Infection and Resistance

With asexually reproducing parasites, there is a direct, and very important, correlation between the two kinds of resistance, and the two kinds of infection. Consider the first infection of any plant host individual. It can only be an allo-infection. There are no other possibilities. In terms of the gene-for-gene relationship, this allo-infection is either a matching or a non-matching infection. Vertical resistance thus controls allo-infection.

Now consider the consequences of a matching allo-infection. The parasite reproduces asexually, and all of its progeny constitute a clone. All members of this clone have a vertical parasitic ability that matches the vertical resistance of the host. And all parts of the one host have the same vertical resistance. It follows that all of this auto-infection is matching infection.

Vertical resistance cannot control auto-infection. It can control allo-infection only. And it does this by reducing the frequency of allo-infections that are matching infections. Auto-infection, and all the consequences of a matching infection, can be controlled by horizontal resistance only.

Some pathosystems lack a vertical subsystem (see 4.11). In these, both allo-infection and auto-infection are controlled by the horizontal subsystem.

Some parasites, particularly insects, lack an asexual reproduction and, again, these rules concerning resistance and infection do not apply. However, vertical resistances are rare against plant parasites that lack an asexual reproduction. Examples include Hessian fly (Mayetiola destructor) of wheat, and golden nematode of potato (Heterodera rostokiensis), and auto-infection is not a significant factor in the population increase of these parasites.

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