Population immunity

Population immunity is a term coined by Vanderplank (1968) to describe the fact that a plant population may be effectively immune to a crop parasite, even though the individuals in that population are less than immune. This effect also suggests that, when breeding plants for horizontal resistance, we probably need considerably less resistance than we may think.

Population immunity is a consequence of population growth. Unlike an individual's growth, a population's growth can be positive or negative. If there are more births than deaths, the population size is increasing, and its growth is described as positive. If the births and deaths cancel each other out exactly, the population size is unchanging, and its population growth is zero. And if there are more deaths than births, the population size is decreasing, and its population growth is negative.

If a parasite population growth is positive, this means that, on average, each parasite individual spawns more than one new individual. In the case of an r-strategist parasite, each individual may spawn very many new individuals, in a very short time, and the positive population growth is then so rapid that it becomes a population explosion (see 1.7).

Now suppose that the crop in question has a level of horizontal resistance such that it severely restricts the reproductive rate of the parasite. On average, each parasite individual spawns only one new individual before it dies. The parasite population growth is then zero. Finally, suppose a slightly higher level of horizontal resistance. On average, each parasite individual now spawns less than one new individual (i.e., most individuals spawn one new individual, while a few spawn none). The parasite population is now decreasing. Its population growth is negative.

An epidemic can develop only when the parasite population growth is positive. And a damaging epidemic can develop only when the population growth is strongly positive. If the parasite population growth is zero or negative, there is no epidemic, and the host population is effectively immune, even though the individuals in it are less than immune. This is population immunity.

One of the dangers of measuring horizontal resistance in the laboratory is that population immunity cannot easily be taken into account. A level of horizontal resistance that looks like susceptibility in the laboratory may prove to be population immunity in farmers' fields. For this reason, laboratory measurements of horizontal resistance should be relative measurements. That is, the level of resistance should be described as being either higher or lower than that of other cultivars of known field performance.

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