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 spot 5 



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 



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. 854). 

 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. 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). 



FIG. 354. A section through a pus- 

 tule in late summer, showing the winter 

 spores or teleutospores. X about 200. 



