Agriculture Reference
In-Depth Information
CHAPTER 7
PATHOGEN POPULATION DYNAMICS
M.W. SHAW
7.1 INTRODUCTION
Population dynamics is the study of how the size and structure of populations
respond to the forces that act on them. This chapter begins with attempts to clarify
concepts of time-scale, of population structure, of how populations can be measured
and of regulation of a population. The chapter continues by considering how
pathogen populations change in fixed host populations over short time-scales, then
how populations change when the host population changes on a time-scale
comparable with the pathogen and, finally, how populations change over many
generations of both host and pathogen. The argument uses both reasoning about
drastically simplified settings and generalisations from experimental data.
7.2 THE MEASUREMENT OF POPULATIONS
Before it is possible to study changes in populations, it is first necessary to decide
what the individuals are that are being counted. A population is composed of
individuals at various stages in their life-cycles. It seems sensible to start by
discussing a relatively easy example,
Blumeria graminis
(cause of cereal powdery
mildew) on a cereal crop. Three components of the population can be immediately
distinguished. First, there is a population of suspended or recently deposited and
ungerminated conidia, each of which is potentially capable of infection. Second,
there is a population of physiologically independent mycelia growing on leaf
surfaces. Some of these are too young to sporulate, others are sporulating, still others
have exhausted the food available to them and have ceased to sporulate. Finally,
there may be a population of cleistothecia. A measure of the population will have to
include several numbers, describing the number of each type of individual per
square metre (or per square metre of host). The conidia could be measured by
various techniques (Gregory, 1973; Kennedy
et al.
, 2000) and the sporulating
mycelia counted visually. More sophisticated methods using staining and
microscopy would be necessary to count the immature mycelia directly. There is
still the problem that a count of visually distinct colonies is not necessarily a count
of physiologically distinct pathogen individuals; one colony may contain several
individuals. A final problem would be sampling cleistothecia, since dead host tissue
will be difficult to survey adequately. An independent categorization could divide
193
