Different Requirements in Breeding for Horizontal Resistance

The breeding of crops for resistance to their parasites during the twentieth century has involved vertical resistance almost exclusively. The requirements of breeding for horizontal resistance are so different from those of breeding for vertical resistance that some comment is necessary. The main differences are as follows:

  • No source of resistance is required. It is possible to start the breeding with susceptible parents.
  • Both the inheritance and the measurement of horizontal resistance are quantitative.
  • Many crosses are necessary and male gametocides are useful in some of the cereals.
  • No transfer of resistance genes is possible. All horizontal resistances to all locally important parasites must be accumulated simultaneously. Selection pressure must also be maintained for all other agriculturally desirable characteristics. A balanced, holistic screening is essential.
  • Recurrent mass selection is used in place of pedigree breeding and back-crossing. This is a major departure when breeding autogamous crops such as beans, wheat, and rice.
  • Large screening populations should be employed in order to allow high selection coefficients.
  • Vertical resistance is characterised by 'big space, high profile, short life, high expense, few cultivars', while horizontal resistance is 'small space, low profile, long life, low expense, many cultivars'.
  • Horizontal resistance breeding is so easy that it can be undertaken by plant breeding clubs made up of amateur breeders (see 11.17).
  • On-site screening is essential. That is, the screening should be conducted in the agro-ecosystem of future cultivation. This means, (i) in the area of future cultivation, (ii) in the time of year of future cultivation, and (iii) according to the agricultural system of future cultivation.
  • Breeding for horizontal resistance involves recurrent mass selection employed with three simple rules.
  1. First, the sole selection criterion should be yield, on the grounds that susceptible plants cannot yield well in the presence of parasites.
  2. Second, the screening population should be inoculated appropriately to ensure that the high yields are due to resistance and not to chance escape.
  3. Third, the one-pathotype technique (see 7.5) should be employed when necessary to ensure that the resistance is horizontal and not vertical.
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