Agriculture Reference
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group were highly related for virulence yet
were very distinct from other clusters. Burdon
and Roelfs (1985) showed that isozyme variation
in wheat stem rust populations was completely
correlated with the race groupings. The dis-
tinct groups of wheat stem rust races that were
described in 1975 in North America were likely
ancestral groups of races that existed before
barberry eradication. Removal of the sexual
cycle would have prevented any further genetic
exchange between the different groups of races.
In 1999 a new stem rust race initially desig-
nated as Ug-99 was found in Uganda (Pretorius
et al., 2000). This race was notable since it was
the fi rst to have virulence to gene
Sr31
. Race
Ug-99 (designated as TTKS on wheat stem
rust differentials) was also virulent to gene
Sr38
.
Gene
Sr31
is found in about 30% of CIMMYT
germplasm, and
Sr38
is present in European,
Australian, and a few CIMMYT wheats (Singh
et al., 2006). In addition to these virulences, Ug-
99 is virulent to a number of other
Sr
genes
present in US spring and winter wheat (Jin and
Singh 2006). As of 2007, Ug-99 was found in
Uganda, Kenya, Sudan, Ethiopia, Yemen, and
Iran. This stem rust race has the potential to
spread through the Middle East and then to India
and Pakistan, where it could cause devastating
grain yield losses in wheat. Since it is widely viru-
lent to many wheat cultivars in this region, it will
be imperative to fi nd sources of resistance and
incorporate these into current wheat breeding
programs.
However, Marquis and the other bread wheat cul-
tivars were very susceptible to both stem rust and
leaf rust. The stem rust epidemics of the early
1900s caused such losses that durum wheat, which
was generally resistant to stem rust, had replaced
bread wheat in Minnesota. Efforts were thus
made to develop bread wheat and durum wheat
cultivars that were resistant to stem rust.
The fi rst hard red spring wheat bred for stem
rust resistance in North America was 'Ceres',
released by the North Dakota Agricultural Exper-
iment Station in 1926 and grown in both the US
and Canada. Ceres was developed by crossing the
stem rust resistant wheat cultivar Kota with
Marquis, and selecting stem rust resistant
progeny. However, stem rust race 56, with viru-
lence to the resistance in Kota and Ceres, increased
throughout the Great Plains region after the
release of Ceres. Widespread epidemics of stem
rust on Ceres occurred from 1935 to 1947 due to
the increased presence of race 56.
Ceres was largely replaced by Thatcher wheat,
developed by the Minnesota Agricultural Experi-
ment Station and released in 1935. Thatcher was
developed from a cross between two lines, derived
from Kanred/Marquis and from Iumillo durum/
Marquis, which combined the good quality char-
acteristics of Marquis with stem rust resistance in
Kanred (
Sr5, Sr16
) and Iumillo durum (
Sr9g
and
Sr12
) (Kolmer 2001b). The Thatcher resistance
of
Sr5
,
Sr9g
,
Sr12
, and
Sr16
was highly effective
against race 56 and the other stem rust races.
Many cultivars subsequently released in Canada
and the US were derived from Thatcher (Kolmer
et al., 1991). None of the known race-specifi c
genes in Thatcher could account for the adult-
plant fi eld resistance in Thatcher, which became
apparent with the increase of stem rust races with
virulence to the seedling genes in Thatcher. The
adult-plant resistance in Thatcher appeared to be
nonspecifi c and durable (Kolmer et al., 1991).
Genetic analyses indicated that stem rust resis-
tance in Thatcher was complex. At least two
genes that condition resistance in adult plants
were likely derived from Iumillo durum and were
independent of the race-specifi c genes (Nazareno
and Roelfs 1981; Gavin-Vanegas et al., 2007).
Stem rust resistance in wheat
In the early part of the 20th century, wheat breed-
ing in the spring wheat region of the northern
Great Plains was largely a struggle against stem
rust, the most important disease of wheat at the
time. Marquis was the most widely grown spring
wheat cultivar in the north central US and in
Canada. Marquis, developed by the Canada
Department of Agriculture in Ottawa and released
in 1907, had early maturity and was popular with
the milling industry since it was the fi rst high-
quality bread wheat developed in North America.
