THE RUSTS 237 



If you will compare this yearly history of the wheat rust 

 with that of the asparagus rust, you will see that the chief 

 difference is that the sporidia develop upon another host 

 from which the aecidiospores return to the wheat plant. 

 This difference is typical of many other rust fungi, and 

 special names are given to indicate the group to which the 

 various sorts belong : — 



When the different stages of a rust fungus develop on 

 one kind of plant, it is said to be an autcecious species ; ex- 

 ample, the Asparagus Rust. 



When the different stages of a rust fungus develop upon 

 two or more kinds of plants, it is said to be a hetercBcious 

 species; example, the Wheat Stem Rust. 



The stem-rust on wheat and barberry is the typical ex- 

 ample of a hetercecious rust. It is interesting because of 

 the alternation of host plants and because aU three forms 

 of spores are present. The Black Stem Rust, however, 

 often occurs in regions where barberries are either rare or 

 not found at all, and there is no dpubt that the fungus can 

 live from year to year without going through the cluster- 

 cup stage on barberry. 



Other Grain Rusts 



There are other rusts of grain even more destructive 

 than the Wheat Stem Rust. The Orange Leaf Rust of 

 Wheat is perhaps the worst of these. Technically it is 

 called Puccinia rubigo-vera. It is believed by good au- 

 thorities to be the most abundant of all grain rusts in the 

 United States. The uredospore form is the one generally 

 present, and in the South at least the fungus can live 

 through the year with no other spores present. Teleuto- 

 spores have been found, but the aecidium stage is unknown. 



