On-site screening means that the screening is conducted in the agro-ecosystem of future cultivation, in the time of year of future cultivation, and according to the farming system of future cultivation. This is important because the epidemiological competence of parasites varies considerably from one agro-ecosystem to another. A cultivar with horizontal resistances that are in perfect balance with one agro-ecosystem will then have too much resistance to some parasites, and too little to others, when taken to a different agro-ecosystem, or when cultivated out of season, or when cultivated with different cultivation practices.
Fortunately, with potatoes, the agro-ecosystems are usually large, and this is not a serious problem. For example, bacterial wilt (Pseudomonas solanacearum) is a tropical disease which lacks epidemiological competence entirely outside the tropics and sub-tropics, apparently because it cannot survive a winter. There is no need to breed for horizontal resistance to this disease in temperate regions. Equally, the temperate virus diseases of potato seem to lack epidemiological competence in the tropics, and there is no need to breed for horizontal resistance to these diseases in tropical countries.
However, the principle of on-site screening becomes important with greenhouse work. Greenhouse screening is permissible when there is a 180-day breeding cycle, but only if it is alternated with an on-site field-screening at the proper time and place.
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