Septoria and Stagonospora Diseases of Cereals - CIMMYT ...

irrigated and moisture stress
conditions, respectively. A further
increase in grain yield in disease
free conditions of about 5% was
observed in the irrigated trials
through the introgression of
7DL.7Ag translocation carrying the
Lr19 gene (from Agropyron
elongatum). This yield increase was
attributed to higher rate of biomass
production in the 7DL.7Ag lines.
However, under moisture stress
conditions 7DL.7Ag lines were
associated with a 16% yield
reduction, possibly due to
excessive biomass production in
early growth stages. This would
suggest that the effect of the
1BL.1RS translocation is genotype
specific and that 7DL.7Ag could be
a useful translocation for
enhancing yield potential at least in
irrigated conditions.
Recent efforts to generate newly
accessible genetic diversity have
involved the reconstitution of
hexaploid wheat by producing
“synthetic wheat” by crossing
durum wheat (T. turgidum), the
donor of the A and B genomes,
with T. tauschii, the donor of the D
genome (Mujeeb-Kazi et al., 1996).
Villareal (1995) and Villareal et al.
(1997) showed that lines derived
after two backcrosses to T. aestivum
showed increased morphoagronomic
variation, and resistance
to Karnal bunt and head scab.
Under full irrigation in
northwestern Mexico, the yield of
this material was nearly 8 t/ha.
When tested under drought
conditions for two years, nearly all
of the synthetic derivatives had
significantly higher 1000-kernel
weight, with grain yield varying
between 84 to 114%, when
compared with the bread wheat
checks.
Historical Aspects and Future Challenges of an International Wheat Program 13
The more focused the breeding
objective, the more restricted a
breeder is in the choice of suitable
parents. The use of genetically
diverse material will continue to be
a prime genetic source for
increasing yield potential, a
complex trait still not well
understood genetically or
physiologically. As long as breeders
have no other readily accessible
tools, genetic diversity and the
opportunity for its recombination
through crossing will be important
to break undesired linkages and
increase the frequency of desirable
alleles. Future breakthroughs in
yield potential will likely come
from such genetically diverse
crosses.
Hybrid wheat
Pickett (1993) and Picket and
Galwey (1997) evaluated 40 years
of wheat hybrid development and
concluded that hybrid wheat
production is not economically
feasible because of a) limited
heterotic advantage; b) lack of
advantage in terms of agronomic,
quality, or disease resistance traits;
c) higher seed costs; and d)
probably most importantly,
heterosis could be “fixed” in
polyploid plants and consequently
hybrids would have no advantage
over inbred lines.
The use of hybrid crops is
usually targeted to higher yield
potential environments. Results
from South Africa (Jordaan, 1996),
however, show that hybrids
outyield inbred lines by 15% at a 2
t/ha mean production potential
when narrow row spacing and low
seeding rates (