Agriculture Reference
In-Depth Information
Many studies on disease gradients have been done with polycyclic diseases
spread by wind-dispersed spores, such as potato late blight (Minogue, 1986), yellow
rust of wheat (Zadoks and Schein, 1979) or powdery mildew of barley (Welham
et al.
, 1995). Typically, such diseases are first observed in a crop as primary disease
foci resulting from a single lesion; by the time a yellow rust focus 1 m
2
in diameter
is observed, four pathogen generations of infection, latent period and sporulation
have occurred (Zadoks and Schein, 1979). Initially disease gradients away from
these foci are steep but spores which escape from the crop canopy soon establish
secondary foci; primary disease gradients become more shallow as foci expand and,
with the expansion of secondary foci disease, disease is soon distributed uniformly
across the crop (Gregory, 1968, 1973).
Less work has been done with polycyclic diseases spread by splash-dispersed
spores, such as wheat glume blotch (caused by
Phaeosphaeria nodorum,
teleomorph
,
Stagonospora nodorum
, anamorph; Jeger
et al.
, 1983) or barley leaf
blotch (caused by
Rhynchosporium secalis
; McCartney
et al.
, unpublished). The
concept of isopaths, to describe the spread of these diseases in time and space, was
introduced by Berger and Luke (1979) and reviewed by Minogue (1986). Often the
rate of spread of isopaths was faster for pathogens spread by wind-dispersed spores
(e.g.
P. infestans
, 3-4 m day ) than for pathogens spread by splash-dispersed spores
(e.g.
S. nodorum
, 0.3 m day ).
Spread of other polycyclic diseases involves both wind-dispersed ascospores,
which initiate epidemics at the beginning of the growing season, and splash-
dispersed conidia, responsible for subsequent cycles of disease spread. For example,
initial horizontal gradients of white leaf spot (causal agent
Mycosphaerella
-
1
-
1
capsellae
) in winter oilseed rape (McCartney and Fitt, 1998) are caused by wind-
dispersed ascospores but subsequent horizontal spread and vertical spread up the
crop canopy is achieved by splash-dispersed conidia, with an estimated 9-13
pathogen generations per season (Inman, 1993). A similar pattern of disease spread
is observed for septoria tritici blotch in winter wheat crops (Shaw, 1987). Other
pathogens with both ascospores and conidia are apparently monocyclic because
either the ascospores (e.g.
O. yallundae
) or the conidia (e.g.
L. maculans
; anamorph
Phoma lingam
) seem to play little part in epidemics in practice.
6.3.2 Measurement of gradients
To measure a spore dispersal or disease gradient in a natural or experimental
situation, measurements of spore numbers per m
3
(spore concentration gradient) or
per m
2
(spore deposition gradient) (
C
) or disease incidence or severity (
Y
) at
different distances (
x
) from the source are needed. Spore numbers can be estimated
with artificial samplers but the choice of sampler and timing of sampling depend on
the size of the spores, their mode of dispersal and concentration and the objective of
the investigation (Fitt and McCartney, 1986; McCartney
et al.
, 1997). Generally,
samplers need to be simple and easy to use because the measurement of gradients
requires the use of at least 10-20 identical samplers simultaneously. To measure
spore deposition gradients, passive samplers such as horizontal slides under
