Agriculture Reference
In-Depth Information
Races with virulence to
Yr1
differed for RAPD
markers from races avirulent to
Yr1
, indicating
different origins of the two groups of races. More
recently, Markell et al. (2004) used AFLP markers
to study the new wheat stripe rust races in the
south central states of the US. The new group of
races that was collected after 2000 was virulent to
Yr8
and Yr9 and was clearly different for AFLP
genotype compared with races collected before
2000. The results, together with virulence
data (Chen et al., 2002; Chen 2005, 2007), indi-
cate that the new group of races was introduced
to the US.
In Europe, wheat stripe rust isolates with iden-
tical virulences and AFLP phenotypes have been
found in the UK and Denmark (Hovmoller et al.,
2002). A single population of
P. striiformis
f. sp.
tritici
is present in northwest Europe with regular
migration of the rust from the UK to Germany
and France. In France two distinct populations of
P. striiformis
f. sp.
tritici
in the north and south of
the country were described based on AFLP and
virulence variation (Enjalbert et al., 2005).
Vogel, a USDA-ARS scientist at Pullman, Wash-
ington. This cultivar, and the sister-line cultivar
Nugaines, laid the foundation for non-race-
specifi c HTAP resistance that is widely distrib-
uted among wheat cultivars grown in the Pacifi c
Northwest and other regions. Since the early
1960s, US scientists have conducted studies to
identify, characterize, and map stripe rust resis-
tance genes and to develop cultivars with stripe
rust resistance (Röbbelen and Sharp 1978; Line
2002; Chen 2005).
Race-specifi c resistance
Stripe rust resistance genes up to
Yr40
have been
designated (McIntosh et al., 2007) and more than
30 genes with provisional designations have been
listed. Races of stripe rust with virulence to most
of the race-specifi c Yr genes have been found
(Chen 2005). Currently, only a few race-specifi c
Yr
genes that are expressed in seedling plants are
effective against all identifi ed races in the US.
These genes include
Yr5
,
Yr15
,
Yr26
, and
Yr40
(Chen 2005; Kuraparthy et al., 2007).
Although
Yr
genes that are race-specifi c are
generally not durable, the deployment of multiple
gene combinations either in homogeneous pure-
line cultivars, multiline cultivars, or cultivar mix-
tures can prolong the effective life span of the
Yr
genes. The most successful example of using a
multiline cultivar to control stripe rust is Rely, a
club wheat cultivar widely grown in the US
Pacifi c Northwest since its release in 1991 (Allan
et al., 1993). Cultivar mixtures combine two or
more cultivars with different resistance genes in
the same fi eld to reduce disease damage (Finckh
and Mundt 1992). Recently more than 30% of the
wheat acreage in Washington was planted to mix-
tures of two or three cultivars. This approach
reduces yield losses caused by stripe rust; it also
minimizes abiotic stresses such as winter and
drought damage. Many wheat breeding programs
in the US are currently combining resistance
genes, such as
Yr5
and
Yr15
, into elite lines to
develop new cultivars with a high level of resis-
tance. In Australia, cultivars with combinations
of genes
Yr6
,
Yr7
,
Yr17
,
Yr18
, and
Yr33
are
common (Bariana et al., 2007).
Stripe rust resistance in wheat
Working with stripe rust of wheat, Biffen (1905)
was the fi rst to show that disease resistance in
plants was inherited according to Mendel's laws.
His research provided the scientifi c basis for
breeding resistant cultivars to control stripe rust
and other plant diseases. Much of the early
research on resistance and genetics of stripe rust
resistance in wheat was conducted in Europe,
which was reviewed by Röbbelen and Sharp
(1978). Lupton and Macer (1962) determined the
genetics of stripe rust resistance in seven wheat
genotypes and used the
Yr
symbols to designate
resistance genes in wheat.
Breeding for stripe rust resistance in wheat in
the US had little incentive from insignifi cant
yield losses during the 40-year period after stripe
rust was discovered in 1915 (Line 2002). After the
widespread stripe rust epidemics in the early
1960s, wheat breeders and geneticists started
work on developing stripe rust resistant cultivars.
The fi rst cultivar, Gaines, was released with
moderate stripe rust resistance in 1961 by Dr. O.
