Agriculture Reference
In-Depth Information
spore dispersal of a large number of pathogens (Fitt
et al.
, 1989). Environmental
physics and meteorology have introduced several concepts and methods which are
very useful for modelling rain-splash and for understanding spore dispersal by
splash. Techniques to estimate drop size distributions or integral rainfall parameters
over a consistent time scale should be investigated further. Above the crop canopy, a
reference splash index could eventually be defined. A simple parameter that changes
with rain intensity and rain type, such as the mass median diameter, could be used to
predict splash (Ntahimpera
et al.
, 1997). Simple means for adapting this method to
different crop targets are needed because mechanical interactions between raindrops
and target characteristics are extensive.
Effects of ground cover and plant obstruction on rain-splash have important
implications for crop management practices. For example, the advantages of using a
thin film of plastic mulch with horticultural crops (e.g. increase in net radiation and
soil heat flux and consequently the soil temperature around plant roots) must be
balanced against the disadvantage that it favours redispersal of pathogen spores by
multiple splashing after they have been removed from infected plant surfaces by
wash-off or splash (Madden
et al
., 1993; Yang
et al
., 1990). Thus, strategies for
disease management require compromise based on the best available information.
As the understanding of disease spread by rain-splash is improved through
more experimental work with different plant/ground cover combinations and the
appropriate modelling, cultural practices can be modified to improve disease
management.
Because suitable instruments for measuring rain-splash or characterizing spatial
variability of rainfall accumulation have become available, mapping potential rain-
splash should make rapid progress (Workneh
et al.
, 2005) and overcome the lack of
site-specific weather information. Besides the advantage of measuring rather than
predicting the rainfall power, it is still difficult to convince meteorological services
to include potential rain-splash in meteorological networks in addition to rain
intensity and/or surface wetness duration measurements. Since rain intensity
measurements cannot accurately estimate potential rain-splash and the density of the
rain gauge network is too low to measure temporal and spatial variability of rainfall,
it is likely that microwave imagery to provide maps of rainfall radar measurements
will be the technology used to provide spatial information about rain-splash in the
long-term.
Improvements in the understanding and prediction of rain-splash will be made by
development of conceptual models based on physics and probability theory. There is
also a need to improve models of spore dispersal processes from a point source or
spatially distributed sources by combining physics and probability theory (Pielaat
et al
., 1998; Saint-Jean
et al
., 2004; Yang
et al
., 1991a). Knowing the crucial role of
canopy architecture, one key modelling question concerning disease spread by
splash dispersal is the choice between 1-D and 3-D modelling of mass transfer in the
canopy. To describe upward spread of light leaf spot on winter oilseed rape, a
refined one-dimensional approach taking into account both splash dispersal and stem
expansion was developed (Pielaat
et al.
, 2002). Combining water transfer by rain-
splash in a 3-D canopy structure (Saint-Jean, 2003) and incorporation of spores in
splash droplets may lead to a versatile analysis of canopy architecture effects on the
