Septoria and Stagonospora Diseases of Cereals - CIMMYT ...

their resistance in either a recessive
or dominant fashion (Eyal, 1999).
However, in most cases, resistance
appears dominant, with the F1
from a cross between a resistant
and a susceptible parent expressing
an intermediate level of resistance
that is more similar to that of the
resistant parent. A few genes may
be enough to confer resistance that
will hold up in farmers’ fields
(Dubin and Rajaram, 1996). While
heritabilities tend to be only of
moderate magnitude, progress in
breeding for resistance is evident.
Although the number of genes
available may be quite high,
accumulating a few key ones may
be sufficient to achieve resistance.
As has been shown for durable
resistance to leaf rust (Singh et al.,
1991), several of the alleged
components of partial resistance to
S. tritici may also be controlled by
only one or a just a few genes
(Jlibene and El Bouami, 1995). It
would seem that those components
that are genetically different could
be combined into the same genetic
background by crossing. Once
available, molecular markers will
be of tremendous use for
accumulating resistance to such
environmentally sensitive diseases.
The expression of S. tritici
resistance was studied in T. tauschii
accessions, synthetic wheats
derived from them, plus
derivatives of synthetic wheats
crossed to common wheats. The
results obtained all pointed to
inheritance based on one or a few
dominant genes (Appels and
Lagudah, 1990; May and
Lagudah, 1992).
Chromosome 5D of T. tauschii
contributed a high level of resistance
to S. nodorum in a synthetic cross
with a T. dicoccum line and in
derived substitution lines
(Nicholson et al., 1993).
Chromosomes 5D, 3D, and 7D
contributed resistance in decreasing
order of importance. Aegilops
longissima and Ae. speltoides were
shown to contribute S. nodorum
resistance based on two to four
partially dominant to overdominant
genes (Ecker et al., 1990a
and b). Triticum timopheevii
transmitted one S. nodorum
resistance gene located on
chromosome 3A to its progeny from
a cross with a susceptible durum
parent (Ma and Hughes, 1995).
In most published accounts of
quantitative analyses, additive gene
effects or general combining ability
(GCA) effects contributed more to
resistance than dominance or special
combining ability (SCA) effects (van
Ginkel and Scharen, 1987; Bruno
and Nelson, 1990; Danon and Eyal,
1990; Wilkinson et al., 1990; Jonsson,
1991; Jlibene et al., 1994). However,
significant non-additive effects were
often identified in the same studies.
SCA effects tend to be an order of
magnitude weaker than GCA
effects. Indirectly Nelson and Fang
(1994) confirmed these conclusions
when they observed an absence of
heterosis for components of S.
nodorum resistance, indicating a
general lack of over-dominance
effects that enhance resistance.
Jlibene et al. (1994) found a small
cytoplasmic effect in a few of
their crosses.
Breeding for Resistance to the Septoria/Stagonospora Blights of Wheat 119
Triticales were found not to be
any more resistant than most
wheats, and a range of reactions to
both S. tritici and S. nodorum was
observed (Scharen et al., 1990).
Tritordeum amphiploids (doubled
derivatives from crosses between
Hordeum chilense and Triticum spp.)
expressed the S. tritici resistance of
the barley parent both as seedlings
in greenhouse tests and adult
plants in field trials (Rubiales et
al., 1992).
Tolerance to S. nodorum was
shown to be a mechanism that is
genetically different from (partial)
resistance and involves several
chromosomes (Rapilly et al., 1988).
Distinct characters were identified
within each of these two defense
mechanisms.
Although the introduction of
alien cytoplasm from wild wheat
relatives reduced partial resistance
somewhat, it did increase tolerance
as measured by reduced yield
losses (Keane and Jones, 1990).
Agronomic Traits
and Resistance
After a period in the 1960s and
1970s of seemingly increased levels
of disease susceptibility in the
semidwarf wheats (Santiago,
1970), modern semidwarf
germplasm carrying high levels of
resistance was subsequently
identified. Many are now widely
grown as commercial varieties.
As in previous studies,
Camacho-Casas et al. (1995)
showed that it is possible to breed
normally maturing semidwarf
wheats if proper disease conditions