Multiplicity of Pathosystems

In this book, an agro-ecosystem is defined by the epidemiological competence of the various crop parasites. Each of these subsystems may have to be further subdivided on the basis of rainfall, soil acidity, and other ecological factors, which may further determine the suitability of cultivars. Within reasonable limits, a separate breeding program will be necessary for each subsystem. In other words, each breeding program will have biological boundaries determined mainly by the epidemiological competence of all the locally important parasites.

Pucciniapolysora, which causes tropical rust of maize, exhibits marked variation in epidemiological competence, linked to agro-ecosystems (see 7.2.2). As we have seen, this parasite loses epidemiological competence with increasing altitude and latitude. At the equator, it loses epidemiological competence entirely at an altitude of 4000 feet. At sea level, it loses epidemiological competence entirely at the tropics of Cancer and Capricorn. Between each of these extremes of altitude and latitude, there is every degree of difference in epidemiological competence between zero and the maximum (Fig. 6.2). For example, in Kenya, the disease was at its most damaging at sea level near the equator. It lacked epidemiological competence entirely in the Highlands, which are above 4000 feet in altitude.

It is important not to confuse epidemiological competence and resistance. For example, maize from close to the Tropic of Capricorn in Malawi was reported to be highly resistant to P. polysora. But, when taken to sea level areas in equatorial Kenya, it was highly susceptible. The low level of disease in Malawi was due to a low epidemiological competence in the parasite, and it was not due to a high resistance in the host.

The susceptible maize crops of subsistence farmers in low altitude equatorial Kenya were open-pollinated and genetically flexible (see 1.15). Consequently, they responded to these many differing degrees of selection pressure for resistance during the cultivation process. Most crops can show such a response only during the breeding process.

This adjustment of subsistence farmers' maize to the local epidemiological competence of tropical rust is an elegant example of self-organisation. In each agro-ecosystem, the maize accumulated enough horizontal resistance to control the disease, but no more. Obviously, once there was enough resistance to stop the disease from affecting the reproductive capacity of the maize, the selection pressure for resistance disappeared.

If plant breeding clubs are to produce a similar effect with other crops, there will have to be at least one plant breeding club for each crop in each of its agro-ecosystems. Ideally, there should be more than one club in order to provide constructive competition. However, farmers who have clubs for breeding their own crops do not need the spur of competition from other clubs.

In practice, some reduction in the multiplicity of clubs is possible. This is because cultivars with too much resistance may be acceptable outside their own agro-ecosystems. For example, maize with adequate resistance to P. polysora at sea level on the equator, would have too much resistance at higher altitude or latitude. But, if all other things are equal, this need not matter. The final test will rest with the self-organising system itself.

Given the continuous variation between zero and maximum epidemiological competence, for many different species of parasite, it is difficult to determine how many breeding clubs will be required. This requirement can probably be determined only by practical experience.

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