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dispersal. Droplet size distributions suggested (Fig. 16.1) were log-normal for
droplets obtained by splashing of drops falling onto solid surfaces or leaves (Levin
and Hobbs, 1971; Macdonald and McCartney, 1988) or Weibull for droplets
produced by drops impacting on a strawberry fruit surface (Yang et al. , 1991b).
1.0
0.8
0.6
0.4
0.2
0.0
0
500
1000
1500
2000
2500
Figure 16.1. Comparison between measured cumulative numbers ( ) of Pyrenopeziza
brassicae spores dispersed from spore suspensions by falling drops with increasing splash
droplet diameter and numbers predicted (---) by a spore incorporation model using the
droplet diameter frequency distribution (— —) expressed as a percentage of the mode, the
proportion of spore-carrying droplets (---) and the mean spore concentration (— . —) in
different droplet diameter categories (modified from Huber et al ., 1996).
For water drops falling onto thin water films, the log-normal distribution was
slightly better than the Weibull distribution; it also has properties that can be used to
calculate statistical moments of variables related to droplet diameter based on a
power law (Huber et al. , 1996). With both models, the droplet size distribution is
positively skewed, with more small droplets than large droplets. This description is
important because droplets of different sizes travel different distances.
Measurements and modelling showed that 100 µm diameter droplets travel 10 cm,
whereas 600 µm diameter droplets can travel as far as 1 m (although mean distances
are less) (Macdonald and McCartney, 1987). Since this difference has important
consequences for the dispersal of spores and subsequent epidemic development,
more work is needed on the incorporation of spores into splash droplets using a joint
distribution describing both the droplet diameter and the number of spores per
droplet. This can be done by a modelling approach (Huber et al. , 1996) based on
three functions of droplet diameter: diameter distribution of droplets (log-normal),
proportion of droplets carrying spores (exponential function) and number of spores
per spore-carrying droplet (power law). Fig. 16.1 illustrates the shape of these three
functions for an experiment on dispersal of Pyrenopeziza brassicae conidia by
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