Minnesota Plant Diseases. 



159 



Rusts (Uredineai), The rust fungi constitute a larger group 

 of plants than the smuts and exhibit more variety of structure 

 and habit. They may be considered as relatives of the smuts 

 in that the winter spores of the latter may be compared with 



FIG. 73. Spores of rust fungi. l.-A cluster of winter spores of wheat rust (Puccinia 

 graminis) on wheat plant. 2. A winter spore germinating to a thread of four cells 

 (promycelium basidium), each of which bears a small spore (sporidium) on a stalk. 

 The winter spore germinates in the spring while still in the straw or on the ground. 

 The sporidia are blown by the wind to another plant and there germinate as seen 

 in 3 and 4. 4. Shows the germination of a sporidium on a barberry leaf; here 

 infection will soon take place. 5. A germinating summer spore of wheat rust, showing 

 germ tubes which on a wheat plant can cause infection (as shown in Fig. 29). 6. A 

 rare grass rust spore (amphispore of Puccinia vexans) germinating; it germinates 

 as a summer spore, but has a thick coat and rests over winter as a winter spore. All 

 highly magnified. 1-5, after Ward; 6, after Carleton. (See also Fig. 74.) 



the smut spores. The rusts, however, exhibit a great number 

 of accessory spore forms. They are all parasites and are of great 

 economic importance on account of the large number of dis- 

 ease-causing forms. The life-story of a rust plant is often very 



