Virulence Frequency in the Bean Rust Fungus: Comparison of Phenotypic vs. Genotypic Polymorphism. John W. McCain, Postdoctoral Research Associate, Department of Plant Pathology, University of Minnesota, St. Paul 55108. E. A. Ozmon, and J. V. Groth. Junior Scientist, and Professor, Department of Plant Pathology, University of Minnesota, St. Paul 55108. Plant Dis. 74:496-501. Accepted for publication 4 December 1989. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/PD-74-0496.

A collection of urediniospores of the bean rust fungus, Uromyces appendiculatus, made in 1982 from west central Minnesota on a highly susceptible cultivar, Pinto U. I. 111, was polymorphic for virulence (produced both uredinia and flecks) on numerous differential snap and dry bean lines. Four years later, another collection of the bean rust fungus from the same region was phenotypically monomorphic for virulence on many of the same differential lines. However, bulk-fertilization of the 1986 collection revealed genotypic polymorphism that was being maintained in the heterozygous condition. The temporal difference in phenotypic variability between the 1982 and 1986 populations may have originated from differences in founder populations or selection in favor of virulence hybrids (overdominance). Other populations of bean rust from Minnesota in 1986, collected 5, 145, and 180 km away from the first 1986 site, all differed from each other in frequency of virulence on several differential bean lines. This genetic divergence of U. appendiculatus in space and time within one state means that spore collections from different sites should not be pooled and single sites should not be used for testing new bean lines.

Keyword(s): heterozygosity, unnecessary virulence.