284 



PLANT DISEASES 



kinds of distinct rusts of wheat, and two of each of 

 the other common small grains. The life stories of 



these various rusts are, how- 

 ever, somewhat similar. At 

 least they are not so different 

 as to demand such difference 

 in treatment as is found in 

 the case of the smuts. 



The life story of one of 

 these, namely, the stem rust of 

 wheat, commonly called black 

 rust, may be cited by way 

 of example. Just before the 

 wheat plants in the field be- 

 gin to head out there may be 

 found on the stems and on 

 the lower parts of the leaves 

 brownish red powdery masses 

 which are formed in streaks. 

 These are the so-called sum- 

 mer spores of the stem rust. 

 The spores are blown off by 

 the wind and new ones formed 

 from the same pustule, or 

 streak. The wind-blown 

 spores fall on other wheat 



plants, and if a drop of dew collects around them or 

 a drizzling rain provides some water, they start to grow 

 by sending out a fungous thread. This thread digs its 

 way into the leaf or stem, commences to branch, and 

 forms a tangle of threads, and this tangle soon breaks 

 open the outer covering of the leaf and again forms red, 

 or summer, spores. This spore, therefore, is particularly 



FIG. 146. Wheat Rust (Pucdnia 

 graminis). 



Stems of wheat showing opened and un- 

 opened black clusters of winter spores. 

 This is commonly known as " black 

 rust " or " stem rust." Slightly mag- 

 nified. 



