Agriculture Reference
In-Depth Information
spring wheat cultivars in Canada and the US
(Kolmer et al., 1991) have had stem rust resis-
tance derived from Thatcher. Dyck (1987) deter-
mined that isogenic lines of Thatcher with
Lr34
had better seedling and adult-plant resistance to
stem rust than Thatcher. The Canadian wheat
cultivars Roblin (Dyck 1993) and Pasqua (Liu
and Kolmer 1998a) have some Thatcher in their
pedigrees and also have
Lr34
. In crosses derived
from both Roblin and Pasqua, progeny lines with
Lr34
have been associated with higher stem rust
resistance. A stem rust resistance suppressor on
chromosome 7DL (Kerber and Green 1980) is
present in Thatcher and other Thatcher-derived
lines. Kerber and Aung (1999) determined that
Thatcher lines with
Lr34
had the same stem rust
response as did Thatcher lines nullisomic for
7DL that lacked the suppressor. Hence
Lr34
appeared to inactivate the stem rust resistance
suppressor.
Gavin-Vanegas et al. (2007) determined that
progenies derived from a Thatcher line with
Lr34
crossed with a stem rust susceptible line segre-
gated for two effective stem rust resistance genes
in adult plants in the absence of
Lr34
, and segre-
gated for three genes when all progeny lines were
fi xed for
Lr34
. In this study segregation of resis-
tance to stem rust races that had high infection
types to seedlings of Thatcher, but low infection
types to Thatcher lines with
Lr34
, was strongly
correlated with segregation of stem rust resistance
in adult plants to a mixture of stem rust races.
The presence of
Lr34
allowed the expression of
additional stem rust resistance gene(s) in Thatcher
that were most likely derived from 'Iumillo'
durum.
Thatcher and the cultivar Chris had low seed-
ling infection types to the stem rust isolate Ug-99,
which has appeared in eastern Africa and is highly
virulent to many US and CIMMYT wheat culti-
vars (Jin and Singh 2006). The stem resistance in
Chris is most likely due to the presence of stem
rust resistance derived from Thatcher that is
enhanced by the presence of
Lr34
. Thatcher lines
with
Lr34
show good resistance to Ug-99 in fi eld
plots in Kenya. Since so few current wheat culti-
vars have effective resistance to Ug-99, the
Thatcher stem rust resistance enhanced by
Lr34
may be an important future source of stem rust
resistance.
Leaf rust resistance in durum wheat
Cultivated durum wheat is generally highly resis-
tant to the
P. triticina
isolates found on common
wheat (
T. aestivum
L.). Genes
Lr14a
derived
from 'Yaroslav' emmer (
T. turgidum
ssp.
dicoc-
cum
) and
Lr23
derived from 'Gaza' durum are
present in common wheat. Genes
Lr10
and
Lr33
(Dyck 1994) may also be present in durum wheat.
Genetic studies of leaf rust resistance in durum
wheat to
P. triticina
isolates from common wheat
have often indicated the presence of one to two
seedling resistance genes that were expressed in a
recessive or dominant manner (Statler 1973;
Zhang and Knott 1990, 1993). Phenotypes of
P. triticina
that are virulent to durum cultivars
were described in Mexico (Singh 1991). Using
isolates collected from durum wheat, Singh
et al. (1993) determined that a collection of
CIMMYT durum cultivars varied for seedling
resistance and adult-plant resistance genes. The
durum cultivar Altar C84 and three other durum
cultivars had a single gene that conditioned seed-
ling resistance, in addition to two adult-plant
resistance genes.
A severe epidemic of leaf rust on durum wheat
occurred during 2001-2003 in northwest Mexico
(Singh et al., 2004). New phenotypes of
P. tritic-
ina
had emerged that were highly virulent to Altar
C84. Increased levels of leaf rust infections were
also noted in France (Goyeau et al., 2006) and
Spain (Martinez et al., 2005). Isolates of
P. tritic-
ina
from durum wheat collected during 2002-
2004 in France, Spain, Mexico, Argentina, and
Chile were highly similar for virulence to the
Lr
genes in the Thatcher isogenic lines, a collection
of durum cultivars (Ordoñez and Kolmer 2007b),
and for molecular SSR variation (Ordoñez and
Kolmer 2007a), suggesting a recent common
origin. Singh et al. (2004) identifi ed CIMMYT
durum germplasm that was resistant to the new
P. triticina
durum virulent phenotypes in Mexico.
The resistant lines were also resistant in the other
