus grows and causes the familiar " cedar apple." 



In the spring the cedar apple produces gelatinous, horn- 



FiG. 28. — Apple leaf affected with rust. 

 Original. 



