Modelling plant diseases for decision making in crop protection

A plant disease model is a simplification of the relationships (between a patho-gen, a host plant, and the environment) that determine whether and how an epi-demic develops over time and space. This chapter describes an approach for de-veloping mechanistic, weather-driven, dynamic models which are suitable to be applied in precision crop protection. Model building consists of four steps: (I) defi-nition of the model purpose; (II) conceptualization; (III) development of the mathe-matical relationships; and (IV) model evaluation. Conceptualization is based on systems analysis; it assumes that the state of the pathosystem can be quantitatively determined and that changes in the system can be described by mathematical equations. A conceptual model describes the system (both conceptually and mathematically), and a set of driving models accounts for changes caused by the external variables. Two main types of conceptual models are described: plant- and pathogen-focused models. Model evaluation is the judgement of the overall adequacy of the model, which includes: verification, validation, uncertainty analysis, sensitivity analysis, and judgement of utility. Finally, the chapter briefly considers how models can be used as tools for decision making at different scales of time and space: from warning services to precision agriculture.