Septoria and Stagonospora Diseases of Cereals - CIMMYT ...

equires a predetermined set of
“differentiating cultivars,” which
was not agreed to by Septoria tritici/
Stagonospora nodorum workers,
suitable methodology (sampling,
culturing of pathogen, inoculum
age and dosage, incubation
categories, disease assessment,
etc.), and controlled environmental
conditions to ensure reliability of
results. The lack of a uniform
methological approach and, in
particular, of a common and
agreeable set of differentials
introduces difficulties in comparing
and drawing conclusions on a
wider basis.
The suggested speciation of M.
graminicola on durum (Triticum
durum) and common wheat (T.
aestivum) may require the inclusion
of differentiating cultivars of both
species when they are grown
together or in studies where both
are tested (Kema et al., 1996;
Saadaoui, 1987). Isolates from one
species may not be pathogenic to
the other Triticum species; still, they
may show a significant interaction
term when a proper set of cultivars
of a species is being inoculated
with isolates from the same species.
Cross infection of the two species
was reported for isolates secured
from either Triticum species (Eyal,
1999; Kema et al., 1996). The
specific pathogenicity towards T.
durum may have bearing on the
structure of S. tritici populations
(Saadaoui, 1987). Alternate
cropping of bread wheat cultivars
in a durum wheat management
system may express a lower than
expected infection level on the
former than under uninterrupted
bread wheat management.
Today it is generally accepted
that the specificity in virulence is
low in S. nodorum, but can be
The Septoria/Stagonospora Blotch Diseases of Wheat: Past, Present, and Future 179
detected in S. tritici populations
provided proper differentiating
germplasm is used (Eyal, 1995).
The magnitude of specificity in the
S. tritici - wheat pathosystem and
its implications for breeding for
disease resistance requires
elaboration. Few dominant single
genes conferring resistance were
identified (e.g. Stb 1 - Bulgaria 88,
Stb 2 - Veranopolis; Stb 4 - Tadinia)
(Somasco et al., 1996). There are
several reports that these resistant
sources are not providing the
claimed protection when moved
across S. tritici populations with
wide virulence patterns (Ballantyne
and Thomson, 1995; Eyal, 1999).
It is proposed that specificity
and inheritance studies should be
conducted with germplasm that is
agronomically relevant to breeding,
preferably exhibiting resistance to a
wide virulence pattern (such as
“Bobwhite“S”, IAS20-IASSUL, or
other Frontana derivatives,
Kavkaz/K4500 L.6.A.4, and other
sources identified through
multilocation testing) (Eyal et al.,
1987). Special attention should be
given to resistant wheat accessions
developed in wide-cross programs.
Some of these accessions may be
susceptible to other foliar diseases
that may need attention when
incorporating Septoria/Stagonospora
resistance. Germplasm used in
virulence studies is usually derived
from disease nurseries naturally or
artificially infected with the
pathogen of interest. Only in a few
cases has the virulence spectrum of
these populations been categorized.
It is therefore likely that
identified resistant germplasm may
have only limited use either as a
breeding source or as
“differentials.” Resistant
germplasm that has withstood
prolonged testing to variable
pathogen populations can be
considered a potential source for
breeding for resistance. Special
attention should be given to avoid
selecting tall stature and late
maturing germplasm that may
introduce such non-genetic factors
into germplasm evaluation, or
germplasm with poor agronomic
characteristics.
Isolates possessing virulence to
important resistance sources can
become the “core virulence
spectrum” for screening
germplasm. There is no criterion as
yet for selecting “relevant” isolates
in artificially inoculated breeding
trials. The criteria dictating the
choice of isolates for genetic studies
of virulence (Kema et al., 1999) may
use considerations other than
breeding. The choice by Kema et al.
(1999) of S. tritici isolates such as
IPO323 (avirulent on Veranopolis,
Kavkaz, and Shafir) and IP094269,
which is virulent on these cultivars,
in studying the genetics of
avirulence in this pathogen merits
adoption by other Septoria tritici /
Stagonospora nodorum investigators.
The correlation between
seedling and adult host response
has been substantiated by several
investigators (Eyal, 1999; Kema and
van Silfhout, 1997). Screening for
resistance at the seedling stage
does not provide an integral view
of the tested germplasm. Seedling
tests can serve as a supplementary
measure and are an excellent tool
for detailed, controlled studies on a
multitude of biological issues
associated with host-pathogen
interactions. The seedling test as a
screening measure is hampered by
not knowing whether the used
isolates are relevant to the
virulence spectrum of the