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Ecology and Epidemiology

Field Assessment of Fitness of Isolates of Phytophthora infestans. P. W. Tooley, Department of Plant Pathology, Cornell University, Ithaca, NY 14853, Present address: USDA-ARS, Plant Disease Research Laboratory, Fort Detrick Bldg. 1301, Frederick, MD 21701; W. E. Fry, Department of Plant Pathology, Cornell University, Ithaca, NY 14853. Phytopathology 75:982-988. Accepted for publication 20 March 1985. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/Phyto-75-982.

The fitness of isolates of Phytophthora infestans was estimated on potato cultivar Norchip in two different field environments. Fitness differences among isolates were evident in an environment marginally favorable for pathogen increase, while no differences were detected in a more favorable environment. Absolute fitness estimates expressed as early-epidemic growth rates based on the logistic model were significantly higher than those based on the exponential model. Fitness estimates based on visual disease assessments were significantly higher than those based on assessments of pathogen population size (expressed as areas or numbers of late blight lesions). Furthermore, differences in estimated fitness of isolates varied with the method of estimation. Our results suggest that fitness estimates based on different models or methods of assessment should not be used interchangeably.