THE BASIDIUM FUNGI (BASIDIOMYCETES) 243 



233. Black rust of grain (Puccinia graminis). Wherever wheat 

 and oats are grown, black rust, sometimes called red rust, is a 

 dreaded pest. It also grows upon many other grasses, includ- 

 ing barley and other cereals. The damage done to the world's 

 crops by this fungus is very large indeed, and 

 in the United States it has been estimated to 

 exceed $15,000,000 in a single year. Much 

 money has been expended m makmg stud- 

 ies of the life habits of this parasite, with 

 the hope that means of preventing its ravages 

 may be discovered. 



The first conspicuous appearance of rust 

 in the late spring or early summer is in the 

 form of reddish-brown patches upon stalks 

 and leaves of wheat and oats (Fig. 198). The 

 patches are composed of large numbers of 

 "summer spores" (uredospores). A section cut 

 through the host leaf (Fig. 199,^) enables 

 one to see that the uredospores are formed 

 upon the ends of hyphse. The spore-bearing 

 ends of hyphse are contmuations of hyphse 

 which have pushed their way among the leaf 

 cells from which they have absorbed their 

 nourishment. At the time uredospores are 

 formed the host plant is usually thoroughly 

 infested with the mycelium. The uredospores 

 are readily carried about by currents of air 

 or contact with animals. If placed upon wheat 

 or oat plants, these spores germinate, and the 

 young hyphse penetrate the host and produce 

 new mycelium. 



Later in the summer the same mycelium 

 which produced uredospores may produce a 

 heavy -walled two-celled spore (Fig. 199, C) 

 known as the " winter spore " (teleuto spore). 

 When formed in large quantities these spores 



Fig. 198. Apiece 

 of a stalk of wheat 

 upon which spots 

 of the rust para- 

 site have formed 



