18 THE RUSTS OF GEAIXS IN THE UNITED STATES. 



The wheat stem rust was carried directly to wheat, rye, and barley, 

 but not to oats. 1 The figures show plainly that it goes with great 

 ease to barley and wheat and very rarely (1 out of 32 inoculations) 

 to rye. This rust can infect barley with about the same ease with 

 which it goes to its own host. Although this may be interpreted as 

 indicating the identity of barley and wheat stem rusts, it is not con- 

 clusive, since the barley rust behaves differently from the wheat 

 rust toward the same cereals. When the wheat rust is taken to 

 barley and then transferred to the other cereal hosts, it is seen that 

 the barley has a decided influence on the rust. Diagram 3 sum- 

 marized from diagram 1 shows its effect. 



Diagram 3. — Summary of successful transfers of ivheat rust through barley. 



16 2 



B 16 



-| 



The wheat rust from barley infects the wheat and barley again 

 with great ease, and the rye with greater ease than the direct infec- 

 tion from wheat. Finally, from the second barley host the wheat 

 rust may even infect oats, a result rarely obtained directly from 

 wheat. In brief, the wheat rust, after passing on to barley, is capable 

 of infecting all of the four cereals, but when transferred from the 

 wheat without passing to barley, only barley and wheat are usually 

 infected, rye being rarely infected and oats very rarely, indeed. 

 Among the cereals, therefore, the stem rust of wheat in this country 

 is not confined to wheat as closely as Eriksson has found it to be in 

 Sweden, nor is it confined to barley and wheat, as found by Carleton. 



Diagram 4, summarized from diagram 1, shows the actual course of 

 infection of wheat rust taken, in succession, on all of the small grains. 



Diagram 4. — Summary of successful inoculations of diagram 1, showing succession. 



w j, 26 R 28 16 _8_ 1 1 !_ 



W 13 31 B ^ B 16 K u K . W 7 O ^ 



The effect on the wheat-rust parasite when barley is taken as a 

 host is clearly shown to be that of enabling a wider range of infection. 

 An interesting feature of this diagram is seen in the fact that the final 

 successful inoculation of oats was directly from the wheat, but the 

 rust had previously passed on to three barley plants and two rye 

 plants. 



Diagrams 5 to 10, inclusive, present summaries of inoculation 

 experiments with Puccinia graminis hordei (stem rust) from barley. 



1 Mr. H. B. Derr, of the United States Department of Agriculture, reports having obtained in the labora- 

 tory the following successful inoculations: W >0 >B >0 >0, Thenumberof successful 



infections in each case was not recorded. 



216 



