Agriculture Reference
In-Depth Information
Lr2a
were widely grown in this region starting in
the mid-1970s. Isolates with virulence to both
genes increased rapidly and were over 65% in the
early 1990s (Fig. 5.1c). Virulence to
Lr16
has
increased from the mid-1990s since many current
spring wheat cultivars have this gene. In Australia
cultivars with either combination of genes
Lr13
,
Lr23
, and
Lr34
or genes
Lr1
,
Lr13
, and
Lr23
are
currently resistant to leaf rust (Bariana et al.,
2007). Cultivars with various combinations of
Lr13
,
Lr24
,
Lr34
, and
Lr37
are considered mod-
erately susceptible to leaf rust.
Leaf rust resistance genes up to
Lr60
have been
designated (McIntosh et al., 2007). Genes
Lr1
(Cloutier et al., 2007),
Lr10
from common wheat
(Feuillet et al., 2003), and
Lr21
from
Ae. tauschii
(Huang et al. 2003) have been sequenced. The
three genes have NBS-LRR regions typical of
resistance genes with isolate specifi city. Genes
Lr1
and
Lr10
are widely ineffective, and
Lr21
has
provided effective resistance in spring wheat cul-
tivars in the US and Canada. Isolates with viru-
lence to
Lr21
would be expected to increase if this
gene was used in a winter wheat cultivar in the
US. Many of the other
Lr
genes for which viru-
lent isolates of
P. triticina
have not been found
have also not been widely used in wheat improve-
ment programs.
as
Lr34
, and mapped to chromosome 7DS
(Dyck 1987). Singh and Rajaram (1992) deter-
mined that Frontana also carries other genes
besides
Lr34
that condition adult-plant leaf rust
resistance. Frontana was used as a leaf rust resis-
tant parent in spring wheat programs in Minne-
sota and also at CIMMYT. The Minnesota
cultivar Chris was derived from crosses with
Frontana and released in 1966 as the fi rst spring
wheat in the US to have
Lr34
. The CIMMYT
cultivars Penjamo 62, Lerma Rojo, and Nainari
60 also had
Lr34
.
Wheat lines and cultivars with
Lr34
optimally
express leaf rust resistance in the adult-plant
stage. Isolates of
P. triticina
with complete viru-
lence to lines with
Lr34
have not been found in
North America (Kolmer et al., 2003, McCallum
and Seto-Goh 2006), despite the presence of
wheat cultivars with
Lr34
for over 40 years. In
fi eld plots lines with only
Lr34
can have moderate
to high levels of leaf rust severity, although these
can usually be distinguished from completely sus-
ceptible lines if leaf rust readings are made when
known susceptible lines are at near-terminal
severity (Color Plate 9a). Lines with
Lr34
can also
express resistance in seedling plants at cooler
temperatures (Singh 1992b; Pretorius et al.,
1994). The presence of
Lr34
enhances the
response of other effective resistance genes in
the same wheat genotype (German and Kolmer
1992). The presence of
Lr34
is also associated
with a distinctive leaf-tip necrosis (Singh 1992a)
that can vary between genotypes and environ-
ments. Wheat cultivars with other
Lr
genes com-
bined with
Lr34
are often more resistant than
lines with only
Lr34
or the other genes singly.
Spring wheat cultivars with combinations of
Lr13
,
Lr16
, and
Lr34
were highly resistant in
Canada (Samborski and Dyck 1982; Liu and
Kolmer 1997) and the US (Ezzahiri and Roelfs
1989).
Diagnostic molecular markers closely linked to
Lr34
have been developed (Bossolini et al., 2006;
Lagudah et al., 2006) that will greatly simplify
selection of breeding materials with
Lr34
. In a
survey of wheat classes in the US using the
Lr34
marker
csLV34
, the allele associated with the
presence of
Lr34
was completely absent in soft
Durable leaf rust resistance in wheat
The development of wheat cultivars with high
levels of effective durable resistance will depend
on genes that confer nonspecifi c resistance or
gene combinations that have proven to be effec-
tive over time. The cultivar Frontana released in
Brazil in 1946 has been a valuable source of
durable nonspecifi c leaf rust resistance. Dyck et
al. (1966) backcrossed leaf rust resistance from
Frontana into Thatcher. Backcross lines with the
adult-plant gene
Lr13
were characterized, yet
none of the lines was as resistant as Frontana
because an additional gene was needed to recover
the original resistance in Frontana. Dyck and
Samborski (1982) characterized gene
LrT2
in a
group of wheat cultivars that included 'Terenzio'
and Frontana. Later,
LrT2
was determined to
be the additional gene in Frontana, designated
