Two kinds of epidemic

Epidemics may be continuous or discontinuous (see 4.11). Single-gene resistances are suited to discontinuous epidemics, while many-gene resistances are suited to continuous epidemics.

Plant pathosystem continuity is governed by the host species. Annual hosts, the leaf tissue of deciduous trees, and the aerial parts of many perennial herbs, have discontinuous pathosystems. At regular intervals, during an adverse season such as a winter, or a tropical dry season, the parasitism stops completely because there is no host tissue available to the parasite. In order to survive, an obligate parasite must enter a dormant phase, find an alternative host, or migrate to another region where host tissue is available. Alternating parasites (see 8) are obliged to switch to their alternate host. A facultative parasite has the additional option of saprophytism.

Discontinuous pathosystems suffer epidemic parasitism. This kind of parasitism has two characteristics. Firstly, there is discontinuity. Every epidemic is separated from the previous epidemic, and the next, by a period in which there is no parasitism. Between epidemics, the host and parasite individuals are also separated from each other, and this imposes special difficulties on the parasite because, at the start of every epidemic, each parasite individual must find a host individual. In terms of the gene-forgene relationship, it must also find a matching host individual.

Secondly, the duration of a discontinuous epidemic is finite. In order to survive, the parasite must achieve a minimum reproduction within a defined period. For this reason, most discontinuous, or epidemic, pathosystems involve r-strategist parasites. These are 'quantity breeders' with a very cheap, often asexual reproduction, and they exhibit a sigmoid growth curve with a population explosion during the log phase. Typically, there is also a parasite population extinction when host tissue ceases to be available, and the parasite survives usually as a small population of a specialised or dormant form.

In complete contrast, a continuous pathosystem occurs with an evergreen, perennial species of host. Some perennial plants live for centuries, even millennia, and they provide continuously available host tissue for parasites. Although the parasite growth rate may fluctuate with seasons, the parasitism does not stop between seasons.

Continuous pathosystems suffer endemic parasitism. They usually involve K-strategist hosts. The size of the host population (i.e., its total biomass) is more or less constant and is governed by K, the carrying capacity of the environment. The size of the parasite population is governed by the size of the host population, and it too is more or less constant. (Note that the term endemic has two quite distinct meanings in biology. In epidemiology, the term 'endemic' means that a disease is continuously present, and it is the converse of 'epidemic' disease which is discontinuously present. In its ecological sense, the term 'endemic' is the converse of exotic, and it means that a species is indigenous. It is often taken to mean that a species is uniquely indigenous; that is, its natural distribution is limited to the area in question).

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