July 8 ,1918 Can Biologic Forms of Stemrust Change Rapidly? 
119 
with rust from the hybrid (PI. 15, B, C). A total of 115 seedlings of 
the remarkably resistant (nearly immune) emmer parent were inoculated , 
54 with rust from stock cultures, and 61 with rust from the susceptible 
hybrid, but not a single uredinium was formed. Very small light-green 
flecks appeared, but never developed into uredinia. In so far as indi¬ 
cated by these experiments, this emmer may be described as “once 
resistant;always resistant” (PI. 15, D, E). 
TabIvE III .—Results of inoculations with P. graminis tritici on resistant and susceptible 
parents with rust from susceptible F 2 hybrid and from stock cultures 
[Marquis X Kubanka (Cl 2094)] 
Source of rust. 
Plant inoculated. 
Result. 
Character of infection. 
Stock cultures.... 
Marquis. 
55 
Moderately heavy; large, vigor¬ 
ous uredinia. 
55 
F 2 hybrid... 
.do. 
47 
Do. 
Moderate; uredinia smaller, 
flecks present. 
Stock cultures.... 
Kubanka (Cl 2094). 
47 
48 
48 
F 2 hybrid. 
.do. 
5 2 
Do. 
52 
Table III gives the results with the other F 2 hybrid [Marquis X Kubanka 
(Cl 2094)]. The F 2 plant from which urediniospores were used to 
inoculate seedlings of the two parents, is shown at A in Plate 16. Uredinia 
are present on leaf, sheaths, and glumes. The results were similar to 
those previously described (PI. 16, B, C, D). 
Having determined the effect of an F t hybrid, and two F 2 hybrids, it 
seemed advisable to try a hybrid of a later generation which was breeding 
true for such morphological characters as presence of awns, hairy chaff, 
durum-like shape of spike, etc. For this work, seed from the F„ plants 
of the cross Haynes Bluestem (Minnesota 169) X Kubanka (Cl 2094) was 
chosen. Seedlings were grown, and 66 plants were inoculated with rust 
from the stock cultures and the resulting urediniospores were used to 
inoculate seedlings of the two parent varieties. The results are given 
in Table IV. The inoculations on this hybrid show that it is not yet 
homozygous for the character of rust resistance, even though it has for 
several years been breeding true for other characters. Here again, as in 
previous trials, a sojourn on the hybrid did not increase the virulence of 
the rust on either parent perceptibly (PI. 17). 
