Quantitative Vertical Resistance

Quantitative vertical resistance has always been something of an enigma. It occurs in the some of the small grain cereals (e.g., Hessian fly, Mayetiola destructor, of wheat), and it is known in wild plant pathosystems such as the powdery mildew (Erysphe fischeri) of groundsel (Senecio vulgaris) in Britain (Clarke, et al, 1987). Quantitative vertical resistance is qualitative in its inheritance, which is controlled by single genes. But it is quantitative in its effects, in that it provides incomplete protection against non-matching allo-infections, and no protection whatever against matching allo-infections.

So long as we assume that the function of vertical resistance is to control allo-infection, it is difficult to explain quantitative vertical resistance, because its control of allo-infection and parasitism is incomplete. In fact, this apparent enigma possibly reveals the true function of the vertical subsystem. This function is to control the population explosion of an r-strategist parasite. Quantitative vertical resistance does this by controlling the reproduction of the parasite. It does not prevent the parasitism of a non-matching allo-infection, but it does prevent, or greatly reduce, the reproduction of the non-matching parasite.

Qualitative vertical resistance, which is much more common, also controls the population explosion of an extreme r-strategist parasite. It prevents parasite reproduction by killing the parasite that makes a non-matching allo-infection. This method has the added benefit of controlling some of the parasitism as well. This may explain why qualitative vertical resistance is so much more common than quantitative vertical resistance.

When breeding crops for horizontal resistance, quantitative vertical resistance can be both dangerous and confusing. However, quantitative vertical resistance can be inactivated during the breeding process by using the one-pathotype technique (see 7.5).

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