How does it work

The designated pathotype is used to identify the original parents in the recurrent mass selection program. Each of these parents must be susceptible to the designated pathotype. This is one of the ways in which breeding for horizontal resistance differs from breeding for vertical resistance. The breeding program starts with screening for susceptibility. But is it vertical susceptibility. All the original parents must be susceptible to the designated pathotype. In the each screening generation, the designated pathotype is used to inoculate the screening population.

In each breeding cycle, the selected parents are crossed, either randomly, or in a half-diallel cross (see 7.10). During this crossing, the various vertical resistance genes will recombine in combinations that are new to the program. Provided that only one designated pathotype is used for each parasite, all of these new combinations will be matched by that pathotype. However, if two or more designated pathotypes were used, as a mixed inoculant, some of the new vertical resistance gene combinations will not be matched. The breeding program will then produce vertically resistant cultivars. This problem is difficult to explain in words, and Fig. 7.1 offers an alternative explanation, which may be more readily comprehended.

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