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Quantification of Factors Influencing Potato Late Blight Suppression and Selection for Metalaxyl Resistance in Phytophthora infestans: A Simulation Approach. M. A. Doster, Department of Plant Pathology, Cornell University, Ithaca, NY 14853; M. G. Milgroom, and W. E. Fry. Department of Plant Pathology, Cornell University, Ithaca, NY 14853. Phytopathology 80:1190-1198. Accepted for publication 17 May 1990. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/Phyto-80-1190.

A computer simulation model was validated and used to evaluate strategies for management of potato late blight and resistance in Phytophthora infestans to the systemic fungicide metalaxyl. Subjects investigated included the frequency of metalaxyl applications (same total dosage per season), comparison of mixtures of metalaxyl and the protectant chlorothalonil versus alternations of the two fungicides, dosages of component fungicides in a mixture, timing for a limited number of metalaxyl applications, use of host resistance, and pathogen fitness. The study investigated disease management for foci initiated by single pathogen genotypes. Metalaxyl/chlorothalonil mixtures performed better than alternating metalaxyl and chlorothalonil sprays in suppressing both the metalaxyl-sensitive and -resistant pathogen when the same total dosages and number of sprays were applied during the season. More frequent sprays applied at lower rates (same total amount of fungicides per season) resulted in less disease caused by either metalaxyl-sensitive or -resistant strains for both mixtures or alternations. Fewer applications of the mixture (replaced by protectant sprays) achieved substantially improved control of the metalaxyl-resistant pathogen. An application of the mixture once in the middle of the season (about 8 wk after emergence in a 12-wk-long season) resulted in the least final disease for the metalaxyl-sensitive strain. Field experiments performed with a metalaxyl-sensitive strain indicated that a mixture application in the middle of the season (5?7 wk after emergence in a 11- or 12-wk season) was most effective in suppressing disease. However, simulations using the metalaxyl-resistant strain showed that application of the mixture once late in the season resulted in the least disease. Adjustments in fungicide rates or timings were less effective in suppressing the metalaxyl-resistant pathogen than use of moderately resistant cultivars. Small differences in aggressiveness between the strains had little influence on the selection for metalaxyl resistance. However, large reductions (such as 50%) in the aggressiveness of the metalaxyl-resistant strain could nullify selection for resistance. Identification of the ?best? strategy for suppression of metalaxyl resistance depended somewhat on whether evaluation was based on the ratio of resistant to sensitive individuals or on the total number of metalaxyl-resistant individuals.