(...)The temporal difference in phenotypic variability found 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
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John W. McCain (1990) studied this question.