PROTOBASIDIOMYCETES 



409 



Probably the most important species of the rust family, both 

 from an economic, point of view and also from the point of view 

 of the development of mycological research, is the common species, 

 Puccinia graminis, upon cereals. It was upon this species that 

 the classical researches of.De Bary (1865 et seq.) were based, 

 throwing light upon many phenomena of parasitism. In more 

 recent times this species has 

 served further as a means of 

 developing a knowledge of 

 biological and physiological 

 forms, or specialized races. It 

 has been the means, also, of 

 showing the relation of the 

 summer spore, or uredo stages, 

 to the continual propagation of 

 certain rust forms, and Eriks- 

 son's mycoplasm theory sought 

 evidence in phenomena ob- 

 served in this species. 



Distribution and occurrence. 

 It would appear that this fungus 

 is distributed, in one or more 

 of its numerous forms or races, 

 throughout the world wherever 

 certain grasses may be found. 

 It is not in all regions the most common cereal rust, but in 

 general it is so considered. The economic work upon rust fungi 

 in such widely separated and important cereal-growing countries 

 of the world as Australia, Russia, Western Europe, and the 

 United States has been largely concerned with this species. It is 

 therefore the fungus which is commonly known as rust of wheat, 

 oats, barley, and other cereals and many grasses. It is not at all 

 restricted by minor climatic conditions, and in the United States 

 it is found in its various forms upon certain grasses from the 

 moist Atlantic seaboard to the most arid portions of the Great 

 Plains, and from the Gulf Coast to the Great Lakes. The annual 

 losses throughout the world amount to a stupendous figure, often 

 estimated to reach one hundred million dollars, 



FIG. 201. RUST OF OATS 



