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Keywords:

  • Plant disease;
  • incidence-severity relationship;
  • epidemiology;
  • spatial scales;
  • disease density;
  • disease models

Summary

The literature concerning relationships among different measures of plant disease intensity is reviewed. Some previous confusion over the definition of the terms “incidence” and “severity” is noted and clarified. The review highlights the common features of relationships between incidence and severity, incidence and disease density, and incidence at pairs of scales in a spatial hierarchy. These relationships often show a similar saturation curve form that can frequently be described empirically using the complementary log-log transformation. A catalogue of alternative functional forms is provided. Practical applications in varietal evaluation are discussed.