Evergreen trees and shrubs have persistent leaves and continuous pathosystems and, consequently, a gene-for-gene relationship and vertical resistance will not evolve in them. A gene-for-gene relationship can evolve only in a discontinuous pathosystem. See also: Deciduous. Evolution

The results of natural selection, often described as the survival of the fittest. Macro-evolution (or Darwinian evolution) occurs during periods of geological time, and involves genetic changes that are both new, and irreversible. New species are formed by macro-evolution. Macro-evolution also produces an increase in complexity, and new genetic code. micro-evolution occurs during periods of historical time, and it involves genetic changes that are not new, and that are reversible. It does not increase complexity, but merely re-organises existing complexity. Nor does it produce new genetic code; it merely rearranges existing code. The formation of ecotypes is microevolution by natural selection, and the production of cultivar or agro-ecotypes, by plant breeding is micro-evolution by artificial selection.

The mechanism of evolution has long been disputed and is now thought to be the result of natural selection operating on emergents at all systems levels. Examples of horizontal resistance See: horizontal resistance, examples. Exobasidium vexans

The fungus that causes blister blight of tea. There is great scope for selection for horizontal resistance within existing crops grown from true seed, as these crops constitute a vast hybrid swarm. Exoskeleton

The hard external surface of all arthropods, including the insects. Because the exoskeleton cannot expand or grow, it must be shed or moulted at several stages during the growth of the individual arthropod. See also: Instar. Extension service

The service that provides technical and specialised information to farmers. In the USA, the extension officers are known as county agents.

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