A population of a parasite in which all individuals have the same horizontal parasitic ability. The various members of a horizontal pathotype may differ in other respects, such as pesticide resistance. Horizontal resistance
Resistance that does not result from a gene-for-gene relationship. Horizontal resistance is the resistance of the Biometricians; its inheritance is normally controlled by polygenes. It results from many different resistance mechanisms; it is quantitative in both its inheritance and its effects; it controls all the consequences of a matching infection including Auto-infection; it also controls allo-infections in a continuous pathosystem that lacks a gene-for-gene relationship; and it is durable resistance.
The level of horizontal resistance can be at any degree of difference between the minimum and the maximum. The minimum level of horizontal resistance usually means that there is a total loss of crop in the absence of crop protection chemicals. Conversely, the maximum level of horizontal resistance usually means that there is a negligible loss of crop in the absence of crop protection chemicals.
In plants that lack a gene-for-gene relationship, horizontal resistance is the sole protection, and the only resistance. In plants that have a gene-for-gene relationship, the function of horizontal resistance is to control all the consequences of a matching allo-infection, including all Auto-infection. Horizontal resistance thus occurs in every plant against every parasite of that plant.
Horizontal resistance requires population breeding and recurrent mass selection . For this reason, Mendelian breeders do not like it, and it has been seriously neglected during the twentieth century. However, these population breeding techniques are so easy to use in most crops that amateur breeders can undertake the breeding for horizontal resistance, usually when organised into a plant breeding clubs, or a university breeding club.
Once adequate horizontal resistance is accumulated, the environmental and human hazards, as well as the labour and costs of applying crop protection chemicals are eliminated. Because a good horizontally resistant cultivar need never be replaced, except with a better cultivar, breeding for horizontal resistance is cumulative and progressive. As horizontal resistance is accumulated, the crop losses from pests and diseases decline, and the biological anarchy that was induced by the use of crop protection chemicals also declines, as biological control agents return and increase in numbers. The improved biological control enhances the effects of the horizontal resistance. The two phenomena are mutually reinforcing.
See also: Horizontal parasitic ability, vertical resistance, Comprehensive horizontal resistance, Laboratory measurements, Relative measurements, Partial resistance, Field resistance, Race-non-specific resistance. Horizontal resistance, comprehensive
The horizontal resistance to one species of parasite does not normally function against any other species of parasite. Comprehensive horizontal resistance means that a cultivar has high levels of horizontal resistance to all the locally important species of parasite. This is achieved during breeding by selecting for the one character of 'good health' (i.e., the holistic approach).
Because the epidemiological competence of parasites varies considerably between agro-ecosystems, the horizontal resistances that are comprehensive in one agro-ecosystem may be too high, or too low, in another agro-ecosystem. This is why On-site selection is important. Horizontal resistance, examples
There are numerous examples of deliberate and successful breeding for horizontal resistance. The best summary is by N.W. Simmonds, entitled Genetics of Horizontal Resistance to Diseases in Crops, and published in Biol. Rev. 66: 189-241. See also: Phaseolus vulgaris.
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