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 uredinio- 

 spores. The uredinio- 



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). 



spores, which are pro- 

 duced in great numbers, are scattered by the wind, thus reaching 

 other host plants into which they grow hyphae and thereby in- 

 fect. They are chiefly re- 

 sponsible for the rapid spread 

 of the disease during sum- 

 mer. 



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 

 FIG. 354. -A section through a pus- or teleospores (Fig.^ 354). 

 tule in late summer, showing the winter These spores are dark in color, 

 spores or teleospores. x about 200. 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 



