Variable levels of horizontal resistance

In discontinuous pathosystems that have both a vertical subsystem and a horizontal subsystem, the level of horizontal resistance often changes during the course of the epidemic. The level is low in the young seedlings, but it increases with increasing age of the host. For this reason, the breeders of small grain cereals have often called horizontal resistance 'adult plant resistance'. This is because their traditional breeding method involved the testing of young seedlings, in which the effects of vertical resistance are prominent. If cereal breeders want to measure the level of horizontal resistance, they should use mature (i.e., adult) plants.

This phenomenon means that, in the wild pathosystem, the level of horizontal resistance increases as the intensity of the parasitism increases, and it reaches its maximum at the climax of the epidemic. This emphasises the different roles of the two kinds of resistance. When the first matching allo-infections occur, at the start of the epidemic, with the n/2 model, the frequency of parasitism is low, but the injury from parasitism is high. The high injury results from two factors. The first is the fact of being matched so early in the epidemic. This ensures the maximum period of parasitism. The second is the low level of horizontal resistance in the young seedlings.

Similarly, the end of the epidemic is characterised by a high frequency of parasitism, but a low injury from parasitism. The high frequency results from the repeated cycles of allo-infection, with an increased amount of allo-infection in each cycle, and an increased rate of matching in each cycle. The decreased injury from the parasitism results from two factors. The first is the fact of being matched so late in the epidemic. This ensures a short period of parasitism. The second is the high level of horizontal resistance in the adult plant.

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