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A Simulation Analysis of the Epidemiological Principles for Fungicide Resistance Management in Pathogen Populations. M. G. Milgroom, Department of Plant Pathology, Cornell University, Ithaca NY 14853; W. E. Fry, Department of Plant Pathology, Cornell University, Ithaca NY 14853. Phytopathology 78:565-570. Accepted for publication 20 October 1987. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/Phyto-78-565.

Three epidemiological principles governing the buildup of fungicide resistance in populations of plant pathogens were derived from a simple mathematical model. These principles are that resistance will build up more slowly if: 1) the initial frequency of resistance is reduced, 2) the apparent infection rates of both fungicide-resistant and fungicide-sensitive genotypes (rR and rS, respectively) are reduced, and 3) rR is reduced relative to rS. These principles can be used to define the basic strategies for managing fungicide resistance. To illustrate these principles, we used a model for potato late blight that included a subpopulation of Phytophthora infestans that was resistant to the fungicide metalaxyl. We simulated the effects of initial frequency of resistance, favorable and unfavorable weather for late blight development, protectant fungicides, cultivar resistance, frequency of metalaxyl applications, metalaxyl dose, metalaxyl weathering rates, fitness of metalaxyl-resistant genotypes, and levels of resistance to metalaxyl. All of the simulated treatments conformed to theoretical predictions, although the magnitudes of the effects were specific to the late blight system. Recommended tactics for management of metalaxyl resistance were also evaluated with the simulation model; most were shown to reduce the buildup of resistance in the pathogen population. Contrary to recommendations, eradicant use of metalaxyl resulted in the least buildup of resistance; however, eradicant use also resulted in unacceptably high levels of disease.