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It must be clearly recognised that horizontal resistance can be eroded in a number of ways. But this quantitative loss of horizontal resistance is very different from the qualitative breakdown of vertical resistance, and it is important not to get the two of them confused. At first sight, the very thought of an erosion of horizontal resistance is upsetting, even alarming. Horizontal resistance is supposed to be durable, and to persist indefinitely, or at least during the foreseeable, agricultural future.

For people who understand horizontal resistance, and who are working with it, erosion is important only occasionally, and these occasions can usually be avoided, or they are easily controlled. However, for people who do not understand horizontal resistance, such as pedigree breeders working exclusively with vertical resistance, the mere possibility of an erosion of horizontal resistance often provides an adequate excuse to deride it, and to neglect it experimentally.

Host Erosion

The erosion of horizontal resistance that occurs most commonly is a result of genetic changes in the host population. This kind of erosion is called the host erosion of horizontal resistance, and it is the converse of the accumulation of horizontal resistance that occurs when there is positive selection pressure for it.

A host erosion can occur either during breeding, or during cultivation. Horizontal resistance can be lost during breeding because of the absence of a parasite. As we shall see in later chapters, such an absence can occur naturally (Chapter 20), or because of a functioning vertical resistance (Chapter 18), or because of protection with pesticides (Chapter 18), or during breeding of a crop with parasites that accumulate only slowly, such as the potato viruses (Chapter 18). In other words, horizontal resistance is eroded if there is no selection pressure for it during the breeding process. Most of the current susceptibilities in modern crops are the result of a host erosion during breeding.

A host erosion of horizontal resistance during cultivation can occur only if the crop is genetically flexible, as happened with the open-pollinated, subsistence maize crops in tropical Africa (Chapter 20). This kind of erosion occurs either when the parasite is absent from the agro-ecosystem in question, as with tropical rust of maize, or when the parasite has a very limited, patchy distribution, as with maize streak virus (Chapter 20). In practice, these days, very few commercial crops are genetically flexible during cultivation, although many subsistence crops are flexible. In those commercial crops that are genetically flexible during cultivation, such as alfalfa (Medicago sativa), the selection pressures for resistance must be continuously maintained in populations that are being used for seed production.

A host erosion of horizontal resistance can also occur in special circumstances. For example, there is a North American insect parasite of the roots of grapes, called Phylloxera (Chapter 20). In the 1860s, Phylloxera was found in France and the European wine industry was faced with total ruin. The problem was solved by grafting the very susceptible, classic wine grapes on to rootstocks of wild American grapes which have very high levels of horizontal resistance to Phylloxera. That resistance has now endured for more than a century in Europe.

In California, however, there is a different situation. Because the resistant rootstocks depress the yield of grapes somewhat, Californian vines are often grafted on to hybrid rootstocks. These hybrids are half wild American, and half European, and their use increases the yield of grapes. Unfortunately, they are also moderately susceptible to Phylloxera, and this pest has recently become a serious nuisance in some Californian vineyards that have these hybrid rootstocks. It is important not to misinterpret a situation such as this, and to attribute it to a breakdown of vertical resistance, or to a parasite erosion of horizontal resistance (see below).

Parasite Erosion

An erosion of horizontal resistance can occasionally occur as a result of population changes in the parasite. This is called the parasite erosion of horizontal resistance. It is an apparent erosion which, in fact, is not due to any change in the resistance itself. There is an increase in the level of parasitism, resulting from an increased parasitic ability in the parasite.

Most species of parasite have a strict limit to their parasitic ability and they cannot increase it beyond that limit, at least during the foreseeable agricultural future. (This argument follows logically from the fact that any parasite which endangers its host's ability to survive, also endangers its own survival). In practice, a parasite erosion of horizontal resistance is normally important only with a special category of parasite called a facultative parasite. This is a parasite that can change between the ability to extract nutrients from a living host, and the ability to extract nutrients from dead plant material. These two abilities are inversely proportional. That is, the greater the one, the less the other.

For example, there is a soil-inhabiting fungus called Fusarium oxysporum f.sp. lycopersici that causes a wilt disease of tomatoes. If tomatoes have not been grown in that soil for many years, the non-parasitic form of the fungus predominates. Under these circumstances, tomatoes can be grown with very little loss from wilt disease. However, if tomatoes continue to be grown in that soil, season after season, the parasitic ability of the fungus increases. This causes an increase in the frequency of wilt disease, and an apparent loss of resistance in the tomatoes.

A parasite that can obtain nutrients only from a living host is called an obligate parasite. There does not appear to be a single known example of a significant parasite erosion of horizontal resistance occurring with an obligate parasite.

Environment Erosion

In addition to host and parasite erosion, an environment erosion of horizontal resistance is possible. This again is an apparent erosion of resistance, and it occurs when someone takes a cultivar from an area where the parasite has a low epidemiological competence, to an area where its epidemiological competence is considerably higher. Typically, this happens when a cultivar that is suited to a dry climate is taken to an area with a humid climate. This happened when the coffees of arid Harrar were taken to the much wetter areas of south-west Ethiopia (Chapter 21). Environment erosion also accounts for many susceptibilities in ancient clones being grown in new areas (Chapter 23), and it is also the main reason for practicing on-site selection (Chapter 12).

False Erosion

Finally, there can be a false erosion of horizontal resistance. This can result from sloppy experimental work, inaccurate measurements, mixing of labels, and so on. It can then transpire that a genetic line that was believed to be resistant is, in fact, susceptible. This happened typically with some new sugarcane cultivars that had not been adequately tested for resistance to mosaic virus (Chapter 22). These cultivars were mistakenly believed to be resistant. When they later became severely diseased with mosaic, in farmers' fields, some scientists concluded, quite incorrectly, that there had been a breakdown of vertical resistance.

A false erosion of resistance can also result from psychological errors. For example, there may be a cultivar that is the standard of resistance, against which all other lines are compared. As resistance accumulates in the entire breeding population, during a number of years of breeding, the resistance of that standard cultivar appears to decrease, relative to the population as a whole. This is obviously an illusion, but it can be an alarming one, if its cause is not understood.

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