Horizontal Resistance is Quantitative

Because it is normally polygenically controlled, horizontal resistance is quantitative in both its inheritance and its effects. For the purpose of practical definition, we may say that the minimum level of horizontal resistance will result in a complete loss of crop in the absence of crop protection chemicals. The maximum level of horizontal resistance will result in a negligible loss of crop in the absence of crop protection chemicals. (However, there may be exceptions to this rule). Every degree of difference between these two extremes of horizontal resistance can usually be observed. Many modern cultivars cannot be cultivated economically without crop protection chemicals, and this is an indication of their relatively low levels of horizontal resistance.

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