THE WHEAT RUST 



69 



99. Other Rusts of Cereal Grains. As a group, the rusts have 



been remarkably successful in adapting themselves to a parasitic life 



partly, no doubt, because of ^___ 



the number and variety of 



spores that they produce. So 



successful have tney been that 



most of the species of seed- 

 bearing plants are attacked 



each by one or more species of 



rust. Some rusts seem to have 



become so specialized as to be 



able to live upon only one or a 



very few hosts ; in other in- 

 stances, a single rust may 



attack a considerable number 



of hosts. 1 Besides the common 



wheat rust which has been 



described in this chapter and 



which is called Puccinia grami- 



nis, there are at least two 



other rusts that infect the 



wheat. All the other cereal 



grains, too, are attacked by 



FIG. 30. A branch of the red cedar 

 bearing two "cedar apples" swellings 

 caused by the presence of a rust in 

 the winter condition. After Jones and 

 Bartholomew. 



rusts, some of which cause large 



losses. The known grain rusts, 



in addition to Puccinia grami- 



nis are : the crown rust, one 



of whose races or varieties 



causes a serious disease of oats ; 



the yellow rust, races of which attack wheat, barley, and rye ; the brown 



rust of rye ; the brown rust of wheat ; the dwarf rust of barley ; and 



1 A word of caution should accompany this statement. In some cases the rusts 

 appearing upon two or more hosts seem to be exactly alike, but the spores produced 

 by the rust upon one host will not infect the other host, and vice versa. Thus, there 

 are rusts on rye, oats, barley, and several common grasses which appear to be the 

 same as that upon the wheat and which are called by the same name (Puccinia 

 graminis). But the summer spores of the wheat rust will not infect the oat at all, 

 or only with great difficulty, and in the same way summer spores from Puccinia 

 graminis on oats will not infect the wheat, although both the rust on wheat .and 

 that on oats pass their spring stage upon the barberry. These facts have given 

 rise to the notion of specialized races of parasitic fungi. Two such races may seem 

 to have exactly the same structure, but they differ in that they have adapted them- 

 selves in some way to life upon different hosts. 



