Minnesota Plant Diseases. 



1 65 



nest broom of the red 

 cedar and the great bush- 

 like brooms of the bal- 

 sam fir. These have al- 

 ready been described as 

 stimulated portions of 

 the host plant which, 

 with the fungus parasite, 

 live in partnership at the 

 expense of the neighbor- 

 ing parts of the host. 

 Besides this deforming 

 power of many rusts 

 these parasites are injuri- 

 ous in the stealing of 

 nourishment which they 

 accomplish at the ex- 

 pense of the host and in 

 the w r ounding of plant 

 parts. The host, as a re- 

 sult, becomes impover- 

 ished and may finally en- 

 tirely succumb. Thus 

 wheat rust annually robs 

 farmers of enormous 

 sums of money by im- 

 poverishing wheat plants. 

 Practically all classes 

 and groups of flowering 

 plants are attacked by 

 rusts as are also certain 

 fern plants. The lower 

 plants as mossworts and 

 algae seem to be free 

 from these parasites. 

 The favorite hosts of the 

 rust fungi seem to be the 

 grasses, for on these 

 plants are found an enor- 



FIG. 77. Spores of a grass rust fungus (Puc- 

 cinia vexans) ; above, winter spores; in the 

 middle, summer spores; below, amphispores. 

 (Summer-spore-like in germination, but rest- 

 ing over winter.) Highly magnified. Micro- 

 photograph by E. W. D. Holwav. 



