Agriculture Reference
In-Depth Information
impacting drop (Yang
et al.
, 1991b), increasing rain intensity might be expected to
increase spore dispersal and disease spread. However, it has been difficult to show a
clear relationship between rain intensity and dispersal (Madden
et al.
, 1996; Madden,
1997). Field studies have been hindered by the considerable variability in rain
intensity during rain episodes, differences in rain type (e.g. showers), and by the
variation in total rainfall volume and duration between different episodes. Until
recently, controlled studies did not generate rain with a great enough range of
intensities to determine the functional relationship between intensity and dispersal
parameters (e.g. mass of water splashed, number of spores transported).
Madden
et al.
(1996) generated rain with a wide range of intensities that matched
the theoretical Marshall-Palmer drop size distributions of natural rain to elucidate
the effects of rain intensity on splash dispersal of
Colletotrichum acutatum
spores;
increasing rainfall intensity increased the mass of water splashed from the surface
(with mass reflective factors of
c.
2-6%), spore removal from lesions and total
number of spores dispersed (measured as colony forming units on a selective growth
medium) (Fig. 16.7a). However, infection of susceptible strawberry fruit exposed to
rain increased with increasing intensity up to 15-30 mm h
-1
and then decreased with
further increases in intensity (Madden
et al.
, 1996; Figure 16.7b) because dispersed
spores were then removed from potential infection sites by wash-off or run-off.
Specifically, the rate of wash-off of spores increased with increasing intensity,
presumably because the physical processes involved in spore removal from lesions
are also involved in removal of spores from infection sites. This disease result is
consistent with the observed spore deposition results (Yang
et al.
, 1991a; Madden
et al.
, 1996), which showed an initial increase in spores per unit area with time but
then a decline at longer times. Ntahimpera
et al.
(1997) confirmed these results in
work with rain of different drop size distributions (that could not be represented by
the simple Marshall-Palmer model) and found analogous results. Specifically,
increasing the median diameter of drops (on a volume or mass basis) increased spore
dispersal, but the incidence of fruit infection was not affected by changing the
median diameter (or drop size distributions). Rain splash dispersal of
Giberella zeae
spores within a wheat field can be described by a power law relationship between
spore flux density at a given canopy height and rain intensity, with a power
independent from factors such as year and location (Paul
et al
., 2004). Thus, rain
intensity has a complex relationship to splash dispersal.
The complexity depends on the role of the physical 3-D structure. For example
in the case of strawberry production, Ntahimpera
et al
. (1998) demonstrated the
effects of a Suddangrass cover crop on splash dispersal of
Colletotrichum acutatum
conidia. Effects of cover crop architecture can be complex; for example rain splash
intensity might be increased and splash droplets obstructed. In experiments on
splash dispersal of
Septoria tritici
conidia in a wheat-clover intercrop (Bannon and
Cooke, 1998) in controlled conditions using simulated rain, the cover crop decreased
spore dispersal in both horizontal and vertical directions. Field work on septoria
tritici blotch in wheat showed how the spatial distribution of lesions influences the
risk of inoculum dispersal by rain-splash in canopies (Lovell
et al.
, 2003).
