Septoria and Stagonospora Diseases of Cereals - CIMMYT ...

Session 1: Pathogen Biology
Biology of the Septoria/Stagonospora Pathogens: An
Overview
A.L. Scharen
Department of Plant Sciences, Montana State University, Bozeman, Montana, USA
Abstract
More than 2000 form-species of fungi, mostly plant parasites, comprise the genera Septoria and Stagonospora. The two
most important pathogens in wheat production are Mycosphaerella graminicola (Septoria tritici) and Phaeosphaeria
nodorum (Stagonospora nodorum). Primary inoculum for the wheat diseases caused by these pathogens is most often airborne
ascospores, but may also be wind- and rain-borne conidia. Field diagnoses may be augmented and made more exact by
use of rapid immunological tests and molecular genetic methods. Infection processes of S. tritici and S. nodorum are similar,
but penetration by S. tritici is known only via stomata. Patterns of occurrence of the two pathogens have changed
dramatically in recent years. Septoria tritici has become more important in northern Europe, and S. nodorum incidence has
increased in parts of North Africa. Changes in cultivars and cultural practices are thought to be responsible for the shifts in
pathogens and diseases. Genetics of resistance in the wheat host and virulence in the pathogen populations continues to be
unclear. Some gene-for-gene interactions have been shown, but in field situations resistance is generally observed as nonspecific
and pathogen populations vary most in aggressiveness.
“Scientists build on foundations
laid by their predecessors…, but
they show great reluctance to
inspect those foundations”
(Ainsworth, 1965). Such reluctance
was evident for many years in the
case of the Septoria/Stagonospora
diseases of cereal grains.
Nineteenth and early twentieth
century workers in Europe and
North America described the
pathogens and the diseases they
cause in considerable detail. But,
only in the years since the advent
of cultivars bred for dwarf stature
and high yield under conditions of
intensive culture have the Septoria/
Stagonospora diseases been
recognized as having major
impacts upon yield and quality.
The causal organisms are still
called by several names, as are the
diseases. A concise summary of the
latest information on taxonomy
and nomenclature was published
in 1997 (Cunfer, 1997) and 1999
(Cunfer and Ueng, 1999). I believe
that we should urge from this
venue, as did those attending the
Fourth International Workshop on
Septoria of Cereals (July 4-7, 1994,
IHAR, Radzikow, Poland), that all
workers accept and use the latest
nomenclature.
More than 2000 form-species of
fungi, most of them plant parasites,
have been assigned to the genera
Septoria and Stagonospora. Two of
these that attack wheat are of most
concern to us. Losses of potential
yield world-wide from just these
two are estimated in the millions of
metric tons of grain and billions of
U.S. dollars each year. The common
names of the diseases are septoria
tritici blotch and stagonospora
nodorum blotch. Classification and
19
nomenclature of these and other
pathogens of cereals are given in
Table 1 (Eyal, 1999). The Septoria
anamorphic states have a
teleomorphic state assigned to the
genus Mycosphaerella, while the
Stagonospora states have
teleomorphic forms in the genus
Phaeosphaeria. The teleomorphs are
very important because they
furnish the primary inoculum for
disease development under specific
environmental conditions.
Several periods of ascospore
deposition (both Mycosphaerella and
Phaeosphaeria) from the atmosphere
during the months of August to
October in the northern
hemisphere and February to April
in the southern hemisphere have
recently been shown to have critical
importance in epidemic
establishment (Arseniuk et al.,