Approximately synonymous with habitat, the environment can be defined as all the external conditions that affect the survival and growth of an organism. Enzyme

An organic catalyst, which can both promote and control a specific biochemical reaction. Ephemeral

Short-lived, temporary. Epidemic

Parasitism at the systems level of the population. An epidemic may be continuous or discontinuous, and this determines the relative importance of the two kinds of resistance, and the two kinds of infection. A continuous epidemic is sometimes called an endemic but this usage is best avoided. See also: Epiphytotic, Epizootic. Epidemic cycle

An epidemic cycle occurs with a discontinuous epidemic, and it concerns the overall development of an individual epidemic, from the initial inoculum of the parasite to its population extinction. An epidemic cycle normally coincides with a growing season, such as a summer in temperate regions, or a rainy season in the tropics. However, the epidemic cycle of rubber in the Amazon Valley is defined by the deciduous nature of the rubber tree, whose leaf-fall is independent of season in this continuously warm and wet environment. Epidemiological competence

A parasite can cause an epidemic only if it has epidemiological competence in the area in question. The level of epidemiological competence can vary from one area to another, and from one season to another, and it is controlled mainly by climatic factors such as temperature and humidity. For example, the maize disease called 'tropical rust' (Puccinia polysora) lacks epidemiological competence outside the lowland tropics. Maize cultivar in Europe are highly susceptible to this disease, but they are not vulnerable to it, because of its inability to cause an epidemic in a temperate climate. The susceptibility of these European maize cultivars becomes apparent only if they are cultivated in the lowland tropics. Variation in epidemiological competence explains the need for On-site selection when breeding for horizontal resistance.

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