Septoria and Stagonospora Diseases of Cereals - CIMMYT ...

Session 5: Epidemiology
Epidemiology of Mycosphaerella graminicola and
Phaeosphaeria nodorum: An Overview
M.W. Shaw
Department of Agricultural Botany, School of Plant Sciences, The University of Reading, Reading, England
Abstract
The within-season and between-crop methods of multiplication and survival, and their environmental relations are
reviewed. Mycosphaerella graminicola multiplies within a season by conidia which are primarily but not exclusively
dispersed by rain. Arguments are given that the influence of ascospores within a crop will be minor, but they are the
major source of movement of the pathogen into new crops. Phaeosphaeria nodorum also multiplies within a season by
conidia, but has clearer associations with wet weather. The role of ascospores in movement between crops may vary
geographically; seed transmission seems to be very important in some areas.
In this contribution, I have tried
to summarize what is understood
of the epidemiology of these two
diseases. My aim has been to
produce a concise summary to
introduce the detailed and novel
contributions that follow. I have
tried to give slightly more extended
discussion of those areas where
new ideas have arisen or our
understanding has changed
substantially in the last few years.
The relevant questions for both
diseases fall into two classes. First,
qualitative: what conditions allow
inoculum transfer, permit infection,
and encourage sporulation?
Second: quantitative: in a given
agro-ecosystem, what factors in
practice regulate pathogen
population size? The two questions
are related, but both need to be
answered if the diseases are to be
managed most effectively. The
answers also depend greatly on
scale: within a region and over
several years, very different
processes and factors may need to
be considered from those operating
within a crop and within a season.
The paper is restricted to wheat.
Mycosphaerella
graminicola
Within a crop
As discussed later, infection of a
crop is usually initiated by airborne
ascospores (Shaw and Royle, 1989).
The density of initial infections is
such that once a few sporulating
lesions per square meter exist, a
polycyclic epidemic on successive
leaf layers follows (Shaw and
Royle, 1993). Pycnidia are produced
within roughly 14 to 40 days,
depending on both temperature
and host cultivar. Conidia may be
dispersed by single rain splashes
within a circle of about 1 m radius,
the number dispersed decreasing
exponentially with distance, with
half distances of the order of 10 cm
(Bannon and Cooke, 1998; Brennan
et al., 1985a; Brennan et al., 1985b).
Initial dispersal is to the ground or
to surface water on a leaf, whence
further dispersal is possible.
However, spores contacting leaf
surfaces are bound to the surface
within a short time. The average
number of splashes moving a spore
during rain of given intensity and
duration is hard to estimate, but is
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unlikely to be large, so half
distances for effective horizontal
dispersal will be of the order of 20-
50 cm at most. Each initial infection
may plausibly produce 50,000 to
500,000 conidia (10-100 pycnidia of
ca. 5000 spores) (Eyal, 1971).
Although most of these are not
dispersed far, considering them as
evenly dispersed over a circle of 0.5
m radius gives 5-50 spores per
square centimeter from initial
infections spaced at about 1/m 2 . If
they had 2-20% infection efficiency,
the crop would be saturated with
latent lesions. Fortunately, infection
efficiency is usually lower than this,
but if few spores are present, 1% of
those applied may cause infection
under good infection conditions.
The actual environmental
conditions permitting infection are
lax because the pathogen tolerates
extended breaks in humidity
during the infection process (Shaw,
1991a; Shaw and Royle, 1993).
Certainly, within two infection
cycles the pathogen population in
crops with moderate initial
amounts of disease will be limited
by the rate of growth of leaf area