Agriculture Reference
In-Depth Information
Foliar pathogens and (non-waterborne) root pathogens have very different
mobility. Splash-borne or airborne pathogens move on scales of metres or much
more, as explained in the previous paragraph. Although infections are most likely on
the immediate neighbours of an infected host, pathogen propagules may reach some
distance and the approximation of random contact between healthy and infected
individuals is useful, although it will overestimate the rate of disease progress.
In a soilborne disease, however, propagule movement may be very limited and
infection rates will be very rapidly limited because most inoculum will reinfect an
already infected host (this is an extreme form of the phenomenon discussed above).
Thus, within a season, soil-borne disease typically has lower rates of increase than
aerially dispersed disease. Furthermore, the movement is usually more by the host
than the pathogen, putting a premium on the ability of the pathogen propagules to
survive until a host root comes sufficiently close. The total population size of a
pathogen like this will evidently have much smaller annual fluctuations than that of
a typical foliar pathogen.
Because of the limited movement of soilborne pathogens, it is quite common for
an area of study to include many effectively separate populations. In this case, it
may be possible to model some effects of the averaging over populations by
specifying that infection rates in the model should scale as some power of the
population densities of host and pathogen. For example, if this power is more than 1
for the host population, then the infection rate of hosts by the pathogen is more
efficient at high density than would be predicted from the performance at low
density. An example of the application of this model is an analysis of data on the
lettuce pathogen
Sclerotinia minor
, attacked by the hyperparasite
Sporodesmium
sclerotivorum
(Adams and Fravel, 1990). Gubbins and Gilligan (1997) showed that
it was necessary to include an assumption of heterogeneity, in this implicit form, in
order to adequately fit these data. Without the assumption, the hyperparasite had to
be assumed to be so efficient in order to fit the overall data, that sclerotia of the
pathogen would have been eliminated from the soil; in fact it survived for a long
time at a low density.
Soil-inhabiting pathogens are notoriously patchy on very small scales. This is in
part a function of the limited opportunity for smoothing out fluctuations in population
density because of the difficulty of moving through soil and partly as a result of
heterogeneity in the soil itself. However, it is also possible that variability may be
generated by population processes within the soil itself. Kleczkowski
et al.
(1996), in
experiments with damping-off of radish (
Raphanus)
seedlings by
Rhizoctonia solani
in
the presence of the antagonistic and non-pathogenic fungus
Trichoderma viride
,
showed how the development of host resistance amplified very small initial variations
in rates of attack and parasitism into very large final variations in pathogen population
density. In the field, such patchiness could then persist for long periods, influencing
both the vegetation and the location of pathogen populations. The dynamics of soil-
borne pathogens are further discussed in Chapter 14.
If hosts exist in patches, it may be sensible to regard the patch as an individual in
a population on a larger scale, the metapopulation. Then the host patches can be
regarded as reproducing, becoming infected, dying and so forth. The metapopulation
can be studied as an entity in its own right (Hanski and Gaggiotti, 2004). Although
