The theoretical basis and practical application of relationships between different disease intensity measurements in plants

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.