Diseases Which Challenge Global Wheat Production—the Wheat Rusts - Wheat Science and Trade - page 107

entials with single genes in Chinese Spring or in
other backgrounds (McIntosh et al., 1995). The
stem rust resistance genes present in the current
differential set used in the US and Canada include
Sr5 , Sr6 , Sr7b , Sr8a , Sr9a , Sr9b , Sr9d , Sr9e ,
Sr9g , Sr10 , Sr11 , Sr17 , Sr21 , Sr30 , Sr36 , and
SrTmp (Roelfs and Martens 1988). Wheat and P.
graminis f. sp. tritici interact in a gene-for-gene
manner (Loegering and Powers 1962; Green
1964); thus the frequency of races with virulence
to a resistance gene is related to the frequency of
virulence alleles in the rust pathogen.
The alternate host of stem rust, common bar-
berry, was prevalent throughout the north central
US in the late 1890s and early 1900s. Pycnial-
aecial infections on the barberry plants contrib-
uted to the initial inoculum and also to the race
diversity of stem rust. The barberry eradication
program in the 1920s removed millions of bar-
berry from this region, thus delaying the initial
onset of epidemics in the spring wheat region via
reduction of initial inoculum and also reducing
the race diversity of stem rust (Campbell and
Long 2001). From 1919 to the 1950s, 10-38 stem
rust races were detected annually in the US,
whereas from the 1960s to the present time, fewer

than 10 races were usually detected (Groth and
Roelfs 1987) (Fig. 5.3). The total amount of wheat
stem rust inoculum has been reduced since the
1960s due to the increased use of highly resistant
winter and spring wheat cultivars and of winter
wheat cultivars with shorter maturity, thus reduc-
ing late onsets of stem rust in the southern Great
Plains.
In 2006 a single race designated as QFCS
accounted for 25 of the 27 total collections, along
with single collections of race MCCD (= race 56)
and race TTTT (Long et al., 2007). Race QFCS
is widely avirulent on many of the Sr genes com-
monly present in spring and winter wheat in the
US. The combination of smaller population size
and the lack of sexual recombination has stabi-
lized selection of races in the wheat stem rust
population and thus has made the resistance of
the Sr genes much more effective and durable.
The use of highly resistant wheat cultivars in
Australia (Park 2007) has reduced the number
of stem rust races and the overall levels of
inoculum.
Roelfs and Groth (1980) determined in 1975
that there were six distinct groups of P. graminis
f. sp. tritici races in North America. Races in each

Fig. 5.3 Frequency (%) of
races of Puccinia graminis f. sp.
tritici collected from wheat in
the US Great Plains. The four
letter code race nomenclature
is based on infection types to
single-gene differentials in
Roelfs and Martens (1988).