Agriculture Reference
In-Depth Information
The wheat cultivars Renown with gene
Lr14a
(1937) and Pawnee with
Lr3
(1943) were the fi rst
cultivars with race-specifi c leaf rust resistance
genes to be released in Canada and in the
US, respectively. Previous to the release of
these cultivars, leaf rust Race 9 (International
Standard race designation), which is avirulent
to both genes, was the most common race
throughout both Canada (Johnson 1956) and
the US (Johnston et al., 1968). Race 9 declined
in frequency during the 1940s and is currently
only rarely found on common wheat in North
America, occurring almost exclusively on
Ae.
cylindrica
. Isolates with virulence to
Lr3
and
Lr14a
rapidly increased in Canada and the US.
Subsequently the release of winter and spring
wheat cultivars with additional race-specifi c
Lr
genes has resulted in a highly diverse
P. triticina
population.
The cultivar Mediterranean with
Lr3
is a major
ancestor to soft red winter wheat. Genes
Lr9
,
Lr10
,
Lr11
,
Lr12
,
Lr18
,
Lr24
, and
Lr26
have been
present in soft red winter wheat cultivars that are
grown in the southern and eastern US (Kolmer
2003). The presence of these genes over time has
selected leaf rust isolates with corresponding vir-
ulences in the southern and eastern US. Many of
the early hard red winter wheats were derived
from crosses with 'Hope' (
Lr14a
), Pawnee (
Lr3
),
and Mediterranean (
Lr3
). Currently, hard red
winter wheat cultivars in the southern to central
Great Plains region have genes
Lr9
,
Lr16
,
Lr17
,
Lr24
,
Lr26
,
Lr41
, and
Lr42
, and possibly
Lr34
.
Hard red spring wheat cultivars in the northern
Great Plains have genes
Lr1
,
Lr2a
,
Lr10
,
Lr13
,
Lr16
,
Lr23
, and
Lr34
(Oelke and Kolmer 2004).
The selection and increase of leaf rust races with
virulence to these genes in the different wheat
classes has resulted in distinct regional popula-
tions of
P. triticina
virulence phenotypes in the
US. In 2005 (Kolmer et al., 2007b) the frequency
of isolates with virulence to genes
Lr2a
and
Lr16
were highest in the north central spring wheat
region; virulence to genes
Lr11
,
Lr18
, and
Lr26
was highest in the southern soft red wheat region,
and virulence to
Lr24
and
Lr41
was highest in the
hard red winter wheat region.
Molecular variation
Genetic variation in populations of
P. triticina
has
also been examined using various types of genetic
markers. Molecular markers have the attribute of
being neutral and thus not directly selected, as
virulence to specifi c resistance genes is. Molecu-
lar markers can be used to assess the underlying
genetic variation among isolates within and
between populations, providing further insight
into the genetic relationships between different
populations. In Canada phenotypes of
P. triticina
that were identical or closely related for virulence,
had identical or highly related random amplifi ed
polymorphic DNA (RAPD) phenotypes (Kolmer
et al., 1995). Major groups of
P. triticina
isolates
could be determined based on either virulence
polymorphism or RAPD polymorphism since
there was a signifi cant correlation between the
two types of markers. Isolates of
P. triticina
from international collections were also grouped
into distinct groups based on continental region
and virulence and RAPD polymorphism (Kolmer
and Liu 2000). Park et al. (2000) showed that
multiple isolates of the same virulence phenotype
of
P. triticina
from different countries in western
Europe also had identical RAPD phenotypes.
The relationship between virulence phenotype
and molecular polymorphism is maintained
since
P. triticina
reproduces throughout the
world almost exclusively by asexual uredinio-
spores. In an experimental population of
P. tri-
ticina
derived from aeciospores, the disequilibria
between individual virulence and RAPD
markers was often eliminated or reduced (Liu and
Kolmer 1998b).
In 1996 isolates of
P. triticina
with virulence to
Lr17
began to increase in the Great Plains region
of the US (Long et al., 2000) and Canada (Kolmer
1998). These isolates were selected by the winter
wheat cultivar Jagger with
Lr17
, which has been
widely grown throughout Texas, Oklahoma,
Kansas, and Nebraska. The isolates with
Lr17
virulence were unique in that they were also viru-
lent to
Lr3bg
and
LrB
, and avirulent to
Lr28
. By
2001 these isolates had become widespread in
almost all wheat growing regions of North
