Screen for high yields

The main screening criterion must be high yield, for two reasons. First, high yield is an essential selection criterion in its own right. Second, only resistant plants can yield well in the presence of parasites. This rule will ensure that selection pressure for yield and resistance is maintained throughout the breeding program. Selection pressures must also be maintained for all other agriculturally valuable characteristics, which might otherwise be lost because of negative selection pressures. Screening exclusively for horizontal resistance is inadvisable, except for research purposes, because it will lead to a loss of other attributes from negative selection pressures. In practice, this holistic approach means screening for the best 'all-rounders'.

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