20 THE RUSTS OF GRAINS IN THE UNITED STATES. 



The stem rust of barley, like that of wheat, goes with equal ease on 

 barley and wheat. Rye is more easily infected by barley rust than 

 by the wheat rust. Oats are capable of direct infection by barley rust. 

 The oat pustules were very small and weak, and thus precluded the 

 possibihty of very numerous experiments with the barley rust from 

 oats; but diagram 5 shows that successful infections were obtained 

 as follows : 



Diagram 8. — Summary of successful inoculations of oats with stem rust of barley. 



The barley rust, after bekig transferred to rye, was carried to 

 barley and then to all of the four cereals; it was likewise transferred 

 to wheat and then to the other cereals. The rye and wheat rusts, as 

 shown by other diagrams, are usually incapable of direct transfer in 

 this manner. That the barley rust is carried through wheat and then 

 transferred to the other cereals is shown in diagram 9 summarized 

 from diagram 5: 



Diagram 9. — Summary of transfer of stem rust of barley through wheat to other cereals. 



2 29 



B W- 



12 

 Wp- 



^ 4 

 O 27- 



1 



The barley rust, then, after passing through rye and wheat, is still 

 able to infect all four cereals. 



Diagram 10, summarized from diagram 5, sht)ws that barley rust 

 was successfully transferred to all of the four cereals. 



Diagram 10. — Summary of transfers of stem rust of barley directly to other cereals. 



3 8 ^5 „ 3 ,,,15 



B R 3e B 9- O 25 B ^ W yf 



The comparatively large percentages of infection obtained are 

 probably accounted for by the fact that in each case barley inter- 

 vened as a host between rye and oats and between oats and wheat. 



The barley stem rust enjoys the widest range of any of the biologic 

 forms of the cereal rusts. On the other hand, a transfer of any of the 

 other stem rusts to barley widens the range of that rust. We have 

 here, then, a decided reaction of host upon parasite, enabling the latter 

 to adapt itself to hosts not ordinarily congenial; for instance, 

 W >B >0. 



As shown under wheat rust, the barley rust and wheat rust are 

 seen to be not necessarily identical, though the fact that they are 

 i;i6 



