Breeding plants for horizontal (i.e., many-gene) resistance is very easy, while breeding for vertical (i.e., single-gene) resistance is highly technical, very difficult, and very expensive.

Vertical resistance requires a ‘good source’ of resistance, which does not always occur. But it is possible to breed for horizontal resistance to any pest or disease, using only susceptible parents.

Horizontal resistance is durable resistance. It never breaks down to new strains of the pest or disease, as does vertical resistance. This means that the breeding is cumulative. A good cultivar need never be replaced, except with a better cultivar.

Horizontal resistance is a quantitative variable, and it exhibits every degree of difference between a minimum and a maximum. In the absence of crop protection chemicals, the minimum level of horizontal resistance usually leads to a complete loss of crop, while the maximum level of horizontal resistance leads to a negligible loss of crop. It is easy to breed for the maximum level.

Horizontal resistance can be accumulated for every locally important crop parasite. We can thus produce resistance that is durable, complete, and comprehensive. The need for insecticides and fungicides then disappears.

Most heirloom varieties have fairly high levels of horizontal resistance. But most modern varieties have rather low levels because horizontal resistance tends to be lost during breeding for vertical resistance, which has been the resistance of choice among professional breeders for the past century. This is because of the vertifolia effect.