Agriculture Reference
In-Depth Information
the disease is spread by the asexual cycling of
dikaryotic urediniospores on wheat, as suitable
alternate hosts are usually not present. In North
America the native
Thalictrum
spp. are resistant
to basidiospore infection (Jackson and Mains
1921; Saari et al., 1968). Infected plants of
Thal-
ictrum
with aeciospores that were pathogenic to
wheat have been reported in northern Kazakh-
stan.
Isopyrum fumarioides
has been noted as an
alternate host of
P. triticina
in Siberia (Chester
1946).
Worldwide the primary host of
P. triticina
is
common hexaploid wheat. Leaf rust caused by
P.
triticina
has also been observed on tetraploid
durum and emmer wheats in Europe, South
America, Israel, Ethiopia, and Mexico (Ordoñez
and Kolmer 2007b), and diploid
Aegilops speltoides
(Yehuda et al., 2004) in Israel.
Puccinia triticina
is also present on wild goatgrass,
Ae. cylindrica
,
in the southern Great Plains of the US. For
all of these nonhexaploid wheat hosts, only
certain races or virulence phenotypes of
P.
triticina
were pathogenic to these hosts, indicating
a high degree of telial host specifi city in
P.
triticina
. Infections of
P. triticina
have not been
noted in natural stands of wild wheat relatives
such as
Ae. sharonensis, T. timopheevi, Ae. tauschii
(syn.
T. tauschii
), or
T. monococcum
, but infections
can be obtained on these species in inoculated
greenhouse tests. A different species of leaf rust,
designated as
P. tritici-duri
with
Anchusa
spp.
as the alternate host, occurs on durum wheat
in Morocco (Viennot-Bourgin 1941; Ezzahiri
et al., 1992).
of leaf rust differentials and were used in the early
race identifi cation studies. Virulence phenotypes
of
P. triticina
are currently identifi ed in the US
by testing single-pustule isolates for virulence to
near-isogenic lines of 'Thatcher' wheat with genes
Lr1, Lr2a, Lr2c, Lr3, Lr9, Lr16, Lr24, Lr26,
Lr3ka, Lr11, Lr17, Lr30, LrB, Lr10, Lr14a, Lr18,
Lr21, Lr28
, and winter wheat lines with
Lr41
and
Lr42
(Long and Kolmer 1989; Kolmer et al.,
2007b).
Puccinia triticina
and wheat interact in a
gene-for-gene manner (Samborski and Dyck
1968). For each
Lr
gene in wheat there is a cor-
responding locus in
P. triticina
with alleles that
condition avirulent responses in the presence
of host resistance genes and alternate alleles
that condition virulent responses in the presence
or absence of resistance genes (Kolmer and
Dyck 1994). The range of seedling infection
types in the Thatcher isogenic lines is shown in
Color Plate 9b. In the US, up to 70 different viru-
lence phenotypes are identifi ed annually (Kolmer
et al., 2007a), with the three most common phe-
notypes accounting for 25%-30% of isolates.
Similar surveys of virulence phenotypes in
P.
triticina
are conducted in Canada (McCallum
and Seto-Goh 2006), in Australia at the Plant
Breeding Institute at Cobbity, and in France
(Goyeau et al., 2006).
The high degree of virulence variation in
P.
triticina
in North America is directly related to
the presence of susceptible hosts and the contin-
ual use of race-specifi c leaf rust resistance genes
in the different classes of wheat. In the southern
US many winter wheat cultivars that are initially
resistant become susceptible to leaf rust due to the
emergence and increase of virulent leaf rust races.
The susceptible winter wheat cultivars allow a
very large population of
P. triticina
to become
established over a wide geographical area in the
fall and survive during the winter. Mutations to
virulence to leaf rust resistance genes are a recur-
rent event in such a large population. Since iso-
lates of
P. triticina
are highly heterozygous for
virulence alleles (Samborski and Dyck 1968;
Kolmer 1992), a single mutation in an avirulent
isolate would be suffi cient to gain virulence to a
resistance gene.
Genetic variation in
P. triticina
Virulence variation
Annual nationwide surveys of leaf rust virulence
phenotypes have been conducted in Canada since
1931 (Johnson 1956) and in the US since 1926
(Johnston et al., 1968). The wheat cultivars
Malakof with
Lr1
, Webster (
Lr2a
), Carina (
Lr2b
,
LrB
), Loros (
Lr2c
), Brevit (
Lr2c
,
LrB
), Hussar
(
Lr11
), Democrat (
Lr3
), and Mediterranean (
Lr3
)
were designated as the International Standard set
