Agriculture Reference
In-Depth Information
techniques can potentially help in the understanding of inoculum dispersal within
plant communities, particularly at crop boundaries.
6.2.2. Dispersal by rain
Rain or spray irrigation can remove spores from infected leaves in run-off water or
in splash droplets (Fitt
et al.
, 1989; Madden, 1992, 1997). The spores of many plant
pathogens can be dispersed only by water because they are contained in mucilage
which prevents dispersal by wind (Gregory, 1973; Fitt
et al.
, 1989). However,
raindrop impacts can also dislodge 'dry' spores from leaves to allow them to be
dispersed by wind (Wadia
et al.,
1998; Geagea
et al.,
2000). For example, wind
tunnel and field observations on late leaf spot of groundnut suggest that rain may
play an important role in releasing conidia of the causal agent,
P. personata
, into the
air.
Water splash directly removes spores from leaf surfaces by incorporating them
into splash droplets. Such droplets can travel more than a metre from the point of
impact but most travel only a few centimetres (Fitt
et al.
, 1989; Madden, 1992,
1997). Consequently, dispersal gradients for splash-dispersed spores are generally
much shorter than those for wind-dispersed spores. Splash can also transport
inoculum vertically and can play an important role in vertical disease movement, for
example in cereals (Fitt
et al.
, 1989; Shaw and Royle, 1993) and oilseed rape
(Pielaat
et al.
, 2002). The effectiveness of splash in removing spores depends on the
size and velocity of the incident drop and on the orientation and mechanical
properties of the surface, but the physical mechanisms involved are not well
understood.
Raindrop size influences both the removal of spores (Gregory, 1973; Fitt and
McCartney, 1986; Fitt
et al.
, 1989; Madden, 1992) and the distances of dispersal
(Yang
et al.
, 1991; Yang
et al.
, 1992; Butterworth and McCartney, 1992). Large
raindrops are more effective than small ones; they remove more spores and splash
them further (Fitt
et al.
, 1988). Raindrops less than 0.5 mm in diameter contribute
little to direct dispersal by splash but can contribute to the wetting of leaf surfaces
(Madden, 1992). Spore removal and dispersal are dependent on the force of impact
(Walklate, 1989) or the kinetic energy (Yang
et al.
, 1991) of the incident water
drops. Therefore, large slow-moving drops dripping from leaves may remove spores
as effectively as small raindrops falling at their terminal velocity. Sensors that
respond to the kinetic energy of impacting drops have been developed to estimate
the 'splash dispersal potential' of rainfall (Madden
et al.,
1998; Lovell
et al.
, 2002).
The potential for dispersal by rain-splash depends partly on the size distribution
of the raindrops (Fitt
et al.
, 1989; Walklate, 1989; Walklate
et al.
, 1989), which
depends on the type of rainfall (Ulbricht, 1983). For example, spores of
Septoria
tritici
(anamorph of
Mycosphaerella graminicola,
cause of septoria tritici blotch of
wheat) were splashed from the base of a wheat canopy to the upper leaves only
during heavy summer showers (Shaw and Royle, 1993). The texture, angle and
flexibility of leaves and other surfaces in the canopy all influence the splash process
and the amount of water splashed (Huber
et al.
, 1997; Madden, 1997; Ntahimpera
