- The physiologic specialization of nine isolates of stripe rust
(Puccinia striiformis West.) collected in the Pacific Northwest
was determined. The separation of races was based on the differential
susceptibility of the following wheat varieties: Cappelle
Desprez, Chinese 166, Dippes Triumph, Druchamp, Etoile de
Choisy, Flamingo, Gaines, Golden, Ibis, Leda, Michigan Amber,
Moro, Omar, Rubis and Suwon x Omar⁴.
Four race groups were identified on the differential wheat
varieties when grown at a constant 18 C temperature; however,
eight races could be identified when the temperature was lowered
to 2C during the 10 hour dark period (2/18 C cycle). Race groups
could be identified in the 18 C growth chamber using only five
commercially grown Pacific Northwest wheat varieties (Gaines, Omar, Golden, Moro and Druchamp). Seven of the eight isolates
tested under the 2/18 C temperature cycle could be identified using
these same five commercial varieties.
The reaction type of certain wheat variety-race combinations
is changed with a change in incubation temperature from 18 C to
2/18 C. Thus, by growing differential varieties at these two
temperatures and using changes in reaction type as a distinguishing
factor all eight isolates could be separated into distinct races.
Pre- or postinoculation growth temperatures influenced the
reaction types of three of the differential wheat varieties. The
reaction on Dippes Triumph became more resistant when the postinoculation
incubation temperature was lowered (RT, 3 to 2).
Suwon x Omar⁴ gave a more susceptible reaction when the temperature
was lowered (RT, 1 to 2). The reaction type on Flamingo
varied depending on the pre- and postinoculation incubation
Reaction types produced on the first and second seedling
leaves did not differ at either temperature. Rust prevalence on
the first seedling leaves was reduced from usually 100 percent
at 18 C to as low as 12.5 percent with certain race-variety combinations
when incubated at the 2/18 C cycle. There was no change
in prevalence on the second seedling leaves with a change in
incubation temperature. The latent period was extended 6.4 days when the incubation
temperature was changed from a constant 18 C to 2/18 C. There
was no correlation between races of stripe rust and a change in
the latent period. The more resistant varieties exhibited latent
periods of 10.3 t o 14.3 days whereas the latent periods were
9.0 to 10.2 days for the more susceptible varieties.
A determination of the apparent infection rate (r) of three
races of stripe rust under field conditions gave significant differences
on Baart and Federation spring wheats. The differences on
Baart wheat were: r = 0.0953, r = 0.0552, and r = 0.0541 for
races SW-2-1, SB-40 -1 and SW-7-1, respectively.