No source of resistance required

The requirements of breeding for vertical resistance are so deeply ingrained that many people find them difficult to abandon. Of these, the most persistent is the idea that a good source of resistance must be found before the breeding can begin. This is absolutely true of vertical resistance. It is absolutely untrue of horizontal resistance. Provided that the original parents embrace a reasonably wide genetic base, they can all be as susceptible as the maizes of tropical Africa were to tropical rust. While it does no harm to use parents with good levels of resistance, this is not a requirement.

A useful comparison can be made with subsistence cultivars. Subsistence farmers have cultivated their crops without crop protection chemicals, usually for centuries, if not millennia. Their cultivars have high levels of horizontal resistance to all locally important parasites, but the yield and quality is low when compared with commercial cultivars. Commercial farmers are in the opposite situation. Their cultivars have high yields and quality, but they generally have low levels of horizontal resistance.

The question then arises: Is it easier to increase the yield and quality of subsistence cultivars, while retaining their resistance; or is it easier to raise the resistance of commercial cultivars, while retaining their high yields and quality? There seems to be little doubt that it is easier to increase resistance than to increase yield and quality. Therefore, it is normally preferable to start with high-yielding, high quality commercial cultivars as parents. The high yields and high qualities must be retained during the recurrent mass selection that increases horizontal resistance.

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