Plant host resistance

In agriculture, horizontal resistance is stable host resistance, and vertical resistance is unstable resistance. The importance of this distinction does not need to be stressed.

Vertical resistance is unstable only under agricultural conditions, because it is employed on a basis of host population uniformity. However, in a wild pathosystem, where it operates as a system of locking, probably as the n/2 model (see 4.15), vertical resistance provides a stable protection at the systems level of the pathosystem.

There are three levels of suboptimisation involved the agricultural use of vertical resistance. First is the attempt to control a crop parasite with only one subsystem, the vertical subsystem. Second is the use of a single vertical resistance throughout the entire population of a cultivar. Third is the tendency to use a single vertical resistance, conferred by a single gene, because the wild hosts had their many, single-gene vertical resistances broken up during the breeding process.

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