Horizontal Parasitic Ability

In theory, the parasite has a variability comparable to that of the host, and its horizontal parasitic ability can also vary between a minimum and a maximum. However, among obligate parasites, at least, this variability appears to be very limited and there must be an absolute limit to the parasitic ability of the parasite. This is because any parasite that impairs the survival ability of its host impairs its own survival also. Equally, the parasite must not impair its own survival because of a reduced level of parasitic ability. For these reasons, the variation in parasitic ability appears to be very restricted.

However, epidemiological competence is closely related to parasitic ability. And the epidemiological competence varies considerably (Figs. 6.1 & 6.2 and section 6.4). Provided that there is a strict limit to parasitic ability, the considerable variation in horizontal resistance will ensure a balanced level of parasitism in a wild pathosystem. Each locality within an ecosystem will have its own host ecotypes (i.e., horizontal pathodemes) with an adequate level of horizontal resistance to all the locally competent parasites.

Facultative parasites present a somewhat different picture, particularly when they are soil inhabitants. For example, the Fusarium wilts have populations that can vary between almost complete saprophytism, with a very low parasitic ability, to almost complete parasitism, with a very low saprophytic ability. This is demonstrated when a susceptible cultivar is first cultivated, with a relative freedom from wilt. With further cultivation in the same soil, however, there is a steady increase in the frequency of wilt. This happened typically with Fusarium wilt of flax (Linum usitatissimum) in North America. This species was recognised as a crop for pioneer farmers working on newly cleared, virgin land. After a few years, the build-up of pathogenic forms of Fusarium made further flax cultivation impossible. Flax cultivation accordingly moved steadily west with new settlement. Eventually, the flax host accumulated enough horizontal resistance for this problem to disappear (see 7.20.14).

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