Long life Small Space Reconsidered

The general rule for horizontal resistance is 'small space, low profile, long life, many cultivars' (see 6.5). The durability and 'long life' of horizontal resistance are not in dispute. But a horizontally resistant cultivar that is in perfect balance with one agro-ecosystem will be out of balance in another agro-ecosystem. This is mainly because of differing levels of epidemiological competence among the parasites. In the new agro-ecosystem, the cultivar will have too much resistance to some parasites, and too little to others. Hence the phrase 'small space'.

However, it is theoretically possible to develop cultivars that have the maximum level of horizontal resistance to all the known parasites. Such a cultivar would have a cultivation range limited only by the climatic requirements of the host itself (e.g., day-length, rainfall, temperature). It would be free of any constraints concerning epidemiological competence that varies from one agro-ecosystem to another. This would be a difficult breeding program, because there is a possibility that these high levels of resistance would cause conflicts with other selection criteria, such as yield and the quality of crop product. The centre of origin of the crop is likely to be the most suitable screening site for such work.

Nevertheless, the possibility exists. 'Long life, small space' could become 'long life, big space', at least in some crops, in some regions. Even an incomplete realisation would be an improvement. But these possibilities can be determined only by practical experiment.

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