Parasite erosion

Parasite erosion is due to genetic changes in the parasite. It is sometimes argued that, however much horizontal resistance there may be in a host, it will not remain effective because the parasite will increase its parasitic ability. It is then postulated that this process can continue indefinitely in a form of arms race. This postulation is false because, as we have seen (see 6.3), there is an absolute limit to the parasitic ability of a parasite. Were this not so, the self-regulation of a wild pathosystem would fail completely. However, the world is still green.

Some scientists refuse to employ horizontal resistance on the grounds that parasite erosion will nullify their work, just as a breakdown of vertical resistance does. With obligate parasites, it can be safely assumed that the parasitic ability of the parasite is at its maximum during screening work. However, the epidemiological competence of the parasite may be below its maximum. Hence the necessity for on-site screening (see 7.2.11 & 7.6.2).

With facultative parasites, the apparent level of parasitic ability may increase. For example, the frequency of some soil-borne diseases (e.g., Fusarium and Verticillium wilts) can increase with continuing cultivation. With continued breeding, the horizontal resistance will increase also. But, if the same screening site is used repeatedly, this may not be apparent because of the increasing levels of disease. The level of disease will then remain more or less constant. This misleading situation can produce an entirely false impression, suggesting that no progress is being made. Although it is a rather rare phenomenon, it is important to be aware of this possibility. It can be resolved by growing a check cultivar of known susceptibility in the same site.

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