Demonstration of the Horizontal Nature of Resistance

The most valid demonstration of the horizontal nature of resistance is to prove its polygenic inheritance. This can usually be assumed from the breeding method, and the breeding history, particularly if recurrent mass selection and the one-pathotype technique (see 7.5) were used. Alternatively, an experimental cross can be made between the test line and a susceptible host, and a progeny of about one hundred individuals is tested for susceptibility. If the inheritance is quantitative, there will be a normal distribution of resistance.

Another demonstration involves constant ranking, over both time and space. However, it should be noted that minor changes in ranking can occur without necessarily invalidating the demonstration (see 6.1). It should also be noted that a variable ranking is not necessarily a demonstration of vertical resistance, for which a Person/Habgood differential interaction is required (see 4.7.2).

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