Horizontal Resistance
Horizontal resistance is stable resistance. It is durable resistance. It will never break down to new strains of the parasite. Justification for this contention will be found in Return to Resistance (available as a free download at www.sharebooks.ca).
The inheritance of horizontal resistance is controlled by many genes of small effect. These genes are called polygenes. The inheritance of horizontal resistance is thus quantitative. That is, it can be inherited with any degree of difference between a maximum and a minimum.
The effects of horizontal resistance are also quantitative. That is, the resistance may be exhibited at any level between a maximum and a minimum. In the absence of crop protection chemicals, the minimum level of horizontal resistance normally results in a total loss of crop, while the maximum level results in a negligible loss of crop.
Horizontal resistance to one species of parasite does not normally confer resistance to other species of parasite. When breeding for horizontal resistance, it is necessary to accumulate high levels of resistance to each locally important species of parasite. The accumulation of all these resistances must be done more or less simultaneously. This is quite easy to do, and the way to do it is explained under recurrent mass selection.
The mechanisms of horizontal resistance are many, and they are mostly obscure. (For example, horizontal resistance to potato blight leads to a reduced rate of infection, a reduced rate of colonisation, a reduced sporulating zone, with fewer sporophores per unit area of sporulating zone, and fewer spores per sporophore). For the amateur breeder, these mechanisms are unimportant. All that matters is that the level of resistance is increased, and the level of parasitism is reduced.
Because horizontal resistance is quantitative and durable, breeding for this kind of resistance is cumulative and progressive.
That is, a good horizontally resistant cultivar need never be replaced, except with a better cultivar. This progression of better and better cultivars can continue, with diminishing returns, until a final limit to improvement is reached.
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