Agriculture Reference
In-Depth Information
16.5.2 Rainfall modification by the canopy and drip drop production in crops
The modification of rainfall by the crop canopy is important in splash dispersal
(Huber
et al.
, 1995) since drip drops produced by a crop canopy on which rain is
falling may have different effects on spore dispersal by splash in comparison to
raindrops that impact on spore-bearing lesions directly. Transformation of rainfall is
largely influenced by the canopy structure (e.g. LAI vertical partitioning, leaf angle
distribution) in relation to rain angle and direction. For uniform canopies, direct
penetration of raindrops through the canopy is limited when LAI is more than 2
(Schottman and Walter, 1982). A two-layer stochastic rainfall interception model
was developed to take into consideration both primary raindrops (falling at terminal
velocity) and secondary drops dripping from leaves (Calder, 1996). This model
predicts the mean numbers of impacts and drops retained per leaf. Such a model,
with appropriate modification for the splash process, could be used to quantify the
amounts of splash in the canopy and the vertical profile of splash volumes. The
kinetic energy of drip drops rainfall underneath a canopy can exceed the kinetic
energy of the raindrops above the canopy because the canopy has increased the
median drop diameter and the proportion of large drops (Armstrong and Mitchell,
1988; Finney, 1984). The DSD of rainfall transformed by a crop canopy is often
bimodal, with a first mode corresponding to the DSD of the untransformed rainfall
(slightly shifted and skewed towards large drop sizes) and the second mode
corresponding to the 4-7 mm range of drip drop diameters (Armstrong and Mitchell,
1987). Range and variability of drip drop sizes seem quite insensitive to the size
distribution of primary drops (Armstrong and Mitchell, 1988; Moss and Green,
1987) but sensitive to vegetation type (Hall and Calder, 1993). Madden (1992)
provides a prospective synthesis of the effects of canopy on rainfall. Recent work on
3-D modelling of canopy architecture for simulation of rainfall interception
parameters has potential to predict downward spore dispersal by wash-off through
stem flow or splash of large drip drops on lower leaves (Bussière
et al.,
2002;
Bassette and Bussière, 2005). This may be important in dispersal of
Mycosphaerella
fijiensis
and
M. musicola
, cause of sigatoka diseases of banana.
A good example where secondary drip drops reaching the ground dispersed
inoculum from soil and litter is in the spread of cocoa black pod disease (Gregory
et al.
, 1984). Ballistic spore-carrying splash droplets carried spores up from the soil
to infected pods located up to a height of 70 cm but aerosol droplets carried upwards
by vertical air currents created by natural convection were believed to transport
spores to the pods above 70 cm which also developed disease symptoms. Fog is
another source of drip drops; when aluminium or plastic 'leaves' were placed in a
fog wind tunnel for several hours with a fog intensity of 0.3 mm hr
-1
(Merriam,
1973), fog drip drops formed could be as much as 1 mm in diameter even though
there are no primary drops.
16.5.3 Effects of rain and canopy characteristics on spore dispersal by splash
Because the DSD shifts towards larger drop sizes as rain intensity increases (Ulbrich,
1983) and splash droplet numbers and mass increase with increasing size of the
