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Disease Progression as a Function of Plant Growth. N. Lalancette, Graduate research assistant, Department of Plant Pathology, The Pennsylvania State University, Fruit Research Laboratory, Biglerville 17307, Present address: Postdoctoral research associate, Department of Plant Pathology, The Ohio State University, Ohio Agricultural Research and Development Center, Wooster 44691; K. D. Hickey, professor, Department of Plant Pathology, The Pennsylvania State University, Fruit Research Laboratory, Biglerville 17307. Phytopathology 76:1171-1175. Accepted for publication 30 January 1986. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/Phyto-76-1171.

Many plants exhibit susceptibility to infection only during their period of growth. For these pathosystems, disease progression could be expressed as a function of plant growth instead of time. The logistic, Gompertz, monomolecular, power, and other functions can be used to describe plant growth as well as the increase of disease. Functions describing these two processes were combined into single models by deriving the absolute rate of change of the proportion of disease relative to plant growth. Given this approach, four basic types of models were identified. A model was described as being either similar or dissimilar depending on whether or not the disease and plant growth patterns (i.e., their functions) were the same. Similarly, if plant growth occurred prior to onset of the epidemic, the model was considered to have a nonsynchronous temporal structure, while if both processes were initiated at the same point in time, the models were termed synchronous. Graphic comparisons of data from simulations indicated that points of inflection, concavity, and asymptotes could be readily varied through manipulation of model parameters. Although modeling disease progression as a function of plant growth does not require a cause and effect relationship, these models would, nevertheless, be particularly applicable to pathosystems in which the occurrence of disease is dependent on the production of young, susceptible plant tissue.