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A Simulation Model to Describe Epidemics of Rust of Phaseolus Beans II. Validation. L. Amorim, Departamento de Fitopatologia, ESALQ, Universidade de São Paulo, 13418-900 Piracicaba, SP, Brazil; R. D. Berger(2), A. Bergamin Filho(3), B. Hau(4), G. E. Weber(5), L. M. A. Bacchi(6), F. X. R. Vale(7), and M. B. Silva(8). (2)Plant Pathology Department, University of Florida, Gainesville 32611; (3)(6)Departamento de Fitopatologia, ESALQ, Universidade de São Paulo, 13418-900 Piracicaba, SP, Brazil; (4)(5)Biometrie und Populations-genetik, Justus Liebig Universität, 6300 Giessen, Germany; (7)(8)Departamento de Fitopatologia, Universidade Federal de Viçosa, 36570 Viçosa, MG, Brazil. Phytopathology 85:722-727. Accepted for publication 10 February 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/Phyto-85-722.

The FERRUGEM simulation model of bean rust epidemics, based on infections of Uromyces appendiculatus that occur on daily cohorts of bean leaf growth, was validated with data from 35 bean rust epidemics monitored in the southeastern region of Brazil. Subjective and goodness-of-fit tests of data were used. Good agreement was obtained between model output and real data in the majority of epidemics. Linear regression lines of Gompertz-transformed proportions of pustular area predicted by the model did not differ for both slope and intercept from regression lines of observed pustular areas in 60.0% of the epidemics. In 71.4% of the epidemics, there was no significant difference in intercepts. Lack of agreement between model-predicted outcomes and observed epidemics occurred in two situations: when the initial proportion of pustular area observed in the field was very low (<0.000005) and when there were long periods of weather very favorable for infection. In the first situation, it may be possible to improve the fit of the model by more rigorous sampling to obtain accurate estimates of initial pustular area, because solitary pustules and small foci are easily missed by chance. In the second situation, the model overestimated the final pustular area, possibly because long periods of leaf wetness may have had a negative influence on dispersal of the urediniospores. This latter factor was not considered in the model routines. Specific experiments to evaluate the effect of weather variables on the dispersion of U. appendiculatus should be conducted before the model is implemented.