The differential interaction of the vertical subsystem and, more particularly, the Person/Habgood differential interaction, may be described as variable ranking (Figs. 4.1 & 4.4). Variable ranking that is not a Person/Habgood differential interaction can also occur in host-parasite associations (see 4.6). One of the definitive characteristics of the horizontal subsystem is that there is no variable ranking. There is a constant ranking (Fig. 4.2).
This constant ranking is always a relative measurement. One cultivar is either more resistant or less resistant than another, and one horizontal pathotype has more or less parasitic ability than another. The ranking remains constant between seasons, and between localities. That is, the ranking is maintained regardless of the level of parasitism, which can vary between seasons, and localities. Pope (1986) has made the useful distinction between the theoretically ideal constant ranking and a practical constant ranking, which can be flawed by experimental error. He argues that minor discrepancies in the constant ranking do not necessarily disprove a horizontal subsystem.
Demonstration of a constant ranking is a useful method of confirming the horizontal nature of the resistance in a new cultivar. Indeed, this provides the best commercial description of the level of horizontal resistance. The resistance to each locally important parasite can be described as being equal to, superior to, or inferior to the resistance of well-known cultivars.
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