BLACK RUST OF GRAIN (PUCCINIA GRAMINIS) 397 



Rye, and Barley, and occurs on other Grasses. The presence of 

 the mycelium in the host is first known through the appearance of 

 reddish spots or lines on 

 the stems and leaves in 

 late spring or early sum- 

 mer. The reddish spots 

 or lines are regions of 

 spore production. They 

 are pustules or blister- 

 like structures caused by 

 masses of spore-bearing 

 hyphae which push up 

 the epidermis until it is 

 finally ruptured (Fig. 

 353} . The reddish color 

 of the pustules is due to 

 the reddish color of the 

 spores. These spores are 

 known as the " summer 

 spores" or uredospores. 

 The uredospores, which 



FIG. 353. Wheat Rust as it appears on 

 Wheat. Left, portion of a Wheat plant, 

 showing the pustules on the stem and leaf; 

 right, a much enlarged section through a pus- 

 tule, showing the summer spores (X 200). 



are produced in great 

 numbers, are scattered by the wind, thus reaching other host 



plants into which they grow 

 hyphae and thereby infect. 

 They are chiefly responsible 

 for the rapid spread of the 

 disease during summer. 



Later in the summer, when 

 the grain is ripening and the 

 food for the Fungus becomes 

 scarce, the same mycelia pro- 

 duce heavy-walled, two-celled 

 spores, known as winter spores 

 or teleutospores (Fig. 354). 

 These spores are dark in color, 

 giving the pustules a dark ap- 

 pearance whence the name 



Black Rust. They pass the winter on the straw, ground, or wher- 

 ever they happen to fall. The following spring, each cell of the 



FIG. 354. A section through a pus- 

 tule in late summer, showing the winter 

 spores or teleutospores. X about 200. 



