Agriculture Reference
In-Depth Information
distribution is known, indices of potential rain-splash can be calculated with or
without taking into account some kind of crop target characterization (Fig. 16.8).
Based on the existing network of meteorological radars and a specific instrument
measuring potential rain-splash at ground level (that remains to be defined), an
alternative (Walklate, 1989) is to calibrate relationships between rain-splash indices
and back-scattering coefficients in the same way that rainfall intensity is calibrated
against radar reflectance (section 16.5.1).
Figure 16.8. Comparison of splash intensity indices based only on rainfall characteristics
(lines) (Walklate
et al
., 1989) or based on both rainfall and target characteristics (symbols)
from oilseed rape (circles) and tobacco (squares) leaves and assuming a minimum splash
drop size threshold of 2 mm (solid line and empty symbol), 3 mm (dashed line) or 4 mm
(dotted line and solid symbol) (redrawn from Huber
et al
., 1997).
16.6.3 Towards monitoring splash-dispersed pathogen diseases in the field
Most experimental studies on splash have been done in controlled conditions, with
single drops or simulated rain impacting on individual leaves or a small number of
plants. This helps to reduce temporal or spatial variability of input factors (e.g. rain
intensity) and avoids the confounding that occurs in nature where many variables are
often linked (e.g. rain and leaf wetness). Based on controlled experiments for several
air-borne fungal pathogens, it was demonstrated that several concepts (e.g. maximal
splash height, mass of water splashed, truncated integration of DSD), and
methodologies (e.g. splash meter, measurement of DSD) can be very helpful for
assessing spore dispersal by rain-splash (Fitt
et al.
, 1989; Madden, 1992). In the case
of
Mycosphaerella graminicola
(
Septoria tritici
) in winter wheat for example, spore
dispersal from the bottom leaves by rain-splash is well established, and thresholds
for disease control are simply related to daily rainfall amount and plot growth stage
(Hansen
et al.,
1994). However, considerations on penetration of raindrops in the
