The Apparent Conflict between Resistance and both Yield and Quality

There is a popular belief that there is an inversely proportional relationship between the resistance to parasites and either the yield or the quality of crop product. Indeed, the argument that this is necessarily so is the one most often quoted as a reason for not investigating horizontal resistance. However, it seems the only evidence to support this belief is the fact that wild plants normally have high resistance but low yield and quality, while cultivated plants normally have low resistance but high yield and quality. There are a number of refutations.

The first refutation is the general principle that correlation is not proof. When the correlation is imperfect, the contention is even less convincing. There are plenty of examples of cultivars with both good yields and quality as well as good resistances. Some wild plants have qualities that exceed those of cultivars. The taste of wild strawberries, for example, is far superior to that of the high-yielding, large-fruited, cultivated varieties. Secondly, some cultivars have both high quality and high resistance. The classic wine grapes, for example, have qualities that cannot be surpassed and they had high levels of resistance to all their old-encounter parasites. It was not until the introduction of new-encounter parasites from America to Europe, in the nineteenth century, that grape parasites became seriously damaging. Indeed, the classic wine grapes were cultivated for centuries without any need for crop protection chemicals. The same can be said for other antique clones, such as figs, dates, garlic, and olives (see 7.20.1).

A third argument comes from average yields. The world average yield of wheat, for example, is 1.5 tonnes/hectare. The average in North America is 2.2 t/ha. In Western Europe, it 4.5 -5.5 t/ha, but some farmers routinely obtain 10.0 t/ha, though the use of crop protection chemicals. The experimental maximum (but uneconomic) yield is 15.0 t/ha., which is ten times the world average. No one knows what the ultimate potential yield of wheat may be, but it is possibly in the region of 20 t/ha. Many of the factors contributing to the low world average cannot be controlled. These uncontrollable factors include poor soils, insufficient fertilsers, inadequate rainfall, and other environmental variables. But one of the more important of the factors reducing yield is the damage caused by parasites. If this damage were to be controlled by horizontal resistance, the resistance would be contributing to yield, rather than conflicting with it.

The origins of this false belief concerning a conflict between resistance and yield probably lie in the difficulty of breeding simultaneously for yield, quality, and resistance, while working with vertical resistance. These difficulties disappear with the simultaneous screening for all desirable characteristics during recurrent mass selection. Clearly, there is a ceiling to the combined characteristics of high yield, high quality of crop product, and high levels of horizontal resistance to all locally important parasites. But this ultimate level can be determined only by the actual experience of breeding for horizontal resistance. It may well prove to be higher than anyone suspects.

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