Septoria and Stagonospora Diseases of Cereals - CIMMYT ...

Session 2: The Infection Process
Stagonospora and Septoria Pathogens of Cereals: The
Infection Process
B.M. Cunfer
Department of Plant Pathology, University of Georgia, Griffin, GA
Abstract
Definitive information on the infection process has been reported for Stagonospora nodorum, Septoria tritici, and
Septoria passerinii. Like other necrotrophic pathogens, they do not elicit the hypersensitive reaction. A significant difference
in the infection process between Septoria and Stagonospora pathogens is that spore germination and penetration proceed
much faster for S. nodorum than for S. tritici and S. passerinii. The Septoria pathogens penetrate the leaf primarily
through stomata, whereas S. nodorum penetrates both directly and through stomata. Stagonospora nodorum kills the
epidermal cells quickly, but S. tritici and S. passerinii do not kill epidermal cells until hyphae have ramified through the leaf
mesophyll and rapid necrosis begins. Resistance slows host colonization but has no appreciable effect on the process of lesion
development. The mechanisms controlling host response are still unclear. The infection process for ascospores is probably very
similar to that for pycnidiospores. Ascospores of Phaeosphaeria nodorum germinate over a wide range of temperatures and
their germ tubes penetrate the leaf directly. However, unlike pycnidiospores, the ascospores do not germinate in free water.
The infection process has been
studied most intensely for
Stagonospora nodorum and Septoria
tritici. One in-depth study on
Septoria passerinii is available.
Nearly all of the information
reported is for infection by
pycnidiospores. However, the
infection process for other spore
forms is quite similar. The
information presented is mostly for
infection of leaves under optimum
conditions. Some studies were
done with intact seedling plants,
whereas others were conducted
with detached leaves. Infection of
the wheat coleoptile and seedling
by S. nodorum was described in
detail by Baker (1971) and
reviewed by Cunfer (1983).
Although no precise comparisons
have been made, it appears that the
infection process has many
similarities in each host-parasite
system and is typical of many
necrotrophic pathogens.
Information on factors influencing
symptom development and disease
expression are excluded but have
been reviewed by other authors
(Eyal et al., 1987; King et al., 1983;
Shipton et al., 1971). A summary of
factors affecting spore longevity on
the leaf surface is included.
Role of the Cirrus and
Spore Survival on the
Leaf Surface
The most detailed information
on the function of the cirrus
encasing the pycnidiospores exuded
from the pycnidium is for S.
nodorum. The cirrus is a gel
composed of proteinatous and
saccharide compounds. Its
composition and function are
similar to that of other fungi in the
Sphaeropsidales (Fournet, 1969;
Fournet et al., 1970; Griffiths and
Peverett, 1980).
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The primary roles of cirrus
components are protection of
pycnidiospores from dessication
and prevention of premature
germination. The cirrus protects the
pycnidiospores so that some
remain viable at least 28 days
(Fournet, 1969). When the cirrus
was diluted with water, if the
concentration of cirrus solution was
>20%, less that 10% of
pycnidiospores germinated. At a
lower concentration, the
components provide nutrients that
stimulate spore germination and
elongation of germ tubes. Germ
tube length increased up to 15%
cirrus concentration, then declined
moderately at higher
concentrations (Harrower, 1976).
Brennan et al. (1986) reported
greater germination in dilute cirrus
fluid. Cirrus components reduced
germination at 10-60% relative
humidity. Once spores are