Septoria and Stagonospora Diseases of Cereals - CIMMYT ...

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Characterization of Septoria tritici Variants and PCR
Assay for Detecting Stagonospora nodorum and Septoria
tritici in Wheat
S. Hamza, 1 M. Medini, 1 T. Sassi, 1 S. Abdennour, 1 M. Rouassi, 1
A.B. Salah, 1 M. Cherif, 1 R. Strange, 2 and M. Harrabi1 1 Laboratoire de Génétique, Institut National Agronomique de Tunisie, Tunisia
2 Plant Pathology, UCL London, United Kingdom
Abstract
Pathogenic specialization of Septoria tritici was studied by inoculating 14 isolates of the fungus, of which seven were
obtained from durum wheat and seven were isolated from bread wheat. Isolates obtained from durum wheat were more
virulent on durum wheat, while those isolated from bread wheat were more severe on bread wheat, which revealed
physiologic specialization of S. tritici to either bread or durum wheat. The sequence coding for the nuclear 5.8S rDNA and
the internal transcribed spacer (ITS1 and ITS2) were amplified by polymerase chain reaction and sequenced for five isolates
adapted to bread wheat and five isolates adapted to durum wheat. These sequences were identical between both variants,
resulting in the absence of divergence inside tritici species. Septoria tritici and Stagonospora nodorum isolates collected
from Tunisia were tested for amplification with specific primers to these pathogens. This revealed the primer’s efficiency to
distinguish Septoria species and detect Stagonospora nodorum DNA with as little as 2 pg of DNA. Time course
quantification of S. tritici mycelia after inoculation of resistant and susceptible cultivars distinguished those cultivars by the
earliest date of apparition of a PCR product.
Extensive genetic variation for
virulence in Septoria tritici or its
telomorph Mycosphaerella
graminicola, characterized by
differential interaction between
host and pathogen genotypes,
suggested the involvement of
specific factors for virulence and
resistance in the pathosystem
(Kema et al., 1996a). Specific
interaction between M. graminicola
and wheat was well demonstrated
by statistical evidence (Kema et al.,
1996b), which suggested gene-forgene
interaction between resistance
and virulence in host and
pathogen, respectively.
Mycosphaerella graminicola
specialization is much more
pronounced on bread wheat and
durum wheat than differential
specificity on a particular cultivar.
In particular when considering the
presence of pycnidia as a disease
parameter, bread wheat and durum
wheat isolates were particularly
virulent on bread and durum
wheat, respectively. Therefore
bread wheat and durum wheat
variants in M. graminicola were
considered. The identical sequence
of the internal transcribed spacer
ribosomal DNA (ITS rDNA)
observed by Kema et al. (1996a)
showed that both variants could
not be distinguished using their
ribosomal DNA and that both were
therefore from a similar taxonomic
rank.
PCR has been shown to be a
powerful technique to detect small
amounts of fungal plant pathogens.
The technique is now commonly
used by field pathologists and
growers for diagnosing several
plant pathogens. The early
detection of plant disease allows
the judicious use of agricultural
fungicides; early treatment is often
more effective because the
pathogen is less well established in
its host. PCR amplification using
specific primers for S. tritici and
Stagonospora nodorum or Septoria
nodorum was able to distinguish
these pathogens and detect a few
fungal cells in infested wheat
tissues well before disease
symptoms were apparent (Beck
and Ligon, 1995). Since both
pathogens may infect the same
plant and visually distinguishing
them is difficult, the specific
detection of S. tritici and S. nodorum
is well circumvented using the PCR
technique and facilitates the
decision of whether or not to treat
with fungicides and which
fungicides to use.