Spatial discontinuity

The vertical subsystem operates as a system of locking and this means that every host individual within a defined area must have a biochemical lock that is different from that of every other host individual. These differences constitute spatial discontinuity (i.e., genetic diversity within a population). The n/2 model (see 4.15) also reveals the need for spatial discontinuity. Indeed, every n/2 pathotype and pathodeme must occur with both an equal frequency, and a random distribution, if this model is to function.

In a discontinuous wild pathosystem, allo-infection from one host individual to another is usually non-matching infection, because of the system of locking of the vertical subsystem. In most crop pathosystems there is no spatial discontinuity. Host population uniformity means that every allo-infection within a cultivar is a matching infection. It is the equivalent of an autoinfection. This provides at least a partial explanation of why a vertical resistance can fail so dramatically in a crop pathosystem, but not in a wild plant pathosystems. (A further reason is the major difference in the levels of horizontal resistance).

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