Spatial Aspects of Epidemics III Patterns of Plant Disease

When plant pathologists look at patterns of pathogens, disease, vectors or other components of pathosystems, it is the outcomes of dispersal processes and the effects of environmental heterogeneity that are being observed. Although it is not usually a good idea to draw conclusions about processes on the basis of outcomes, it is nevertheless true that one of the reasons why so much attention has been devoted to the study of patterns of plant disease is that these patterns are regarded as realizations of underlying dispersal processes of pathogens. Thus, in one of the first epidemiological studies of field-scale spatial patterns of disease, Bald monitored the incidence of tomato plants showing symptoms of infection with Tomato spotted wilt virus (TWSV). A program of intensive mapping of diseased plants in field plots was undertaken, with the objective of conducting: “…an ecological survey of incidence, to find out something about the natural dissemination of spotted wilt.” The first step in understanding ecological processes is to identify patterns. However, when the objective of an analysis of spatial pattern is an explanation of the underlying processes, statistical methods alone may not suffice. Instead, such methods may be used to formulate biological hypotheses that can be investigated by non-statistical means. For example, both Bald and Cochran, discussing the same TSWV data set, offered biological interpretations of their statistical analyses of pattern in terms of the behavioral ecology of the vector of the virus. Here, we distinguish between such studies, carried out primarily with the objective of finding a biologically based explanation for observed spatial patterns of disease, and studies in which the primary objective is to make statements about disease intensity, where the pattern of disease is of interest in that it affects the sampling distribution of the intensity estimator. Methods for both types of study are discussed in this chapter.

Chapter topics include:

Why We Look at Spatial Patterns

Terminology

Spatial Plant Disease Data

Analysis of Sparsely Sampled Incidence Data

Analysis of Sparsely Sampled Count Data

Relationships between Distributions

Spatial Hierarchies

Sparsely Sampled Disease Severity Data

Analysis of Intensively Mapped Disease Data

Spatial Patterns and Dispersal Functions

Distance-Based Methods