Diseases Which Challenge Global Wheat Production—the Wheat Rusts - Wheat Science and Trade - page 110

may have contributed to reducing the amount of
stem rust present in winter wheat. The SrTmp
gene is present in several current cultivars (Jin
and Singh 2006) and in nearly 10% of breeding
lines in the hard red winter wheat region. Other
major components of stem rust resistance in hard
red winter wheat are Sr24 and resistance derived
from the 1RS.1AL wheat-rye translocation.
Originally derived from Thinopyrum ponticum ,
Sr24 has been common in hard red winter wheat
cultivars since the release of Agent in 1967, and
to a lesser degree in soft red winter and spring
wheat cultivars. Stem rust races with virulence on
Sr24 have not been detected in North America,
and the gene is currently present in nearly 50%
of the current hard red winter wheat breeding
lines and cultivars. Because Sr24 is tightly linked
with Lr24 (McIntosh et al., 1995), selection for
leaf rust resistance produced lines with stem rust
resistance. The 1RS.1AL translocation in 'Amigo',
with the rye chromosome introduced from 'Insave
F.A.' rye via a triticale with greenbug resistance
(Sebesta et al., 1994), gives effective resistance to
stem rust races in North America as well as to race
TTKS.
Derived from Petkus rye, Sr31 is on the
1RS.1BL wheat-rye translocation with Lr26 and
Yr9 (McIntosh et al., 1995). Gene Sr31 is present
in some current hard red winter wheat cultivars
and in a few soft red winter wheat cultivars in the
US, and is present in CIMMYT-derived wheat
cultivars that are grown worldwide. This gene
provided a very high level of resistance to all
known stem rust races prior to the emergence of
TTKS in Uganda in 1999. Derived from T.
timopheevi , Sr36 is the most common stem rust
resistance gene in soft red winter wheat in the US
and has been an important source of resistance in
Australia (McIntosh et al., 1995). In addition,
Sr36 conditions resistance to the current stem
rust races in the US as well as race TTKS (Jin
and Singh 2006).
Genes Sr9b , Sr11 , and Sr17 are also present in
spring and winter wheat in North America. Gene
Sr9b is closely linked with Lr13 and is present in
several spring wheat lines that were selected for
leaf rust resistance from Frontana. Virulence to
Sr9b is common in stem rust races in North

America. Virulence to Sr11 is present in North
America and Australia. The origin of Sr17 is
assumed to be Yaroslav emmer; thus it may also
be present in wheats with Sr2 , though many stem
rust races are virulent to this gene.
In Australia wheat cultivars with Sr24 , Sr26 ,
Sr30 , Sr36 , and Sr38 have been released in the
past 40 years (Park 2007). Gene Sr26 is derived
from Th. ponticum , is present only in Australian
cultivars, and stem rust races with virulence to
this gene have not been detected. Gene Sr30 is
present in a number of Australian cultivars and
also CIMMYT germplasm (McIntosh et al.,
1995). Gene Sr38 , derived from T. ventricosa , is
linked with Lr37 and Yr17 and was selected in
Australian germplasm based on resistance to all
three rusts. Currently gene designations up to
Sr46 have been given for stem rust resistance
genes in wheat (McIntosh et al., 2007).

FUTURE PERSPECTIVES

The emergence of new races of wheat leaf rust,
wheat stripe rust, and wheat stem rust continually
challenge wheat breeders and plant pathologists
to develop effective sources of durable resistance
to these pathogens. The eventual cloning and
sequencing of genes such as Lr34/Yr18 and Sr2
will provide greater insight into how these non-
specifi c resistance genes function, thus offering
the potential of designing new resistance genes
that will offer greater resistance durability. These
genes will likely differ for functional domains and
specifi city in comparison with the race-specifi c
NBS-LRR resistance genes.
The discovery that the herbicide glyphosate
(Anderson and Kolmer 2005) greatly reduces rust
infections in glyphosate-resistant wheat is an
exciting development that could be utilized
immediately to reduce losses in wheat due to
rusts, and it may lead to other transgenic strate-
gies to design rust resistance genes or genes for
tolerance or resistance to chemical applications in
wheat. The characterization of avirulence genes
in fl ax rust ( Melampsora lini ) and the correspond-
ing resistance genes in fl ax ( Linum usitatissimum ),
and an understanding of how their coded func-