Virulence changes in Uromyces appendiculatus after five asexual generations on a cultivar of bean (Phaseolus vulgaris)

Abstract

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.