Agriculture Reference
In-Depth Information
even if it is assumed that the area visibly damaged by a disease is much smaller than
the area from which nutrients are withdrawn and direct competition with another
pathogen individual can occur. This suggests the operation of a density-dependent
factor early on. Perhaps the most obvious factor is that attack by a pathogen will
stimulate various types of acquired resistance: hence the success rate of fungal
spores on a new, juvenile host may be much greater than on a host already attacked
(successfully or unsuccessfully) by other spores. However, Fleming (1980) has
argued forcibly that the role of predation and hyperparasitism in reducing the growth
rate of pathogen populations might be much greater than commonly recognised.
Off-season survival may also be density-dependent because of natural enemies.
For example, sclerotia of
Sclerotinia
spp. are attacked by various hyperparasites,
including both fungi and viruses (Boland, 2004; Budge and Whipps, 2001; Burgess
and Hepworth, 1996). As
Sclerotinia
becomes commoner, so its hyperparasites will
become more common and overwinter survival of the sclerotia will decrease. This
means that natural regulation by such agents will tend to occur only when the
pathogen is common.
Such density-dependent processes can lead to equilibria in which both pathogen
and hyperparasite have an annual cycle in abundance, the pathogen peak populations
being lower than they would otherwise be. If population growth rates are very rapid,
the result could be massive fluctuations between years without simple repeating
patterns: chaotic fluctuations (Shaw, 1994b). Chance effects would be likely to lead
to extinction for the hyperparasite or for both hyperparasite and pathogen, unless the
pathogen is sufficiently widespread (or its dispersal sufficiently restricted) that
cycles in different parts of its range do not coincide.
An exception to the general case that hyperparasites cannot maintain pathogens
at low density has been described in
Salix
species grown for fuelwood by Morris and
Royle (1993). Two pathovars of
Melampsora epitaea
rust infect plantations in the
south-west of England; one infects leaves, the other stems (Pei and Ruiz, 2000). The
stem infecting form overwinters as cankers which are susceptible to the hyperparasite
Sphaerellopsis filum
and support a standing population of this hyperparasite. Where
this pathovar of rust is present, the rate of increase of the leaf-infecting form is much
reduced because the hyperparasite:host ratio is quite high even at the start of the
summer, when leaves appear. This results in much lower peak populations of rust
and there is obvious interest in attempting to manipulate this situation to provide
biological control of the rust. The complexity of the biology in this case is
substantial: an adequate model might assume that there was a fixed population of
trees but that the lifetime of leaves, the host of the leaf-inhabiting form of the rust, is
short. This means that a model would have to include, as distinct variables, the
population of leaves, both forms of healthy rust, and both forms of rust infected by
the hyperparasite.
7.8.4 Competition
Pathogens may compete simply by depleting the space or nutrients available in a
host (exploitative competition), by some more active interaction such as the
