Potential forPhytophthora infestansPopulations to Adapt to Potato Cultivars with Rate-Reducing Resistance

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Abstract
The rate and extent of adaptation of 4 populations of P. infestans to selected potato cultivars that differed in levels of rate-reducing resistance were determined. 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 2 cultivars. Two pairs of subpopulations were developed in the greenhouse and 2 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 2 cultivars planted alternately as it did in plots of either cultivar alone. These results 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.

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