Virulence Changes inUromyces appendiculatusAfter Five Asexual Generations on a Partially Resistant Cultivar ofPhaseolus vulgaris

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Abstract
A field population of U. appendiculatus, the bean rust pathogen, was obtained from the susceptible cultivar Pinto III in southwestern Minnesota [USA], and maintained for 5 asexual generations on a partially resistant bean cultivar, Slimgreen. Changes in latent period, pustule size and urediniospore production on ''Slimgreen'' and ''Pinto III'' 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 4 cultivars (virulence frequencies between 15 and 55%), none of the 4 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.