Two kinds of resistance

The two breeding methods revealed two kinds of resistance to crop parasites, which Vanderplank (1963) called vertical resistance and horizontal resistance respectively. Vertical resistance is a component of the gene-for-gene relationship, and its inheritance is controlled by single genes. It has two very considerable advantages in agriculture, but several grave disadvantages (see 5.5) also. Its chief disadvantage is that it is unstable (see 10.6). It does not endure, because it operates against some strains of the parasite but not others. Consequently, it fails to function when a matching pathotype appears. Horizontal resistance is the resistance of the biometricians. Its inheritance is controlled by polygenes, and it is durable resistance.

Vertical resistance is characterised by being 'unstable, big space, high profile, short life, expensive, few cultivars' (see 5.6) while horizontal resistance is characterised by being 'stable, small space, low profile, long life, inexpensive, many cultivars' (see 6.5). Vertical resistance is suited to large breeding institutes, while horizontal resistance is suited to small breeding clubs (see 11.17).

Vertical resistance operates as a system of locking in a wild plant pathosystem (see 4.15), but it has been employed in the crop pathosystem on a basis of uniformity. This is why it is ephemeral resistance in agriculture. Horizontal resistance in a wild pathosystem is normally adequate to control all the consequences of a matching infection. This is because the system responds to selection pressures. In the crop pathosystem, horizontal resistance is usually inadequate, because it has been lost during the process of breeding for vertical resistance (see 6.6.1), or because of treating the screening population with crop protection chemicals. This loss of horizontal resistance is possibly the most important reason for our current dependence on crop protection chemicals.

Infection is defined as the contact made by one parasite individual with one host individual for the purposes of parasitism. Allo-infection is infection in which the parasite originates away from the host being infected. The parasite has to travel to its host. Allo-infection is analogous to cross-pollination (allogamy). Autoinfection is infection in which the parasite originates on, or in, the host being infected. The parasite has no need to travel. Autoinfection is analogous to self-pollination (autogamy).

The first infection of seasonal tissue (e.g., a seedling of an annual species, leaves of a deciduous perennial species) is normally an allo-infection, and this is where the single-gene, vertical resistance is valuable as a system of locking (see 4.14). Auto-infection is possible only after an allo-infection and, if there is a vertical subsystem, it is possible only after a matching allo-infection. This is where the many-gene horizontal resistance is valuable. The two kinds of infection define the primary functions of vertical resistance and horizontal resistance (see 9.1.6).

If there is a single-gene, vertical subsystem (i.e., a gene-forgene relationship), an infection is either a matching or a non-matching infection. A matching infection succeeds, while a non-matching infection fails. In a normally functioning vertical subsystem, an allo-infection is usually a non-matching infection, while an auto-infection is a matching infection. Auto-infection can thus be controlled only by horizontal resistance, and vertical resistance can control allo-infection only. However, horizontal resistance can also control allo-infection in pathosystems that lack a vertical subsystem.

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