Analytic Models of Plant Disease in a Changing Environment

Abstract

During the last 20 years several epidemiological models, so-called simulators, have been developed that describe the development of plant diseases in time and/or space. Teng (69) and Hau (22) have given overviews of the simulation models published so far and have explained the methods used in their de­ velopment. The starting point for many such models, e.g. EPIDEM (76) and EPIVEN (44), is the infection chain of the pathogen. The individual phases of the life cycle such as germination, incubation, etc, are modeled separately and then later combined. This kind of modeling involves identifying the influenc­ ing variables and expressing their effects in mathematical terms. During a simulation, each time step of the asexual cycle is passed through and this produces a disease progress curve affected by external variables, like tem­ perature or relative humidity. Vanderplank (71) called this approach analytic. As an alternative approach, he proposed synthetic models that describe epidemics by means of one or only a few equations, normally differential equations, with a limited number of parameters. He (71) assumed that these few parameters can be estimated with higher accuracy, and proposed that synthetic models are better suited to describe the popUlation dynamics of epidemics than the sophisticated simulators containing many parameters.