Agriculture Reference
In-Depth Information
expressed among these materials. In a further
evaluation of stripe rust resistance the same
authors (Ma et al., 1995b) evaluated 34 durum
wheat genotypes, 278
Ae. tauschii
accessions, and
267 primary synthetics in the fi eld. Among the
respective groups, 46%, 23%, and 12% were
found to have seedling resistance to stripe rust,
indicating the presence of major genes. Never-
theless, some seedling-susceptible primary syn-
thetics also displayed adult-plant resistance,
indicating the presence of potentially novel
race-nonspecifi c genes, which have yet to be
exploited.
sion was dependent on the specifi c tetraploid ×
Ae. tauschii
combination and that resistance
expressed in the best synthetic wheat was equiva-
lent to that in moderately resistant bread wheat
cultivars. Xu et al. (2004) tested 120 primary syn-
thetics and their respective durum parents for
reaction to
Septoria nodorum
and
Pyrenophora
tritici-repentis
(Died.) Drechs, the causal organ-
ism for tan spot disease, and found that 47% and
30% of the primary synthetics were resistant to
these respective diseases. These diseases are
expected to increase in importance as reduced- or
zero-tillage management systems become even
more widespread, as the fungus overseasons on
stubble and crop debris retained on the soil
surface.
Septoria diseases and tan spot
Septoria
spp. are also important causes of dis-
eases of wheat, affecting almost 50 million hect-
ares worldwide (Gilchrist and Dubin 2002). The
most widely spread is
Septoria tritici
Desm.
(speckled leaf blotch), which is prevalent in
higher rainfall years in many environments. May
and Lagudah (1992) evaluated a wide range of
Ae. tauschii
accessions and a number of primary
synthetics and found that 90% of the
Ae. tauschii
accessions and more than 60% of the primary
synthetics were resistant to
S. tritici
. In crosses
of a few resistant primary synthetics with
cultivated wheat, they concluded that a single
dominant gene conferred resistance in the syn-
thetic materials and that this resistance was dif-
ferent from that found in cultivated types.
However, they did not determine the genetic
basis of resistance in the remaining primary
synthetics.
Stagonospora nodorum
(Berk.) Castellani and
Germano (syn.
Septoria nodorum
) is less widely
spread but still an important disease. Nicholson
et al. (1993) determined the location of resistance
in a primary synthetic developed from
T. dicoc-
cum
and concluded that resistance was associated
with chromosomes 5D and, to a lesser extent, 3D
and 7D. Loughman et al. (2001) tested a set of
433
Ae. tauschii
accessions and found resistance
to be widespread. However, this resistance was
either not expressed or only partially expressed in
primary synthetics. They concluded that expres-
Karnal bunt
Although Karnal bunt (caused by
Tilletia indica
Mitra) is not an important disease globally, its
incidence can limit global wheat trade. For this
reason many countries have imposed zero-
tolerance restrictions on the importation of
Karnal bunt-infected grain, making immunity a
goal for breeders. Partial resistance to Karnal
bunt can be found in wheat cultivars originating
from India and China. However, higher levels of
resistance, either through the discovery of
immunity or new genes with additive effect, are
sought.
Immunity to Karnal bunt was fi rst reported in
synthetic amphiploids by Multani et al. (1988).
Villareal et al. (1994b) later reported that 49% of
the primary synthetics they evaluated were
immune to Karnal bunt. The 3-year mean infec-
tion of the synthetic materials was <1%, com-
pared to 56% for the susceptible check cultivar.
They subsequently determined that resistance
was either dominant or partially dominant among
a subset of the resistant primary synthetics
(Villareal et al., 1994a). However, it has not been
possible to transfer the observed complete immu-
nity to cultivated wheat, indicating that resistance
may be in part due to the tenacious glumes of
most primary synthetics (Valenzuela-Herrera
et al., 2006).
