Agriculture Reference
In-Depth Information
coefficient was derived from knowledge of the primary splash dispersal patterns and
the spore loss defined by a rate parameter. The approach was able to describe both
the increase and decrease in deposition with time at a fixed point and the decrease in
deposition with distance at a fixed time. Pielaat and van den Bosch (1998) used a
'random jump' modelling approach to describe the effects of secondary splash. The
model was based on three parameters: the probability per unit time that a spore is
splashed; a spore dispersal distance probability function (primary gradient function);
and the probability that the spore is not lost from the system. Compared to the
diffusion approximation, this approach appears to be more realistic for short and
medium time scales, although the two models converge at long time scales.
6.3 SPORE DEPOSITION AND DISEASE GRADIENTS
6.3.1 Relationship between spore deposition and disease gradients
For both wind and splash-dispersed plant pathogen inoculum, deposition rates
decrease with distance away from the inoculum source. Under environmental
conditions favourable to infection, dispersed inoculum will produce further
infections on susceptible hosts. The disease pattern that develops will also show a
decrease in disease with increasing distance away from the source, i.e. a disease
gradient. Disease gradients can also result from gradients in host or environmental
factors but these will not be considered in this chapter. The observation of a
gradient, therefore, implies the existence of a local source of inoculum, since
background inoculum from a large number of distant sources produces a uniform
distribution of disease with distance across a crop (Gregory, 1968, 1973). Vertical
disease gradients (i.e. disease decreasing with height) can also be observed when
inoculum sources are at ground level, for example with black pod of cocoa (caused
by
Phytophthora megakarya
) (McCartney and Fitt
,
1998). Disease gradients
produced by splash-dispersed inoculum are usually steeper than those produced by
wind-dispersed inoculum, reflecting the differences in dispersal length scales
between the two mechanisms (Fitt and McCartney, 1986).
Gradients of monocyclic or polycyclic diseases in crops can provide much
information about the role of the wind-dispersed or splash-dispersed pathogen
spores in the development of epidemics. Monocyclic diseases produce only primary
disease gradients, in which all the lesions arise from the same inoculum source. For
example, gradients of the phoma leaf spot stage of stem canker (causal agent
Leptosphaeria maculans
) can be produced by the wind-borne ascospores in winter
oilseed rape crops in the autumn (Gladders and Musa, 1980). However, spores of
pathogens causing monocyclic diseases may be released over long periods of time so
that the disease gradients gradually become less steep as the growing season
progresses. This may explain why gradients of wheat eyespot in inoculated winter
wheat plots became less steep with successive observations, although removal of
inoculum suggested that there was no secondary disease spread (Rowe and
Powelson, 1973).
