Agriculture Reference
In-Depth Information
ment Station in 1966 and was the fi rst US cultivar
to have
Sr6
.
The resistance derived from Hope, Thatcher,
and
Sr6
was the main component of stem rust
resistance in the spring wheat region from the
1930s through the 1970s. However, the frequency
of
Sr2
and of the Thatcher adult plant resistance
has apparently declined in current cultivars given
the following observations. Thatcher was moder-
ately resistant to race TTKS (Ug-99) (Pretorius
et al., 2000) in fi eld tests in Kenya where race
TTKS has been predominant. The Thatcher
stem rust resistance is also expressed in seedling
plants, but seedling tests with race TTKS failed
to detect the Thatcher type of resistance in spring
wheat cultivars from the northern Great Plains
released between 1996 and 2005 (Jin and Singh
2006). Stem rust race TPMK, which became
common after the decline of race 15B (Fig. 5.3),
produces moderate to high infection types on
seedling and adult plants of Thatcher. Selection
of stem rust resistant germplasm in breeding pro-
grams based on resistance to TPMK could have
placed higher selection pressure on the low infec-
tion types conditioned by race-specifi c genes such
as
Sr6
and
Sr9b
than on the nonspecifi c resistance
derived from Thatcher. Resistance due to
Sr2
also appeared to be absent in these cultivars. The
absence of
Sr2
in spring wheat was likely associ-
ated with intense breeding for Fusarium head
blight (caused by
Fusarium graminearum
) resis-
tance, for which a major QTL is located on chro-
mosome 3BS (Anderson et al., 2001) in close
repulsion linkage with
Sr2
(Spielmeyer et al.,
2003).
High levels of stem rust resistance in hard red
winter wheat, in combination with early maturity,
have had an impact in reducing the
P. graminis
population size in North America. The stem rust
resistance gene
SrTmp
, derived from 'Triumph
64', was likely present in some of the initial hard
red winter wheat germplasm (Roelfs and McVey
1979). This gene was effective against the major-
ity of stem rust races in North America, but race
56 and race 15B were virulent to
SrTmp
. The
resistance conditioned by
SrTmp
, combined with
the early maturity of the Triumph background,
Other undesignated race-specifi c seedling resis-
tance genes were also detected in Thatcher
(Nazareno and Roelfs 1981; Knott 2000).
Another source of stem rust resistance in spring
wheat during that period was the adult-plant
resistance gene
Sr2
, derived from Yaroslav emmer
(McIntosh et al., 1995). The stem rust resistant
cultivar Hope and the breeding line H-44 were
developed by selecting progeny from a cross
between Marquis and Yaroslav emmer. Breeding
line H-44 was subsequently used as a parent in
breeding programs in Canada. The resistance
conditioned by
Sr2
is characterized by fewer ure-
dinia and lower overall severity of stem rust at
maturity. The cultivar Renown released in 1937
was the fi rst stem rust resistant cultivar with
Sr2
to be released in Canada. Cultivars with
Sr2
were
resistant to race 56 and the other stem rust races
then present. Gene
Sr2
is present in wheat culti-
vars in Australia and the CIMMYT program
(McIntosh et al., 1995) and in Canadian cultivars
(Liu and Kolmer 1998a). Gene
Sr2
is present in
some hard red winter and soft red winter cultivars
in the US (Roelfs 1988) since Hope was used as
a parent in these programs.
From 1950 to 1954 race 15B of wheat stem rust
was widespread in the Great Plains region. This
race caused high stem rust severities on spring
wheats with
Sr2
and the Thatcher stem rust resis-
tance. In 1954 yield losses in spring wheat were
over 40% in North Dakota and reached 18% in
Minnesota. In response to race 15B, the cultivar
Selkirk with genes
Sr2
,
Sr6
,
Sr7b
,
Sr9d
,
Sr17
, and
Sr23
was highly resistant to race 15B and was
released in 1954 by the Canada Department of
Agriculture. Selkirk was widely grown through-
out the spring wheat region of the US and Canada
through the 1960s. The
Sr6
gene was particularly
effective against race 15B and was actively selected
in spring and winter wheat breeding programs in
the central and northern Great Plains. The gene
is also present with high frequency in CIMMYT
germplasm. The widespread use of
Sr6
likely
contributed to the rapid decline of race 15B in
North America in the late 1950s. Wheat cultivars
from Kenya also had
Sr6
. The cultivar Chris was
released by the Minnesota Agricultural Experi-
