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Selection for Fungicide Resistance Within a Growing Season in Field Populations of Phytophthora infestans at the Center of Origin

First author: Horticultural Crops Research Laboratory, U.S. Department of Agriculture-Agricultural Research Service (USDA-ARS), 3420 NW Orchard Ave., Corvallis, OR 97330; second author: USDA-ARS, 1691 S. 2700 W, Aberdeen, ID 83210; third, fourth, and fifth authors: Cornell-Eastern Europe-Mexico/Programa Internacional Cooperativo del Tizón Tardío de la Papa Potato Late Blight Project, Apartado postal 3-12, Cuauhtemoc Izcalli V, Metepec, Edo. de México 52176, México; and sixth author: Department of Plant Pathology, Cornell University, 334 Plant Science Building, Ithaca, NY 14853

The central highlands of Mexico should provide an optimal testing ground for evaluating the potential threat of selection for resistance to fungicides in the population of Phytophthora infestans. We evaluated the hypotheses that exposure to the fungicides azoxystrobin, cymoxanil, dimethomorph, fluazinam, mancozeb, metalaxyl, and propamocarb hydrochloride would lead to (i) a shift in the sensitivity distributions (i.e., selection) and (ii) a lower genotypic diversity of the population. We compared populations from unsprayed plots with populations that had been exposed to several applications of each of the fungicides within a single field season. This study provides novel baseline data and shows that the Toluca valley P. infestans population has a wide range of sensitivities to the fungicides fluazinam, cymoxanil, dimethomorph, metalaxyl, and propamocarb. Directional selection toward resistance combined with a reduction in genetic diversity of the P. infestans population was observed only for the fungicide metalaxyl. The results obtained provide direct experimental support for continuing vigilance regarding further introductions of exotic strains of P. infestans into the United States.

Additional keyword: evolution.