Agriculture Reference
In-Depth Information
responsible for the upward transport of spores. In another phenomenon, called
outward interactions, air moves upwards at speeds greater than the local average
wind speed (Aylor, 1990). These complex flow patterns have consequences for
spore removal and transport within and out of the canopy. Unfortunately, current
models of air flow within crop canopies do not describe these phenomena well and
new models are needed to improve understanding of spore dispersal close to sources
(Aylor, 1990). However, recent advances in modelling in fluid dynamics may begin
to address these questions.
Wind not only transports spores but also removes them from infected plants.
Although many fungi have evolved active spore release mechanisms to eject spores
directly into the air (Lacey, 1986), spores of a large number of foliar pathogens are
simply passively blown or shaken off their hosts. To remove spores, the
aerodynamic or mechanical forces generated by wind must overcome the forces
holding the spore to the host surface (Aylor and Parlange, 1975). The wind speeds
needed to remove spores are not known for many fungi but can be relatively large
(Grace, 1977). Conidia of
Blumeria
(
Erysiphe
)
graminis
f.sp.
hordei
(cause of
barley powdery mildew), which form in chains above the leaf surface, were released
by wind speeds greater than 0.5 m s (Hammett and Manners, 1974) and conidia of
Drechslera maydis
(cause of southern leaf blight of maize) were removed only by
wind speeds of more than 5 m s (Aylor, 1975). The wind intermittency observed in
crop canopies probably plays an important role in spore removal because it is only in
gusts that wind speeds are large enough to remove spores (Aylor, 1978; Aylor
et al.
,
1981). The importance of gusts in the removal of conidia of
Passalora personata
(cause of late leaf spot of groundnut) has been demonstrated in wind tunnel
experiments (Wadia
et al.
, 1998). Wind gusts can also remove bacterial cells
(
Pseudomonas syringae
) and spores (
Bacillus subtilis
) from leaves on which they
had previously been sprayed (Lighthart
et al.,
1993).
Spores can also be dislodged
by shaking (Bainbridge and Legg, 1976); thus wind gusts may indirectly remove
spores by moving the crop canopy. The removal of spores by gusts of wind has
important implications for dispersal, particularly within crop canopies.
The turbulent nature of wind causes a dilution in the concentration of a spore
plume as it moves down-wind. Within crops, concentrations are also depleted by
deposition of spores on the crop and the ground. Spores can be deposited by
gravitational settling and inertial impaction (Legg and Powell, 1979). The rate at
which spores settle onto surfaces,
S
, is proportional to the spore fall speed,
V
s
,
and
the spore concentration above the surface,
C
(
S
=
CV
s
).
V
s
is generally in the range
0.1 to 3 cm s for most fungal spores (Gregory, 1973). Deposition by impaction
I
is
dependent on
C
and wind speed
u
(
I
=
CuE
); the constant of proportionality,
E
(the
impaction efficiency), increases with increasing spore size and wind speed but
decreases with increasing width of the impaction surface (Chamberlain, 1975;
Aylor, 1982). When spores are released in wind gusts their inertial impaction
efficiency is enhanced because they are travelling at relatively high wind speeds
(Aylor, 1978; Aylor
et al.
, 1981), especially in crop canopies where mean wind
speeds are small (McCartney and Bainbridge, 1987). Simulations using random
walk models (see below) suggest that this enhanced inertial impaction can steepen
gradients when plume depletion is predominantly by deposition (Legg, 1983).
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