Onsite screening

On-site screening means that the screening is conducted (i) in the area, (ii) the time of year, and (iii) according to the farming system, of future cultivation.

The principle reason for on-site screening is that agro-ecosystems vary so widely. In particular, the epidemiological competence of most parasites varies from one agro-ecosystem to another. An agro-ecotype that is in perfect balance in one agro-ecosystem will be out of balance in another. This is because it will have too much resistance to some species of parasite, and too little to others.

Within the maize of tropical Africa, each farm constituted a separate screening population. So long as the local agro-ecotypes were closely similar, an exchange of pollen between farms would mean little, because more distant agro-ecotypes were out of pollination range. Agro-ecotypes taken from above 4000 ft at the equator, or beyond the Tropics of Cancer and Capricorn at sea level, would suffer maximum crop loss when grown on the equator at sea level.

This need for on-site selection, when working with horizontal resistance, is one of the main safeguards against exploitation of subsistence farmers by the corporate plant breeders of the industrial world.

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