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

Abstract

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.