214 Journal of A gricultural Research voi. v. no. s 



EXPERIMENTS WITH BRIDGING HOSTS 



Johnson (4, p. 10) found that by using Avena saliva as a bridging host 

 the timothy rust could be transferred to Hordeum vulgare; by using 

 Festiica elatior it could be transferred to Hordeum vulgare and to Triti- 

 cum vulgare; by using Dactylis glomerata it could be transferred to Triti- 

 cum vulgare. Since the writers were able to infect barley directly, but 

 not wheat, without the bridging hosts, an attempt was made to deter- 

 mine whether or not, with the strain of rust employed, it would be possi- 

 ble to make transfers to wheat after using Dactylis glomerata as a bridging 

 form and whether or not the rust would transfer to barley more readily 

 under the same conditions. Transfers were made from timothy to Dac- 

 tylis glomerata, and heavy infection was obtained. Two series of inocu- 

 lations were then made with spores from Dactylis glomerata to wheat, 

 oats, barley, rye, and timothy. The results were as follows: Wheat, -^\ 

 oats, -^j barley, ■^■, rye, -^i timothy, ^. When oats was used as a 

 bridging host, approximately the same percentage of infections resulted 

 as when the rust was transferred directly from timothy. The writers 

 were, therefore, unable to increase the infection capabilities of the rust by 

 means of first transferring to Dactylis glomerata or oats. Neither was 

 the vigor of infection appreciably greater on barley and oats after using 

 bridging species. It is possible that by confining the rust for a long 

 series of generations on a bridging host definite results might be obtained. 

 Such experiments are now under way. 



The results cited show that different results may be obtained with 

 different strains of rust. That Johnson (4) and Eriksson and Henning (3) 

 worked with difi'erent strains seems entirely probable, in view of the 

 fact that neither was able to transfer the rust directly to barley, while 

 the writers experienced no particular difficulty in making such transfer. 

 The possibility of conflicting results may be clearly shown by results 

 which the writers have recently obtained. Timothy rust and stem rust 

 of oats {Puccinia graminis avenae) transferred very readily to Dactylis 

 glomerata. But the rusts by no means acquired the same capabilities as 

 a result of growing on Dactylis glomerata, at least not in a few generations. 

 When the timothy rust on Dactylis glomerata was transferred to oats less 

 than ID per cent of the inoculated leaves became infected; when the rust 

 was transferred to barley very small pustules were produced on about 

 16 per cent of the inoculated leaves; when it was transferred to rye, 

 small pustules were produced on about 6 per cent of the inoculated 

 leaves; when it was transferred to timothy 95 per cent of the leaves 

 became infected. When, on the other hand, stem-rust of oats (P. gra- 

 minis avenae) on Dactylis glomerata was transferred to oats, 100 per cent 

 of the inoculated leaves became very severely affected; inoculations on 

 barley resulted in 7 per cent of infection; inoculations on rye resulted in 

 no infection (in other experiments the writers have been able to infect 



