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FUNGOUS DISEASES OF PLANTS 



XXV. RUST OF RHODODENDRON AND NORWAY SPRUCE 

 Chrysomyxa Rhododendri (De C.) De Bary 



Occurrence. The rhododendron rust is especially abundant in 

 portions of Europe and America, particularly in regions where the 

 wild species of the rhododendron and of the fir grow together in 

 the same forest areas. In fact, the rust is the most common fun- 

 gus of the rhododendron, and in regions where the fir abounds 

 the rhododendron is seldom free from its attacks. In Europe it is 

 notably abundant on Rhododendron hirsutum and on Rhododendron 

 ferrugineum, these being the hosts upon which the uredo and 

 teleuto stages are found. The spermogonial and aecidial stages 

 are confined to the fir [Pice a excelsd). In the United States this 

 fungus is particularly common in the mountains of the East, and 

 southward as far as the southern limits of the Appalachians. 



The spore forms. The spherical spermogonia appear upon 

 young leaves of the fir in the spring, and these are followed a 

 month or more later, sometimes as late as midsummer, by the 

 secidial stage. The secidia break through the epidermis of the 

 leaf as more or less tuberculate structures arising thickly on 

 the under surfaces of the leaves to a height of two or three milli- 

 meters, each producing numerous aecidiospores. The latter germi- 

 nate readily, and the host is penetrated by means of the stomata. 

 It is claimed that the mature leaves of the rhododendron are those 

 generally infected. The mycelium developed in such persistent, or 

 evergreen, leaves winters over and produces abundantly the uredo 

 stage, followed also by the teleutosporic stage. The uredo stage 

 serves to spread rapidly the fungus from one plant to another 

 during the growing season. The teleutosporic stage, however, ger- 

 minates immediately, and the basidiospores penetrate the young 

 shoots of the fir, thus completing the life cycle of this euheterouredo. 

 The uredosporic pustules appear only on the under surfaces of the 

 leaves, or occasionally on the younger stems, and these spores are 

 borne in chains with alternate sterile cells. The teleutospores are 

 closely adherent in groups. They are more or less cylindrical in 

 form, extremely light in color, and vary as to the number of cells 

 from three to six, those at the center of the group showing the 

 larger number of cells. 



