Agriculture Reference
In-Depth Information
However, Aylor (1987) suggests that deposition will determine the shape of
concentration gradients only when wind speeds in the canopy are low and when
turbulence is slight. Thus, for spores released in gusts the effects of enhanced
turbulence on diffusion may be much greater than the enhanced deposition by
inertial impaction. This conclusion was supported by an analysis of deposition
gradients of wind-dispersed urediniospores of
Puccinia recondita
(cause of brown
rust of wheat) (Aylor, 1987). The escape of spores from a canopy into the
atmosphere also depends on the relative effects of turbulent transport and deposition
and may depend on how they are released (Aylor, 1990). Spores that are removed
passively from leaves are usually released only when turbulence and vertical mixing
are large; such conditions also favour escape from the canopy. Vertical transport
may be affected by spore size; concentrations of ascospores of
P. brassicae
decreased less quickly with height above an oilseed rape crop than concentrations of
larger oilseed rape pollen grains (McCartney, 1990a). Once released into the
atmosphere, spores have the potential to disperse over large distances and heights.
Fungal spores have been found in the atmosphere at heights of 500-1 000 m above
the north sea many kilometres from any potential sources (Hirst
et al.
, 1967) and
spores and pollen from South America have been found in air samples taken in
Antarctica (Marshall, 1996). Long distance aerial transport of inoculum has been
cited as the probable mechanism for the invasion of disease into new territories,
although such events are rare (Brown and Hovmøller, 2002). Long-distance
transport of particles may be enhanced by natural events such as bush fires, which
were implicated in spread of viable bacteria and fungal spores over 1450 km from
Yucatan to Texas and from south east Asia to Hawaii (Mims and Mims, 2004).
Three dimensional time-averaged spore concentration or deposition patterns
round a point source (infected plant) are complex. However, average concentrations
measured in one direction away from the source decrease monotonically with
distance. These dispersion gradients have been described by a number of different
equations (McCartney and Fitt, 1985; Fitt and McCartney, 1986; Minogue, 1986;
Fitt
et al.
, 1987; Aylor 1990). Two of the most commonly used are a negative
exponential equation:
CC x
=
0
exp(
−
α
)
(6.3)
And an inverse power law equation:
CAx
β
−
=
(6.4)
where
C
is the concentration or deposition rate,
x
is the distance from the source and
C
0
,
α
,
A
and
β
are constants. The coefficients
α
and
β
determine the rate of decrease
in spore concentration (or deposition) with distance.
Although the two functions have similar shapes, there is a fundamental
difference between them. The exponential equation implies a constant length scale
(
C
decreases by the same proportion over equal distances), while the inverse power
law implies a length scale which changes proportionally with distance. The fixed
