The Mechanisms of Vertical Resistance

Against foliar pathogens (i.e., fungi, bacteria, viruses), the mechanism of vertical resistance is hypersensitivity in all known cases. This mechanism involves the rapid death of all cells adjacent to the point of the non-matching allo-infection, and the pathogen then dies with them. The result is a small 'hypersensitive fleck' which is just visible to the naked eye. However, in more general terms, not all hypersensitive reactions are the result of vertical resistance. Such reactions can be induced by non-parasites. Equally, not all vertical resistances are due to hypersensitivity. The mechanism of vertical resistance to the wilt pathogens Verticillium, and Fusarium is not hypersensitivity, but it is otherwise poorly understood. This is true also of leaf-parastising aphids. The mechanism of vertical resistance to Hessian fly (Mayetiola destructor) of wheat is apparently antibiosis, which so reduces the growth rate of the parasite that its reproduction is delayed or even prevented.

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