Agriculture Reference
In-Depth Information
2007; Chhuneja et al., 2007; Lin and Chen 2007).
The relative abundance of HTAP resistance genes
should facilitate incorporation of durable resis-
tance into adapted wheat germplasm.
In 1983, Roy Johnson defi ned durable resis-
tance as “resistance that remains effective in a
cultivar that is widely grown for a long period of
time in an environment favorable to the disease”
(Johnson 1983). Based on this defi nition, HTAP
resistance has been durable and therefore should
be widely used in breeding programs. Although
race-specifi c resistance genes are generally not
durable, these genes can be used in combination
to prolong their life span. Wheat cultivars with
more effective and durable stripe rust resistance
can be attained by combining HTAP resistance
with effective race-specifi c resistance.
compared with susceptible cultivars, even though
the predominant races were virulent on the race-
specifi c resistance genes in both cultivars. In fi eld
plots at Pullman, Washington, many European
wheat cultivars such as 'Cappelle Desprez',
'Vilmorin 23', and 'Hybrid 46' have exhibited
slow-rusting resistance. Several spring wheat cul-
tivars, such as 'Eden', 'Macon', and 'Scarlet',
have consistently had susceptible infection types
but have rust severities lower than highly suscep-
tible cultivars. However, the slow-rusting resis-
tance in these cultivars is not as effective as the
HTAP resistance in the cultivars Alpowa, Express,
and Louise.
WHEAT STEM RUST
Slow-rusting resistance
Although there are several defi nitions given for
slow-rusting resistance in the literature, the defi -
nition originally given by Caldwell (1968) and
followed by Parlevliet (1979) and Singh and
Rajaram (1992) is still descriptive. Slow-rusting
resistance was characterized as slow disease
development in the fi eld despite a susceptible
infection type and by one or more resistance
components, such as longer latent period, smaller
uredinium size, low receptivity, and reduced
sporulation. Slow-rusting and HTAP resistances
are similar in that both are expressed mostly in
adult plants and are characterized by low disease
severity in the fi eld. However, cultivars with
slow-rusting resistance have a susceptible infec-
tion type, while cultivars with HTAP resistance
have lower infection types than susceptible cul-
tivars. Slow-rusting resistance usually does not
include expression of a hypersensitive response,
while HTAP resistance involves some degree of
hypersensitivity.
Slow-rusting resistance to stripe rust is present
in wheat cultivars. Stripe rust development is
much slower on the cultivars Heines VII and
Yamhill compared with susceptible cultivars.
This resistance is likely due to a longer latent
period for stripe rust sporulation. In fi eld plots
these two cultivars had lower stripe rust severity
Distribution and epidemiology
Although not the most widespread or common
among the wheat rusts, stem rust of wheat can
potentially be the most damaging. In the US epi-
demics have been most frequent and severe in
spring wheat and durum wheat in the northern
states Minnesota, South Dakota, North Dakota,
and to a lesser degree in winter wheat in the
southern and central Great Plains states and the
Ohio Valley. Stem rust also occurs less frequently
in the Pacifi c Northwest region of Washington
and Idaho. Worldwide, stem rust is mostly found
in regions with a continental climate where
summer temperatures regularly exceed 25 ºC.
Stem rust has caused losses in wheat in Canada
(Kolmer 2001b), the southern Cone of South
America (German et al., 2007), continental
Europe, the Indian Subcontinent, Australia (Park
2007), eastern Africa (Wanyera et al., 2006), and
China (Roelfs et al., 1992).
Stem rust appears as elongated blisterlike pus-
tules, or uredinia, most frequently on the leaf
sheaths of a wheat plant, but also on true stem
tissues, leaves, glumes, and awns (Color Plate 11).
On the leaf sheath and glumes, uredinia rupture
the epidermis and give a ragged appearance.
Masses of brownish-red urediniospores, up to
10,000 per day, are produced in the uredinia and
are easily shaken off plants. The uredinial stage is
