Host erosion

Host erosion is due to genetic changes in the host. In cultivars, these changes normally occur only during the breeding process. They cannot occur during the cultivation process because the host is genetically uniform, and it lacks genetic flexibility (see 1.15). The host erosion of horizontal resistance during breeding for vertical resistance is called the 'vertifolia effect' (see 6.6.1).

In crops that are both open-pollinated and seed-propagated, such as maize in tropical Africa (see 7.2), the host erosion can occur during the cultivation process. This is why this maize was so susceptible to the re-encounter Puccinia polysora. In such a genetically flexible population, horizontal resistance will normally erode or accumulate to the level needed to match the epidemiological competence of the parasites in that ecosystem.

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