40 Diseases of Economic Plants 



smaller than a pinhole, while upon the lower surface of the 

 diseased spot appear some weeks later the fringe-toothed 

 cluster-cups. This spot is sometimes so abundant that its 

 presence upon orchards may be recognized at great distances 

 by the characteristic hue imparted to the foliage mass as a 

 whole. Rust robs the tree of nourishment and renders normal 

 fruitage impossible. 



A peculiar relation exists, 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 ( Juniperus) , the 

 Alternate Host of the apple rust. The fungus summers upon 

 the cultivated apple or the wild crab-apple tree, from which 

 later spores are borne by the wind to adjacent juniper or 

 red cedar trees. There the fungus grows and causes the 

 familiar "cedar apple." 



In the spring the cedar apple produces gelatinous, horn- 

 like projections, each bearing myriads of spores, which, 

 when conveyed by the wind to the susceptible apple tree, 

 under suitable conditions, cause the apple rust. Evidence 

 that this relation really exists is readily found if one ex- 

 amines an orchard having red cedar trees upon its windward 

 boundary. 



It is clear that the more red cedar trees there are in the 

 neighborhood of an orchard, especially to windward, the 

 more probability there is of damage from the rust. The 

 rational treatment, therefore, is to remove these trees in 

 so far as possible, or if the value of the cedar warrants it to 

 hand-pick the cedar galls. 



In rare instances spores may be carried several miles, but 

 orchards are reasonably secure if all possibility of infection 

 from the immediate neighborhood be removed; that is to 

 say, for a radius of two miles. In case the red cedars are too 

 numerous, or if for other reasons it is impossible to cut them, 

 spraying the apple trees at a time corresponding with the 

 spore discharge from the cedar-galls will lessen the evil, but 

 the success of spraying is neither complete nor certain. 



Resistant apple varieties are : Yellow Transparent, Maiden 



