Changing selection pressures

At the beginning of the breeding program, the selection pressures are so high that the death rate among seedlings will be almost total. Three comments are relevant. First, the screening population may look so awful that the club members conclude, wrongly, that there is no point in continuing. If a mere dozen seedlings survive, out of many thousands, the program is working to perfection, and the parasites are doing all the work of screening for you. These few survivors may look terrible, but they are very valuable and, because they become the parents of the next screening generation, they represent a very significant genetic advance in resistance.

Second, there is a very real danger that no seedlings whatever will survive. If this danger is real, it is entirely legitimate to treat the screening population with crop protection chemicals to enable the few survivors to set seed. This requirement may discourage organic farmers from breeding for horizontal resistance, but they can always rent a field on a conventional farm for the early breeding cycles that may require protection.

Third, as the breeding progresses, and more and more resistance accumulates, the survival rates will increase dramatically. The death of susceptible seedlings is no longer the main selection criterion, and other criteria must be used. The most useful of these is the yield of individual plants, provided that the screening population has been inoculated, and the one-pathotype technique has been used (see 7.5).

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