Agriculture Reference
In-Depth Information
disease severities over all varieties, and data on varietal partial resistances, to predict
relative disease severities on different varieties with the same specific resistance.
Furthermore, the ability of this model to provide quantitative risk predictions may be
limited by such unexpected events as the heavy infection of volunteeers of Ida,
mentioned above.
Another issue to be considered is how closely spaced sampling sites need to be
in order to provide risk predictions which are robust over a reasonably wide area. In
effect, we return to the question of what is meant by 'a population'. The spacing
should be related to the pathogen's mobility; in the case of cereal mildews, the large
population of wind-dispersed spores should allow sampling sites to be spaced much
further apart than would be the case for splash-borne spores, dispersed over a very
small area.
3.8 PATHOGEN SURVEYS AND DISEASE MANAGEMENT
Several examples discussed in this chapter have illustrated the value of
epidemiological knowledge in the execution of pathogen surveys, notably in the
effective design of sampling schemes and the application of the resulting data. In
some of the better-studied pathogens, it is becoming possible to begin to integrate
the pathogen's epidemiology and population genetics, in order to develop a deeper
understanding of how populations adapt to crops (Brown and Hovmøller, 2002;
Milgroom and Peever, 2003).
Conversely, surveys of pathogenic variation clearly have the potential to
continue to contribute to improving the efficiency of disease management. They
should be particularly valued in the present climate of increased concern about the
environmental impact of farming, as they can help to optimise the use of resistant
varieties and reduce unnecessary fungicide applications. For this desirable situation
to continue - indeed, for the value of pathogen surveys to be increased further -
management of pathogen surveys must be seen as a long-term exercise. Many
examples cited in this review illustrate the benefit of being able to place new results,
whether on virulences or on responses to fungicides, in a historical perspective of
years or even decades of continuous research on a disease (Bayles
et al.
, 1997;
Kolmer, 2005).
Unfortunately, pathogen survey work has something of the image of a
Cinderella, compared with the much more fashionable subject of molecular genetics.
However, while molecular genetics research is helping to provide medium to long-
term guidance on strategies for crop management and is also beginning to increase
the efficiency of pathogenicity testing, it is no substitute, in terms of its value for
disease control, for the kind of direct knowledge about variation in characters
relevant to pathogenicity that comes from well-managed, long-term surveys.
ACKNOWLEDGEMENT
The writing of this chapter was supported by the Biotechnology and Biological
Sciences Research Council.
