Plant epidemics, models and analysis

Plants are susceptible to many diseases; fungal, bacterial, viral, and other forms of epidemics occur commonly in agricultural systems, resulting in substantial crop yield reductions. There are two primary ways of measuring plant disease, incidence and severity. Plant disease incidence can be defined as the number of diseased plants, or plant parts expressed as a proportion of the total number of plants, or plant parts, assessed. Plant disease incidence is a binary variable, because individuals are either diseased or not. Plant disease severity, on the other hand, can be defined as the area or volume of plant tissue that is diseased, usually relative to the total plant tissue. It is a continuous variable, typically bounded by 0 and 1. In this article, emphasis is placed on incidence. Statistical issues regarding plant diseases in crops or natural systems concern the characterization of spatial patterns of disease, estimation of rates of disease increase over time, and rates of spatial spread. There is a variety of statistical tools available to quantify spatial pattern of disease incidence, which can be categorized based on the general type of analysis – point pattern or geostatistical analysis – or on whether the data consist of mapped or unmapped observations.