Agriculture Reference
In-Depth Information
heights were well predicted. If the structure of the model is assumed to be
unaffected by the nature of the splash surface then, as a first approximation, the
model could be used to predict droplets splashing from leaves. In a more complex
approach, Saint-Jean
et al.
(2004) developed a framework for modelling splash in a
three-dimensional plant canopy. The model used a combination of stochastic and
mechanistic methods and included initial velocity and droplet diameter distributions
and a Newtonian approach to droplet trajectory modelling. Splash dispersal patterns
simulated by the model compared well with measured patterns for simple artificial
canopies consisting of vertical cylinders. Such complex models may give insights
into the important factors governing splash dispersal, but are probably too complex
for routine use in disease risk assessment.
The distances travelled by primary splash droplets splashing directly from
lesions are affected by crop canopy structure, position of the lesion in the crop, the
nature of the water source (rain type, irrigation) and the leaf surface, and the wind
speed. Crop canopy structure affects the deposition of splashed droplets and the
potential for spread by secondary splash (Madden, 1992, 1997). Dispersal gradients of
droplets splashed from strawberries were steeper with a straw surface on the ground
than with a polyethylene surface (Yang and Madden, 1993). Dispersal gradients of
Oculimacula yallundae
(teleomorph of
Pseudocercosporella herpotrichoides,
cause of
eyespot disease of cereals) conidia were steeper in a uniform wheat seedling canopy
than in wheat intercropped with clover, suggesting that the clover was acting as a
source of secondary splash (i.e. conidia previously splashed from the wheat)
(Soleimani
et al.
, 1996). Thus, duration of exposure to rain and rain intensity may
modify 'primary splash' gradients. The spread of the bacterial disease, citrus canker
(
Xanthomonas axonopodis
pv.
citri
), in Florida is enhanced by strong winds
simultaneous with rain in storms or hurricanes. Disease spread from a source to the
nearest newly diseased tree within a 30-day period was estimated at up to 3.5 km
(Gottwald
et al.
, 2002).
Primary dispersal is dominant at the beginning of a rain shower while the canopy
is being wetted and the initial source of inoculum is being dispersed. Most of the
bacteria that cause citrus canker were released within the first 10 minutes in
simulated wind-driven rain experiments (Bock
et al.,
2005). However, as rain
duration continues, secondary spread may begin to be important as previously
splashed inoculum is transported by further splashes. If the rain persists for
sufficient time to deplete the source, inoculum deposited may be lost by wash-off.
This conclusion is supported by the results of experiments on the effects of rain
intensity (increasing intensity for fixed time durations is equivalent to increasing
duration for fixed intensities) on the dispersal of spores of
Colletotrichum acutatum
(cause of black spot or anthracnose) from strawberry fruit (Madden
et al.
, 1996).
After an initial increase, disease incidence declined with increasing intensity and the
intensity at which maximum infection occurred decreased with longer periods of
exposure.
Splash dispersal is a complex process that is difficult to model, particularly if the
effects of secondary splash and wash-off are to be incorporated. Since dispersal may
involve more than one event, Yang
et al.
(1991) developed a diffusion model to
describe effects of multiple splashing on dispersal gradients. The effective diffusion
