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Genetics

Virulence Changes in Uromyces appendiculatus After Five Asexual Generations on a Partially Resistant Cultivar of Phaseolus vulgaris. Helen Miller Alexander, Former research associate, Department of Plant Pathology, University of Minnesota, St. Paul 55108, Present address: Department of Biology, University of Louisville, Louisville, KY 40292; J. V. Groth(2), and A. P. Roelfs(3). (2)Associate professor, Department of Plant Pathology, University of Minnesota, St. Paul 55108; (3)Research plant pathologist, Cereal Rust Laboratory, ARS, USDA, St. Paul 55108. Phytopathology 75:449-453. Accepted for publication 23 October 1984. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/Phyto-75-449.

A field population of Uromyces appendiculatus, the bean rust pathogen, was obtained from the susceptible cultivar Pinto 111 in southwestern Minnesota, and maintained for five asexual generations on a partially resistant bean cultivar, Slimgreen. Changes in latent period, pustule size, and urediniospore production on Slimgreen and Pinto 111 over the generations were small or not detectable. In contrast, there was a large increase in the proportion of the pathogen population virulent on bean cultivar US#3 and large decreases in the proportion virulent on the cultivars Early Gallatin, Roma, and B1349 over the course of the experiment. Although Minnesota populations of U. appendiculatus are polymorphic for virulence on all four cultivars (virulence frequencies between 15 and 55%), none of the four is grown in the state in more than small amounts. These experimental results illustrate that changes in virulence gene frequencies may be independent of pathogen exposure to host resistance.