Agriculture Reference
In-Depth Information
patterns which cause them. This is partly because spore dispersal is a short term
phenomenon compared to most other stages of disease development. For example,
conidia of Pyrenopeziza brassicae , the cause of light leaf spot on oilseed rape
( Brassica napus ssp. oleifera ), take about 18 hours to germinate under optimum
conditions, while splash dispersal of conidia over typical distances of 20-30 cm
takes less than one second and wind dispersal of ascospores of P. brassicae over 100
m takes 1-2 minutes. Even for long distance dispersal, such as for tobacco blue
mould (www.ces.ncsu.edu/depts/pp/bluemold/) that can spread from Cuba to the
southern USA (Aylor, 1999) or cereal rusts in the USA or India (Hamilton and
Stakman, 1967; Nagarajan and Singh, 1990), dispersal events (hours or days) may
be short compared with infection processes. Disease patterns are often the result of
many individual dispersal events from many sources over periods of days or even
weeks. Environmental and biological factors that affect infection and disease
development can add further complications. The development of real epidemics is,
therefore, a complex process; as Waggoner wrote in 1962 “we shall find the real
epidemic muddy and uncomfortable” (quoted by Gregory, 1973). This chapter aims
to make the water a little clearer by considering the underlying processes which
govern spore dispersal, examining the relationship between disease and spore
dispersal gradients, presenting some examples of field studies and considering how
disease spread can be modelled.
6.2 UNDERLYING MECHANISMS: SPORE DISPERSAL
This section will discuss dispersal from the point of view of the spore; from the
source (lesion, pustule, fruiting body) to the new host (or loss to the ground
or non-host surface). It is important to appreciate these 'primary' physical
mechanisms of spore dispersal as a foundation for understanding the spread of
disease epidemics.
6.2.1 Dispersal by wind
Winds are highly variable in both time and space (McCartney and Fitt, 1985; Aylor,
1990). This variability or turbulence causes individual spores, released from the
same source under the same wind conditions, to follow different paths and travel
different distances. Therefore, as spore plumes disperse downwind from sources
their concentrations in the air decrease. The decreases in concentration are
frequently referred to as 'concentration gradients' (Gregory, 1973). Mean wind
speed characteristics above crops are fairly well understood; wind speeds increase
with height depending on the nature of the crop (height, architecture, density) and
the stability of the atmosphere (temperature profile) (McCartney and Fitt, 1985). For
example, in neutrally stratified atmospheres when buoyancy effects can be
neglected, over open terrain with uniform vegetation, wind speed u(z) increases
logarithmically with height z (Grace, 1977; Monteith and Unsworth, 1990):
Search WWH ::




Custom Search