Agro Ecotypes

One of the many features characterising agro-ecosystems is the variation in epidemiological competence of different species of crop parasites (see 7.2.2). This variation may be absolute or it may differ in degree. For example, tropical parasites usually have an absolute lack of epidemiological competence in temperate regions, and vice versa. Alternatively, within one climatic range, the variation in epidemiological competence may be quantitative. The most elegant example of this is tropical rust of maize (see 7.2).

This re-encounter parasite reached East Africa in 1952. Its maximum epidemiological competence is at sea level on the equator. Epidemiological competence declines at sea level with increasing latitude and it ceases entirely at the Tropics of Cancer and Capricorn. Epidemiological competence also declines with altitude, and it ceases entirely at 4000 feet above sea level, on the equator (Fig. 7.1).

Within the region of East Africa there are clearly many different maize agro-ecosystems, defined by the epidemiological competence of this rust. Agro-ecotypes with adequate resistance in one agro-ecosystem are likely to have either too much or too little resistance in another agro-ecosystem. These agro-ecotypes are often called 'landraces'. Because open-pollinated maize responds to selection pressures during cultivation, each local landrace is in a state of balance with its own agro-ecosystem.

It is this variation in agro-ecotypes that makes so many cultivars perform poorly when taken to another country or another region. We really do need to abandon ideas of 'wide adaptability' and the 'universal cultivar' that will perform well throughout a wide climatic range. Decentralisation should be our catchword.

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