150 



Diseases of Economic Plants 



mass of powder nearly the color of iron rust. In early stages 

 of attack these blisters are few and scattered, but they 

 rapidly increase in number. When young, the skin covering 

 the sori is unbroken ; later it ruptures, setting loose the rusty, 

 powdery mass of spores beneath. This, the summer stage, is 



chiefly responsible for the 

 hibernation of rust in the 

 East. Later in the season 

 th(^ sori become black in- 

 st(^ad of rusty-colored, thus 

 constituting the winter 

 stage of the rust. 



In the spring still an- 

 other stage, the spring 

 stage, may occasionally be 

 seen, though it is rare east 

 of the Rocky Mountains. 

 This consists of small, 

 oval, pale spots upon the 

 branches or leaves. In the 

 centers of these spots small 

 round pustules develop in 

 concentric lines. Soon 

 each pustule opens, con- 

 stituting a very minute cup 

 which is sunken into the 

 leaf. From these cups 

 issues a powder quite sim- 



FiG. 79. — Sori ot the asparagus rust 

 upon the stems, enlarged. Original. 



ilar to that from the sori 

 of the summer stage, 

 though more orange in 

 color. All three stages of rust — spring, summer, and win- 

 ter — may occur simultaneously upon the same plant. 



The disease spreads rapidly throughout the field during the 

 spring and summer stages. The winter stage is the typical 

 hibernating condition in which the causal fungus may rest un- 

 til the following spring before it can induce further infection. 



