Quantitative Epidemiological Competence

The horizontal subsystem has two quantitative variables which eclipse all others in importance. These are the level of horizontal resistance in the host, and the level of epidemiological competence in the parasite.

The horizontal resistance is a quantitative variable that is genetically controlled by polygenes. In a genetically flexible population the level of horizontal resistance responds to selection pressures exerted by the parasite. That is, if the epidemiological competence of the parasite increases, the horizontal resistance will increase accordingly, and vice versa. But note that these selection pressures cannot operate in a genetically uniform crop because it lacks genetic flexibility (see 1.15).

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