236 FARM FRIENDS AND FARM FOES 



the tissues of the leaf. Here it develops into a mycelium 

 that soon produces swollen orange-colored pustules, often 

 called cluster cups. In these pustules the mycelium 

 develops aecidiospores or spring spores which in turn are 

 blown through the air by the wind. This is often called 

 the cluster-cup stage of the rust. 



Some of the secidio spores are likely to find lodgment 

 upon the leaves or stalks of wheat plants. When moisture 

 from dew or rain is present, these will germinate and start 

 a growth of mycelium in the tissues. This mycelium will 

 soon produce a crop of rust-colored uredospores or summer 

 spores which scatter through the wheat fields and spread 

 the disease rapidly. This process continues through most 

 of the summer, and consequently this is the stage in which 

 the most damage is done by the rust fungus. 



Finally, as the wheat matures the mycelium in the tissues 

 gives rise to the black rust or teleutospore stage. In this 

 condition it passes the winter. 



Yearly Cycle of Wheat Stem Rust 



I. Early Spring. 



a. Teleutospores develop sporidia. 

 b. Sporidia are blown to barberry. 



c. Sporidia develop mycelium in barberry leaf. 

 II. Late Spring. 



d. Mycelium in barberry leaf develops aecidiospores. 

 e. ^cidiospores are blown to wheat. 



/. ^cidiospores develop mycelium in wheat. 



III. Summer. 



g. Mycelium in wheat develops uredospores. 



k. Uredospores are blown to other wheat plants. 



i. Uredospores develop mycelium in these wheat plants. 



IV. Autumn. 



j. Mycelium in these wheat plants develop teleutospores that 

 live through the winter. 



