Septoria and Stagonospora Diseases of Cereals - CIMMYT ...

20
Session 1 — A.L. Scharen
Table 1. Classification and nomenclature of the Septoria spp. and Stagonospora spp. fungi on small grain cereals. a
Genus Teleomorph Anamorph Common name Host
Septoria spp. Mycosphaerella Septoria tritici Septoria tritici Wheat
graminicola blotch of wheat
1998; Shaw and Royle, 1989). The
anamorphic conidia, also called
pycnidiospores, are most important
as secondary inoculum locally as
the crop is growing and are
disseminated mainly by rain
splash.
Sources of primary inoculum in
areas where the teleomorph is not
known remain a matter of
controversy and speculation,
particularly in the case of septoria
tritici blotch. Both S. nodorum and
S. tritici are found parasitizing a
wide range of graminaceous hosts.
(Krupinsky, 1994; Sprague, 1950).
Several species of grasses have
been suspected as alternative hosts
and inoculum sources, but the
question is yet unresolved. Conidia
from plant debris may act as
primary inoculum for disease
development in some cases. The
fact remains to puzzle us that no
case has been reported in which
septoria tritici blotch and/or
stagonospora nodorum blotch
failed to appear because of a lack of
primary inoculum when a
- b Septoria tritici f. avenae Oats
- Septoria tritici f. holci Holcus
- Septoria tritici f. lolicola Lolium
- Septoria passerinii Speckled leaf blotch
of barley
Barley
- Septoria secalis Leaf spot of rye Rye
Stagonospora spp. Phaeosphaeria Stagonospora nodorum Stagonospora nodorum Wheat
nodorum blotch of wheat
Phaeosphaeria Stagonospora avenae Oats
avenaria f. sp. avenae
Phaeosphaeria Stagonospora avenae Oats, wheat
a (7)
avenaria
f. sp. triticea
f. sp. triticea and triticale
b Teleomorphic stages not found.
susceptible crop and favorable
environmental conditions
prevailed.
As late as the 1960s, diagnosis
of Septoria diseases on wheat was
not well developed even among
plant pathologists and plant
breeders. Leaf chlorosis and
necrosis was often viewed as part
of the natural process of
maturation. Any of several leaf
spotting pathogens could have
been present and contributing to
leaf death. When Septoria was
recognized, it was often called
“head septoria” which we know
now as stagonospora nodorum
blotch and “leaf septoria” which
we know as septoria tritici blotch.
Both often occur together and with
other pathogens, and both can
infect and cause symptoms on all
parts of the wheat plant.
Field diagnosis on the basis of
symptomology is regularly done,
but often laboratory backup, with
microscopic examination of spores,
is necessary for accurate diagnosis.
As can be seen in Table 2 (Eyal,
1997), spore size and appearance
may overlap, so some doubts may
remain even after microscopic
examination. Rapid tests have been
developed that use immunological
techniques mainly for early
diagnosis in intensive production
areas where chemical control is
commonly used. Molecular genetic
methods have been used recently
to show similarities and differences
between species and forma speciales
(Arseniuk et al., 1997; Ueng et al.,
1998.). The karyotype of S. nodorum
suggests 14-19 chromosomes
having approximately 0.5-3.5
megabase pairs (Cooley and Caten,
1991). McDonald and Martinez
(1991) determined 14-16 bands
believed to correspond to
chromosomes of 0.33-3.5 megabase
pairs in S. tritici.
Regardless of the fact that
morphologic and genetic
differences of considerable
magnitude exist between S.
nodorum and S. tritici, histological
studies have shown that the