Diseases Which Challenge Global Wheat Production—the Wheat Rusts - Wheat Science and Trade - page 99

in each cell. Teliospores germinate readily to
produce haploid single-celled basidiospores that
are unable to infect cereals and grasses. Despite
early intensive studies, an alternate host for P.
striiformis was not found (Eriksson and Henning
1894; Mains 1933; Tranzschel 1934; Straib 1937;
Hart and Becker 1939). Alternate host plants for
P. striiformis may not exist (Hassebrauk 1970), or
with the short dormancy of teliospores and readily
produced basidiospores, the alternate host may
escape infection (Wright and Lennard 1978;
Rapilly 1979). In the absence of an alternate host,
the teliospores and basidiospores are not func-
tional in the P. striiformis life cycle.
Puccinia striiformis f. sp. tritici is able to infect
a broader range of grass species than stem rust or
leaf rust. Hassebrauk (1965) listed about 320 grass
species of 50 genera in the Gramineae family that
were naturally or artifi cially infected by wheat
stripe rust. The most susceptible genera are Aegil-
ops , Agropyron , Bromus , Elymus , Hordeum , Secales ,
and Triticum . Some Aegilops spp., especially Ae.
cylindrica (common goatgrass), are highly suscep-
tible to stripe rust. These grasses near wheat fi elds
can contribute to stripe rust development as
sources of early inoculum, but have limited roles
in rust survival because they mature earlier than
wheat crops. The importance of grasses as hosts
in wheat stripe rust epidemics may vary from
region to region. Generally, wild grasses play a
less important role than wheat crops and volun-
teer wheat in the initiation, development, spread,
and survival of stripe rust. However, grasses can
serve as reservoirs in maintaining diversity of
stripe rust races.

ronmental conditions, which compromised conti-
nuity from year to year or among investigators.
The differential set used in Europe did not iden-
tify important races in the US (Line 2002). Line
et al. (1970) fi rst developed a uniform system to
identify and describe races of the wheat stripe rust
pathogen in the US. The current differential set
includes 20 wheat cultivars and lines with various
combinations of genes Yr1, Yr2, Yr3a, Yr4a,
Yr6, Yr7, Yr8, Yr9, Yr10, Yr17, Yr19, Yr20,
Yr21 , and other genes currently undesignated
(Chen et al., 2002). A total of 126 races of P. stri-
iformis f. sp. tritici has been identifi ed since the
establishment of this US differential set (Line
and Qayoum 1992; Chen et al., 2002, 2007; Chen
2005, 2007).
The emergence of the majority of P. striiformis
f. sp. tritici races in the US can be related to selec-
tion by wheat cultivars with race-specifi c resis-
tance. Line and Qayoum (1992) discussed races
selected by widely grown wheat cultivars in the
US before 1987. The appearance and rapid devel-
opment of races with virulence to seedlings of the
cultivar Express caused yield losses in California
in 1999. The widely grown cultivars RSI 5,
Bonus, and Summit in California apparently
selected races with virulence to Yr1 , Yr9 (on
the 1RS.1BL wheat-rye translocation), and the
seedling resistance in Express. When these
cultivars were released they were highly resistant
to the previously detected races but became sus-
ceptible within a few years under commercial
production. The average length of time of a
cultivar with race-specifi c Yr genes retaining
effective stripe rust resistance is 3.5 years
(Chen 2005).
The soft winter wheat cultivar Stephens has
been very popular in the Pacifi c Northwest since
its release in 1978. Wheat stripe rust races with
virulence to seedlings of Stephens were fi rst iden-
tifi ed in 1977 (Line and Qayoum 1992) and since
then have been predominant in this region. Since
2004, races with virulence to Stephens and viru-
lence to Yr9 have been predominant throughout
the US (Chen 2007). The widely grown cultivars
Jagger and Jagalene, which likely have stripe rust
resistance from Stephens, may have contributed
to the widespread occurrence of these races in the

Genetic variation in Puccinia
striiformis f. sp. tritici
Virulence variation
The wheat stripe rust pathogen is highly variable
for virulence to stripe rust resistance genes in
wheat. In 2000, 42 races of wheat stripe rust were
found in the US (Chen et al., 2002). Early studies
of physiological specialization in P. striiformis f.
sp. tritici used several sets of differential cultivars,
different inoculation methods, and varying envi-