APPLE DISEASES 63 



RUST 



Caused by Gymnosporangium Juniperi-virginiance Schw. 



This disease is variously known as cedar-rust, cedar-rust of 

 apple, cedar-apple, apple-rust and cedar-flowers. It affects 

 not only the apple and wild crab, but also the red cedar. Both 

 kinds of plants are necessary in the same immediate locality 

 for the perpetuation of the rust-pathogene. If the cedar is 

 absent, there can be no apple-rust. 



With respect to the apple, a great many lists of susceptible 

 and resistant varieties have been published from various 

 parts of the country. These lists agree in part, but it is of 

 interest to note that certain varieties are classed as susceptible 

 in one state and as resistant in another. The Ben Davis, for 

 example, is said to be resistant in Delaware, Massachusetts, 

 Rhode Island and Nebraska, but is reported as being susceptible 

 in Iowa and Virginia. The Rome is susceptible in Alabama, 

 Indiana, West Virginia and other states, but is resistant in 

 Delaware. Similar examples of such variation are found in 

 the Grimes and Red June. While there may be some variation 

 due to the difference in location, it appears that the judgment of 

 the individual as to what constitutes resistance or susceptibility 

 is a more probable explanation. Different periods of rust- 

 infection may give rise to confusing data, since the leaves of 

 one variety may expand more quickly or may have a shorter 

 or longer period of susceptibility than the leaves of another 

 variety. It has been found that varieties are susceptible only 

 while the leaves are young; after maturity, they are im- 

 mune. It has been learned also that the morphological make- 

 up of the leaf, as, for example, hairiness, has nothing to do with 

 the question of its susceptibility. 



In spite of these uncertainties with respect to varietal resist- 

 ance, it is generally agreed that the York Imperial is very 

 susceptible. Others in this class are the Rome, Wealthy, 



