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Ecology and Epidemiology

Calculation of Apparent Infection Rate in Plant Diseases: Development of a Method to Correct for Host Growth. Ajjamada C. Kushalappa, Visiting professor (titular), Departamento de Fitopatologia, Universidade Federal de Viçosa, Viçosa, MG, 36 570, Brazil; Artémio Ludwig, systems analyst, Centro de Processamentos de Dados, Universidade Federal de Viçosa, Viçosa, MG, 36 570, Brazil. Phytopathology 72:1373-1377. Accepted for publication 12 November 1981. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-1373.

The use of the logistic growth model to calculate the apparent infection rate (r) and to characterize plant disease progress was developed by Vanderplank; however, its use resulted in empirical problems. In various coffee rust epidemics, the estimation of Vanderplank's r as well as his p (infection rate corrected for host growth) for intervals within a disease progress curve often gave negative values. These values resulted from rapid host growth, which reduced the cumulative proportion of disease (x). We developed a new method to adequately correct for host growth in calculating a corrected infection rate (p'). Similar correction for the exponential, monomolecular, and Gompertz growth models also are described. All of these growth models have a basic requirement that the asymptote is constant over the course of the epidemic (A/Y_max = 1, in which A is the maximum diseased area and Y_max is the maximum host area, and it is assumed that all host area can become diseased by the end of the epidemic). In our method of calculating the intrinsic growth rate of X, the diseased area as a proportion of variable host mass is corrected by a factor (Y_t/Y_max), thus representing X as proportion of the asymptote (A or Y_max).