Agriculture Reference
In-Depth Information
infections or over-seasoning stages within the crop and emigration and immigration
can be ignored. How large an area this is depends on how the pathogen propagules
move around. Fungal spores moved by rain splash (see also Chapter 16) move only
a few metres (Madden, 1992), so a field 50 m on each side would be a reasonably
self-contained population. It is the minimum dimension of the area that matters in
this context. A hectare of ground consisting of a 1 m wide strip 10 km long is
unlikely to be a closed system. On the other hand, for a virus moved by whitefly,
such as African Cassava Mosaic geminivirus (Colvin
et al.
, 2004; see also Chapter
20) or a wind-blown pathogen such as powdery mildew of cereals, an important, if
small, fraction of infections may occur hundreds, thousands or tens of thousands of
metres from the source plant (Brown and Hovmøller, 2002). In such cases, areas
measuring many kilometres in both dimensions are necessary in order to have an
isolated system. For many purposes, this is not appropriate and immigration
(especially) needs to be considered explicitly.
7.5 DENSITY-DEPENDENT AND DENSITY INDEPENDENT FACTORS
The factors controlling the growth rate of a pathogen population may be completely
uninfluenced by the population: temperature, rainfall, or developmental changes in
the susceptibility of hosts are good examples. These factors are as likely to increase
a population when it is small as when it is large. If controlled entirely by such
factors, a pathogen population would change by a random walk, with no predictable
long-term trend and the certainty of eventual extinction when a long run of bad
seasons occurred, or when it exterminated its host. However, there are also many
factors that influence population growth rates in ways which tend to
regulate
the
population, that is, to return it to some central value. These operate over both short
and long time-scales. Over short time-scales, rare pathogens will not trigger
secondary defences in their hosts, will not support large populations of
hyperparasites or mycoviruses and will not compete with each other for infection
sites or host tissue; common pathogens will do all three. Over longer time-scales,
rare pathogens will not cause rapid evolution of resistance in their hosts and are not
liable to reduce their host population density, while common pathogens will select
strongly for resistance and may tend to reduce their host population, hence
intensifying competition within the pathogen population. Examples of all types of
factors will be given in subsequent sections.
7.6 SHORT-TERM CHANGE IN A STATIC HOST POPULATION
As discussed in section 7.3, the notion of short-term change encompasses a wide
range of actual time-scales. A lettuce crop may be in the ground for only 12 weeks;
a tree crop 150 years. Whether the host population can be considered constant over
this time-scale depends on the nature of the pathogen: for a systemic virus, a single
lettuce crop may represent a fixed population but for a pathogen whose host was leaf
or root tissue, there would be considerable host turnover during the season.
