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Potential for Phytophthora infestans Populations to Adapt to Potato Cultivars with Rate-Reducing Resistance. R. V. James, Department of Plant Pathology, Cornell University, Ithaca, NY 14853; W. E. Fry, Department of Plant Pathology, Cornell University, Ithaca, NY 14853. Phytopathology 73:984-988. Accepted for publication 23 December 1982. Copyright 1983 The American Phytopathological Society. DOI: 10.1094/Phyto-73-984.

We quantified the rate and extent of adaptation of four populations of Phytophthora infestans to selected potato cultivars that differed in levels of rate-reducing resistance. The initial populations had different degrees of heterogeneity: single isolates, mixtures of isolates, and mutagenized isolates. Infection efficiency, sporulation, and rate of epidemic development in field plots were used as criteria to measure adaptation. Two subpopulations were derived from each initial population as a result of repeated asexual generations on either of two cultivars. Two pairs of subpopulations were developed in the greenhouse and two in the field. Each subpopulation was tested for infection efficiency and sporulation on its "own" cultivar (the one on which it had been cultured repeatedly) and on the "other" cultivar (on which it had not grown previously). Subpopulations differed in infection efficiency, but adaptation was not indicated because the changes were not differential for the "own" vs. the "other" cultivar. In the field, disease progressed as rapidly in plots composed of two cultivars planted alternately as it did in plots of either cultivar alone. We interpret the results of these studies and previous ones, as well as the constancy of the relative resistance of cultivars over time (disregarding R genes) to indicate that rapid adaptation of P. infestans populations to cultivars with rate-reducing resistance is unlikely.