Agriculture Reference
In-Depth Information
America and were the most common virulence
phenotypes in the US. Further analysis with
(amplifi ed fragment length polymorphism AFLP)
markers (Kolmer 2001a) indicated that the iso-
lates with
Lr17
virulence had very distinct molec-
ular phenotypes compared to all other isolates in
North America. This indicated that the isolates
with
Lr17
virulence were most likely introduced
to the Great Plains region from either Mexico or
the Pacifi c Northwest and were not derived by
mutation from the previously existing population.
New virulence phenotypes of
P. triticina
were also
introduced to Australia in the mid-1980s (Park
et al., 1995). In recent years a virulence pheno-
type of
P. triticina
with virulence to many
durum cultivars has been found in France, Spain,
Mexico, Argentina, and Chile (Singh et al., 2004;
Ordoñez and Kolmer 2007b). This virulence
phenotype may have had a single origin and sub-
sequently spread to the other durum producing
regions.
Recently locus-specifi c microsatellite or simple
sequence repeat (SSR) markers have been devel-
oped for
P. triticina
(Duan et al., 2003; Szabo and
Kolmer 2007). These markers can be used to
determine molecular genotypes of
P. triticina
since heterozygotes can be distinguished from
homozygotes. The SSR markers have been used
to differentiate
P. triticina
populations in Central
Asia (Kolmer and Ordoñez 2007) and to describe
genetic diversity in
P. triticina
populations in
France (Goyeau et al., 2007). These locus-specifi c
markers will be extremely valuable for assessing
genetic variation in
P. triticina
and patterns of
migration between populations in different conti-
nental regions.
gene for leaf rust resistance have been quickly
rendered susceptible because of the selection and
increase of virulent leaf rust races. In the south-
eastern states since the mid-1970s,
Lr9
derived
from
Ae. umbellulata, Lr11
derived from Hussar
wheat, and
Lr1
derived from various common
wheats, have been widely used in soft red winter
wheat cultivars and have selected phenotypes of
P. triticina
with virulence to these genes (Fig.
5.1a). Currently
Lr1
and
Lr11
do not provide
effective resistance, and cultivars with
Lr9
are
moderately resistant, but this resistance would
quickly erode if cultivars with
Lr9
were grown
over a larger area.
In Texas and Oklahoma isolates with virulence
to
Lr1
quickly increased in the late 1970s and
early 1980s after the release of cultivars with this
gene (Fig. 5.1b). Virulence to
Lr24
appeared
shortly after the release of the hard red winter
wheat cultivar Agent with
Lr24
in 1971. By the
mid-1970s virulence to
Lr24
was common in the
winter wheat region of the Great Plains. In the
mid-1980s the cultivar Siouxland with
Lr24
and
Lr26
was widely grown from Texas to South
Dakota. Isolates with virulence to
Lr24
and
Lr26
increased up to the early 1990s. Starting in 2002
isolates with virulence to
Lr24
increased again
due to widespread cultivation of 'Jagalene', with
Lr24
. Although
Lr24
was originally derived from
Ae. elongatum
, and
Lr26
from
Secale cereale
, the
nonwheat origin of both genes did little to enhance
their durability of resistance. The cultivar Jagger,
released in the mid-1990s with
Lr17
, selected iso-
lates with virulence to this gene, as these reached
nearly 90% of isolates in Texas and Oklahoma in
2001. Isolates of
P. triticina
with virulence to
Lr41
, derived from
Ae. tauschii
, were found even
before winter wheat cultivars with this gene were
released in the late 1990s in the southern Great
Plains. Isolates with virulence to
Lr41
have
increased such that cultivars with this gene
('Thunderbolt', 'Overley', and 'OK Bullet') are
now susceptible to leaf rust.
Selection of isolates for virulence to specifi c
resistance has also occurred in the spring wheat
region of Minnesota, North Dakota, and South
Dakota, even though leaf rust does not frequently
overwinter in this area. Cultivars with
Lr1
and
Leaf rust resistance in wheat
Race-specifi c resistance
The tremendous amount of genetic variation for
virulence in
P. triticina
populations combined
with the ability of urediniospores to be wind-
disseminated over thousands of kilometers has
made breeding for stable leaf rust resistance in
wheat a continually challenging task. Time and
again wheat cultivars with a single race-specifi c
