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Horizontal (polygenic) and vertical (oligogenic) resistance against blight

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Horizontal resistance is polygenic, and acts against all races of blight. Vertical resistance, conferred by the R-genes, is oligogenic, and can be overcome by a change of race.

Horizontal resistance slows down the rate at which disease increases in the field. Vertical resistance reduces the initial amount of inoculum from which the epidemic begins. A slow rate of disease increase in the field greatly enhances the benefit from reducing the initial inoculum. Therefore, horizontal resistance greatly enhances vertical resistance: horizontal resistance activates the R-genes. The R-genes have fallen into disrepute because they were used wrongly: they were used to replace horizontal resistance, whereas they should be used only to supplement it.

This same conclusion is arrived at independently from the fact of stabilizing selection in favor of simple races of blight on potato varieties without R-genes; and all the evidence indicates that R-genes have a useful future in the potato industry only if breeders are prepared to couple them with high horizontal resistance.


La resistencia horizontal es poligénica y actúa contra todas las razas de tizón. La resistencia vertical conferida por los genes-R, es poligogénica y puede ser superada por un cambio de raza.

La resistencia horizontal disminuye el grado en el cual la enfermedad aumenta en el campo. La resistencia vertical reduce la cantidad inicial de inóculum, en la cual empieza la epidemia. Un lento grado de aumento de la enfermedad en el campo, activa grandemente el beneficio de la reducción del inóculum inicial. Por lo tanto, la resistencia horizontal activa grandemente la resistencia vertical: la resistencia horizontal activa los genes-R. Los genes-R han caído en descrédito, porque fueron usados equivocadamente: Se usaron para sustituir la resistencia horizontal, siendo que debieron haber sido usados sólo para suplementarla.

Esta misma conclusión ha sido derivada independientemente del hecho de la selección estabilizada, en favor de las razas simples de tizón, en variedades de papa sin genes-R; y toda la evidencia indica que los genes-R tienen un futuro útil en la industria de la papa, solamente si los genetistas están preparados para unirlas con la alta resistencia horizontal.

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Van Der Plank, J.E. Horizontal (polygenic) and vertical (oligogenic) resistance against blight. American Potato Journal 43, 43–52 (1966).

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