Agriculture Reference
In-Depth Information
now widely ineffective, but isogenic Thatcher
lines with
Lr16
,
Lr23
, and
Lr34
have effective
resistance in fi eld plots when compared to the
completely susceptible Thatcher (Oelke and
Kolmer 2004). Gene
Lr23
is highly temperature-
dependent in expression of resistance. In ambient
greenhouse temperatures of 15-25 ºC, lines with
this gene expressed variable infection types
ranging from moderate to large uredinia to small
uredinia surrounded by necrosis (Dyck and
Johnson 1983). At 25 ºC in growth cabinets, lines
with
Lr23
expressed very low hypersensitive
infection types to US isolates of
P. triticina
.
Although
P. triticina
isolates with
Lr16
have been
detected in the spring wheat region of the US,
cultivars with this gene still have some resistance
in fi eld plots (Oelke and Kolmer 2004). 'Knudson',
released by AgriPro-Coker in 2002, has also been
very resistant and was determined to have
Lr3
,
Lr10
,
Lr13
,
Lr16
,
Lr23
, and
Lr34
(Kolmer and
Oelke 2006). Spring wheat genotypes with com-
binations of
Lr16
,
Lr23
, and
Lr34
have shown
good levels of resistance that has not been signifi -
cantly eroded by virulence changes in the
P.
triticina
population.
Additional genes from wheat germplasm that
have shown good levels of durable resistance have
also been characterized. Barcellos et al. (2000)
determined that the Brazilian cultivar Toropi had
two genes that conditioned adult-plant leaf rust
resistance which were also associated with leaf-tip
necrosis. Mishra et al. (2005) determined that the
Indian cultivar C 306 had a single adult-plant
resistance gene associated with leaf-tip necrosis
that was independent of
Lr34
. The landrace-
derived cultivars from Uruguay, 'Americano 25e'
and 'Americano 44d', were shown to have unique
adult-plant resistance genes that were not
Lr34
(Kolmer et al., 2007c). The Canadian spring
wheat cultivar AC Taber was determined to have
an effective adult-plant resistance gene other than
Lr13
or
Lr34
(Liu and Kolmer 1997). Navabi
et al. (2003) estimated two to four effective
adult-plant resistance genes were present in fi ve
CIMMYT lines.
Quantitative trait loci (QTLs) that affect resis-
tance in adult plants have also been mapped to
chromosome regions. Xu et al. (2005a,b) identi-
fi ed QTLs that affected fi nal rust severity, infec-
tion rate, infection duration, and latent period on
chromosomes 2B, 7BL, and 2DS in the soft red
winter wheat germplasm line CI 13227. The
Swiss cultivar Forno was determined to have a
major QTL for adult-plant resistance on chromo-
some 1BS and minor regions for resistance on
2DL, 3DL, 4BS, and 5AL (Schnurbusch et al.,
2004). These additional sources of adult-plant
resistance can be used in wheat improvement pro-
grams to diversify germplasm for effective leaf
rust resistance.
Association with other disease resistance genes
An intriguing aspect of adult-plant leaf rust resis-
tance in wheat is an association with resistance to
other diseases of wheat. Lines with
Lr34
also have
adult-plant, nonspecifi c resistance to stripe rust
(caused by
P. striiformis
Westend. f. sp.
tritici
Eriks.) (McIntosh 1992; Singh 1992c). The stripe
rust resistance associated with
Lr34
has been des-
ignated as
Yr18
. The
Lr34
locus also exhibits a
pleiotropic effect on barley yellow dwarf virus
reaction (Singh 1993). Spielmeyer et al. (2005)
showed that lines segregating for adult-plant
resistance to leaf and stripe rust due to
Lr34/
Yr18
also had adult-plant resistance to powdery
mildew. The 7DS chromosomal region of
Lr34
may condition a generalized nonspecifi c response
that acts against biotrophic wheat pathogens. The
adult-plant resistance gene
Lr46
is also associated
with nonspecifi c stripe rust resistance, which has
been designated as
Yr29
(Williams et al., 2002).
Navabi et al. (2005) also showed that adult-plant
resistance genes other than
Lr34/Yr18
condi-
tioned resistance to both leaf rust and stripe rust.
Since adult-plant resistance to leaf and stripe rust
was highly associated, selection of germplasm
with resistance to both rust diseases could be
accomplished by testing for resistance to only one
disease.
The Minnesota spring wheat cultivar Thatcher
was released in 1935 on the basis of resistance to
stem rust and good breadmaking quality charac-
teristics (Hayes et al., 1936). Many subsequent
