Problems with comprehensive resistance

The ideal cultivar has an adequate level of resistance to all the locally important parasites. It will transpire (see 7.2.13) that this is easier to achieve with horizontal resistance, than with vertical resistance.

Most crop species have dozens of pests, and dozens of diseases. Unfortunately, it is difficult to breed for vertical resistance to more than one species of parasite at a time. The basic idea of pedigree breeding is to produce one cultivar with vertical resistance to one species of parasite, a second cultivar with vertical resistance to a second species of parasite, and so on. This results in a series of cultivars, each with one vertical resistance to a different species of parasite. Using gene-transfer methods, these vertical resistances are then all combined in a single cultivar, a 'super-cultivar' with resistance to everything. At least, that is the idea. And it is a neat idea. Unfortunately, it is almost impossible to achieve in practice. The sheer volume of breeding work is so exorbitant that one or more vertical resistances are likely to be matched before the breeding is completed. Furthermore, such a super-cultivar is like a chain, in that it is only as strong as its weakest link. And, like the chain, the super-cultivar would be ruined with the failure of only one weak link, one short-lived vertical resistance.

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