Epidemiological Competence

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Figure 6.1 The Level of Parasitism

The two principle factors governing the level of parasitism in a matched host are the level of horizontal resistance in the host, and the level of epidemiological competence in the parasite. If the epidemiological competence is zero, there will be no parasitism, regardless of the level of resistance. If the epidemiological competence is maximal, the level of parasitism will be maximal if the resistance is minimal; and the parasitism will be minimal if the resistance is maximal. These scales are quantitative, and the various degrees of difference between the minimum and the maximum are shown.

The epidemiological competence of the parasite depends on the ecosystem in question, and it is controlled by environmental factors such as water availability, temperature, and soil characteristics. The epidemiological competence can vary between the minimum and the maximum and, in a wild plant pathosystem, its level governs the level of horizontal resistance in the host.

This is an example of a self-organising system with networks of controls. The climate controls the composition of the ecosystem and, hence, the epidemiological competence of the various species of parasite. The epidemiological competence of the parasites control the levels of the various horizontal resistances in the host which, in their turn, control the levels of parasitism. This is a resilient system that can tolerate occasional swings away from the norm resulting from variable weather patterns.

In practice, it is difficult to measure either horizontal resistance or epidemiological competence. Consequently, it is not normally feasible to give a cultivar a horizontal resistance rating. Nor is it possible to give a parasite an epidemiological competence rating, for a particular agro-ecosystem. All we can do is to assign relative assessments. For example, we can say cultivar 'A' is more resistant than cultivar 'B' to a particular parasite. Or that a particular parasite has a greater epidemiological competence in agro-ecosystem 'X' than it does in agro-ecosystem 'Y'.

The purpose of this discussion is to emphasise two points. The first concerns the importance of on-site screening during the breeding process. Suppose there are several species of parasite in each of two different agro-ecosystems, but that the epidemiological competence of these parasites varies differentially between the two agro-ecosystems. A cultivar that has horizontal resistances that are in perfect balance with the first agro-ecosystem, will then have too much resistance to some parasites, and too little too others, when cultivated in the other agro-ecosystem. It follows that there must be on-site screening (see 7.2.11). This means that all screening must be conducted in the agro-ecosystem of future cultivation. In detail, this means that the screening must be conducted (i) in the area of future cultivation, (ii) in the time of year of future cultivation, and (iii) according to the farming system of future cultivation.

The second point concerns the extremes of horizontal resistance in wild plant pathosystems. If a parasite has a zero epidemiological competence in a given ecosystem, the local ecotype of its host will have a minimal horizontal resistance. Conversely, if the parasite has the maximum epidemiological competence in that ecosystem, its host ecotype will have the maximum horizontal resistance. This means that horizontal resistance is a quantitative variable that can occur at any level between two widely separated extremes.

Such ecological variation in epidemiological competence has been clearly established with Pucciniapolysora of maize in Africa (see 7.6.2), and with bean (Phaseolus vulgaris) pathogens in Mexico, where the wild progenitor of this host is indigenous. In the Tepexi area of the State of Puebla, rust (Macrophomina phaseoli) and anthracnose (Colletotrichum lindemuthianum) both have a low epidemiological competence, while bacterial blight (Xanthomonas campestris phaseoli) and common bean mosaic virus (CBMV)

have very high epidemiological competence. Cultivars that have been bred and selected in this area have excellent horizontal resistance to bacterial blight and CBMV, but they have little resistance to rust and anthracnose. They cannot be cultivated in areas in which the latter pathogens have a high epidemiological competence (Garcia Espinosa, 1997).

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  • Lucius Rizzo
    What is epidemiological competence?
    2 years ago
  • christina
    What is diferential interaction and epidemiological competence?
    2 years ago

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