The designation process

Designation is a critically important aspect of a horizontal resistance breeding program in crops which have vertical resistances. Negligence at this step can easily ruin the entire program. There are six steps in the designation process, as follows:

List all the locally important parasites that occur in the breeding site, and identify each one that has a gene-for-gene relationship. In most breeding programs, there will be only two or three such parasites. (Note that a crop species that is derived from a continuous wild pathosystem will have no gene-for-gene relationships, (but see taro blight at 4.12).

For each species of parasite with a gene-for-gene relationship, choose a once-popular cultivar in which the vertical resistance has broken down. A pure line or a clone of the cultivar, as the case may be, is chosen as the designated host. This designated host is continuously maintained in the form of succeeding, over-lapping generations, for the entire duration of the breeding program.

Choose only one vertical pathotype of each species of parasite in which a gene-for-gene relationship occurs. Each vertical pathotype must be chosen because it matches the designated host. It then becomes the designated vertical pathotype. Each designated pathotype will be cultured on its designated host for the entire duration of the breeding program.

Each designated pathotype is inoculated on to a range of cultivars, which have been chosen as potential parents in the breeding program. Only those cultivars that are susceptible to the designated pathotype of every parasite with a gene-for-gene relationship may be used as parents. Cultivars which are resistant to even one designated pathotype have a functioning vertical resistance and, for this reason, cannot be used as parents.

The aim is to identify some 10-20 good cultivars that are susceptible to each of the designated pathotypes. These cultivars become the original parents of the screening population.

A small seed stock of each of these original parents should be maintained for the duration of the breeding program. These stocks will be required for testing purposes, if a designated pathotype is lost, and must be replaced.

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