Genetic source of resistance essential

The second disadvantage of vertical resistance has already been mentioned. This is the need, indeed the necessity, of first finding a genetic source of resistance. Vertical resistance does not occur universally. It is entirely absent from crops derived from continuous wild pathosystems (see 4.11), and it occurs against only some of the parasites of crops derived from discontinuous wild pathosystems. If a source of vertical resistance cannot be found, for the simple reason that it does not exist, the breeding cannot begin. There are some famous crop parasites, such as Colorado beetle (Leptinotarsa decemlineata) of potato, Take-All disease (Gaumannomyces graminis) of wheat, and bacterial wilt (Pseudomonas solanacearum) of tobacco, tomatoes, and potatoes, for which a source of vertical resistance has never been found, and Mendelian resistance breeding has apparently never been attempted.

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