Even though horizontal resistance will not break down like vertical resistance, it can be eroded quantitatively. This is an alarming thought for anyone who values the idea that horizontal resistance is durable resistance. However, this erosion is easily avoided, and it is unlikely to be serious if it does occur, but it is still important to understand it.

There are four kinds of erosion of horizontal resistance:

Host erosion

A host erosion results from genetic changes in the host population. This can occur during the cultivation of a genetically flexible crop grown in the absence of the parasite, such as maize which is open-pollinated. But it does not occur during the cultivation of a genetically inflexible crop, such as a clone or pure line, even when the parasite is absent.

A host erosion can also occur during the breeding of any crop in the absence of a parasite, particularly if the screening population is protected by a functioning vertical resistance or by a pesticide. It is then known as the vertifolia effect.

Parasite erosion

A parasite erosion results from genetic changes in the parasite population. This is important only occasionally, and only with facultative parasites. For example, a soil-borne Fusarium or Verticillium wilt fungus might have a low parasitic ability. But that parasitic ability could increase if a susceptible host were grown repeatedly on the same land.

A parasite erosion was seen repeatedly in North America as settlers moved west and cultivated flax on virgin land. The native Fusarium wilt would gradually increase its parasitic ability until flax cultivation became impossible, and it moved west to new virgin land with new settlers. It was said that the linseed oil factories had a very high rate of being insured, and then burning to the ground, as flax cultivation moved west. Eventually, the flax accumulated so much horizontal resistance that this problem disappeared.

Environmental erosion

An environmental erosion results when a cultivar is taken from an area of low epidemiological competence, and is cultivated in an area of high epidemiological competence.

For example, bacterial wilt of potatoes lacks epidemiological competence entirely in temperate areas, and potatoes that have been bred in a temperate area are likely to be very susceptible to it. This lack of horizontal resistance becomes obvious only when those potatoes are grown in a tropical or subtropical country where the wilt has a high epidemiological competence.

False erosion

A false erosion results from sloppy experimental work, when a cultivar is thought to be resistant and is later found to be susceptible. It is very tempting to blame nature rather than oneself for this kind of error.

A false erosion can also occur over a long period of breeding when a cultivar used as a standard of susceptibility appears to become increasingly susceptible. This is an illusion resulting from the fact that all the other plants in the breeding program are becoming increasingly resistant, and the discrepancy between susceptible and resistant slowly widens.