120 
Journal of Agricultural Research 
Vol. XIV, No. 2 
Table IV. —Results of inoculations with P. graminis tritici on resistant and susceptible 
parents and F 9 hybrid with rust from partially resistant F 9 hybrid and from stock 
cultures 
Source of rust. 
Plant inoculated. 
Result. 
Character of infection. 
Stock cultures.... 
Hybrid 4 X 38AA— 
66 
66 
Variable; uredinia large and 
numerous on some leaves, 
small and few on others. 
Some leaves show sharp 
flecking, others none. 
Do. 
Haynes Bluestem.... 
59 
Heavy 
59 
F 9 hybrid. 
.do. 
52 
Do. 
52 
Stock cultures.... 
Kubanka 2094. 
55 
Moderate; uredinia small, 
flecks present. 
55 
F 9 hybrid. 
.do.'.... 
49 
Do. 
49 
GENERAL DISCUSSION 
The results of the experiments with P. graminis tritici-compacti show 
that barley which both theoretically and from the results obtained by 
previous investigators might be expected to increase the infection range 
does not do so. Even susceptible varieties of wheat do not change the 
parasitic capabilities of the rust so as to enable it to attack a normally 
resistant variety. Furthermore, the rust does not acquire additional 
virulence when associated for a long time with a given host. Barley is 
moderately susceptible to the rust but the relations between host and 
rust are apparently the same regardless of the length of their association 
with each other. Wheats resistant to the rust remain resistant regard- 
less of the previous history of the r*ust. 
The results of the work to ascertain the effect of hybrids on P, graminis 
tritici are not at all in agreement with those of Pole Evans (3). Bobs, 
the wheat found to be immune under the conditions of his experiments, 
is quite susceptible under our conditions, both in the field-rust nursery 
and greenhouse (PI. 13, A). Possibly the strain of rust in South Africa 
is not the same as ours. 
On account of the far-reaching significance of Pole Evans's results, the 
utmost precautions were taken in the present work to detect any differ¬ 
ences which might appear in either the resistant or susceptible parents 
when inoculated with rust from the hybrid as compared with that from 
stock cultures. In no case, however, was there the slightest evidence of 
any change in the virulence of the parasite, nor any indication that a 
short sojourn on a susceptible hybrid had given it any peculiar ability 
to cause normal infection on a heretofore resistant variety or to cause a 
more than usually virulent infection on a susceptible variety. 
