Agriculture Reference
In-Depth Information
et al.
, 1999). This may then affect the energy imparted to the droplets and dispersal
distances. Distances to which spores are dispersed also depend on the effects of
canopy structure and density and mulching material, if present, on splash drop
trajectories (Fitt
et al.
, 1992; Madden, 1997). The efficiency with which spores are
incorporated into splash droplets also affects spore dispersal gradients and the
number of spores carried per droplet is influenced by spore size (Fatemi and Fitt,
1983; Fitt and Lysandrou, 1984; Brennan
et al.
, 1985). The incorporation of spores
into splash drops can be modelled as the product of three functions of the droplet
diameter, which may take similar forms for a range of pathogens: the diameter
frequency distribution, the proportion of droplets carrying spores and the mean
number of spores in each number category (Huber
et al.
, 1996).
Most splash droplets that carry spores are very much larger than wind-dispersed
spores and are therefore affected less by turbulence. When effects of turbulence are
small, splash droplet trajectories can be computed using conventional Newtonian
dynamics (Macdonald and McCartney, 1987). The trajectory of a splash droplet is
determined by solving the equation of motion which defines its velocity
v
:
dv
mF
dt
=+++
F
F
F
(6.15)
g
A
D
a
where
m
is its mass,
F
g
is the force of gravity,
F
A
is a buoyancy force,
F
D
is the drag
force and
F
a
is a force due to acceleration of the droplet. Functional representations
are available for each of these forces and only the initial velocity (speed and
direction) and the local wind speed are needed to calculate the droplet trajectory
using iterative procedures. However, few studies of splash dispersal from leaves
(Reynolds
et al.
, 1987; Macdonald and McCartney, 1988; Yang
et al.
, 1991; Yang
and Madden, 1993; Ntahimpera
et al.
, 1999) have measured initial velocities of
splash droplets. Initial speeds of up to 10 m s , with median values of 2 m s , have
been measured for splash droplets dispersed from field bean (
Vicia faba
) or barley
leaves
(Macdonald and McCartney, 1988). The greatest speeds were associated with
small droplets (less than 250 µm diameter) while the largest droplets (greater than
900 µm) rarely reached speeds greater than 3 m s . Average initial velocities of
droplets less than 1.2 mm in diameter measured from splashes on a number of
different surfaces were consistent (~1 m s ) but there was greater variability for
larger droplets (Ntahimpera
et al.
, 1999). The angle of ejection, which is important
in determining the droplet trajectory, depends on the flexibility, angle and texture of
the leaf. For flexible leaves, many droplets are ejected downwards or parallel to the
leaf surface (Macdonald and McCartney, 1988).
Physics-based modelling of dispersal of inoculum in splash droplets in plant
canopies is complex due to the absence of detailed information on the splash
process. Pietravalle
et al.
(2001) used a mechanistic trajectory model to predict
maximum splash height from impacting drop kinetic energy for droplets splashed
from a horizontal water surface. The model assumed that droplet trajectories could
be approximated as parabolas. Model parameters were estimated from controlled
indoor experiments and validated against natural rain events. Maximum splash
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