The horizontal subsystem has two somewhat different functions, depending on whether or not a vertical subsystem is present. If no vertical subsystem is present, all aspects of the parasitism must be controlled by the horizontal subsystem. That is, both allo-infection and auto-infection, as well as both frequency and injury, must be controlled by horizontal resistance. If a vertical subsystem is present, however, the function of the horizontal subsystem becomes mainly the control of auto-infection, and the reduction of injury.
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