The relationship between plant disease severity and yield.

The relationship between disease and yield is most often summarized as a simple empirical model that describes average crop performances is the presence of a pathogen. Such models may be robust and useful for surveys but their use is usually constrained to the specific conditions under which the modal was developed. Changes in production system usually invalidate the relationship. The alternative is to base the relationship on an epidemiological analysis of the pathogen population and a physiological concept of host growth and development. This review provides the knowledge and conceptual basis and discusses the limitations to progress in the development of such models. It is shown that a host-based assessment of disease is well suited to yield investigations and to multiple pest constraints, and that disease is logically related to yield via radiation interceptions and radiation use efficiency.