Second consequence a good source of resistance is not always available

The second consequence is that much resistance breeding has never been attempted at all because it has not always been possible to find single-gene resistances. There are no gene-for-gene relationships in continuous wild plant pathosystems, in which there is no break in the parasitism (see 4.11). Such pathosystems are controlled solely by their horizontal subsystems. Furthermore, not all discontinuous host-parasite associations have a gene-for-gene relationship. Consequently, single-gene resistances cannot always be found, and breeding for vertical resistance is then impossible. In the past, this has usually meant that no resistance breeding of any description would be attempted.

Continuous pathosystems are common in the tropics and it has been in these less-developed regions of the world that resistance breeding has been most neglected, on the spurious grounds that a source of resistance could not be found. Similarly, vertical resistances are rare among the insect parasites of crops, and it is no accident that there has been so little breeding for resistance to the insect pests of our crops. It is common knowledge that the most frequently used crop protection chemicals are the insecticides.

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