The very rapid population growth that can occur with an r-strategists species during a favourable season. Many crop parasites are r-strategists, and it is their population explosions that are can be so alarming, and so difficult to control. The function of the gene-for-gene relationship and the vertical subsystem in a wild plant pathosystem is to control the population explosion of a parasite, but it can do this only if it functions as a system of locking based on genetic diversity. Horizontal resistance can also reduce the rate of population growth of the parasite to the point where the epidemic can no longer develop, and this is called population immunity. See also: Population extinction. Population extinction
The death of most of the individuals of an r-strategists population that occurs at the end of a favourable season. With plant parasites, this happens typically in a discontinuous pathosystem, with the loss of host tissue that occurs with leaf-fall in a deciduous host species , or with the death of all plant parts, except the seeds, in an annual host species. With crop parasites, it often occurs with harvest, such as the digging of potatoes, or the combine harvesting of cereals. See also: Population explosion. Population growth
Unlike an individual, a population can have growth that is positive, static, or negative. Positive population growth occurs when each individual, on average, spawns more than one progeny. Static (or zero) growth occurs when each individual, on average, spawns exactly one progeny. Negative growth occurs when each individual, on average, spawns less than one progeny. See also: Population immunity. Population immunity
A host population that is less than immune, but which does not suffer an epidemic. Each host individual may be carrying the parasite, but the level of horizontal resistance is such that the population growth of the parasite is zero or negative. Populus spp.
Poplar trees, used in plantation forests to produce hardwoods. Not receommended for amateur breeders . Positive feedback See: Feedback.
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