Agriculture Reference
In-Depth Information
is only necessary to consider the host and pathogen population at the time of
flowering in each year. If the same host area is sown each year and conditions are
similar on average each year, the pathogen population will tend to change by a
constant factor each year. If only one individual existed in the first year, producing
R
0
individuals in the following year, we would expect exponential growth, with
R
0
2
in the following year and so on. Clearly, for the population to persist,
R
0
must be
more than 1. As stated earlier, an aim of management is to reduce
R
0
as much as
possible.
Oilseed rape is generally grown as part of a rotation. Infection will come from
old sclerotia within the same field (since some sclerotia survive for several years)
and from spores from more distant sources. The distant sources affect
R
0
in a rather
simple way since the ascospores are wind-blown and may be regarded as distributed
more or less at random over a wide area. If the crop area is doubled, twice as many
of these spores will land on susceptible tissue. The number of locally generated
spores depends on the time since the last susceptible crop was grown. Since this
depends on the particular rotation used on a farm, it may depend rather little on the
area of crop grown regionally.
This also provides a way to think about the long-term dynamics of diseases that
have a phase of rapid population growth during the crop season, like those discussed
in sections 7.6 and 7.7. In those sections, the population at the start of the season
was simply assumed to be a known quantity. However, the start of active host
growth is simply the low point of the pathogen population cycle, while the end of
that season is usually the high point. A pathogen population at a single time in the
year can therefore be thought of as being in balance between an increase during the
host's growing season and a decrease during the off-season. This is usually
combined with a change from one phase to another, for example from the conidial
stage to the sexual stage, or from mycelium to sclerotia. The chances that the
increase would balance the decrease exactly over millennia are very small but, if the
factor of decrease were greater than the factor of increase, the pathogen would
become extinct. In fact, this would happen if the decrease outweighed the increase
for more than a few years.
There are two possible ways to explain why pathogens do not become extinct.
In the first place, it is unlikely that conditions over large areas would be exactly
synchronised. Extinction would require simultaneous coincidences of poor
conditions over the whole of the pathogen range. The time-scale for this may be
so long that it very rarely occurs even over millions of years. An alternative
explanation is that the potential increase during the cropping season is on average
greater than the decrease in the off-season but that the population size must be
limited during the growing season by some factor or factors that are density-
dependent. The next two sub-sections discuss such factors.
7.8.3 Parasitism and predation
In very many cases, as already mentioned, it appears that the growth rate of an
epidemic within an annual crop slows long before crowding for space is plausible,
