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The Concept and Measurement of Phenotypic Diversity in Puccinia graminis on Wheat. J. V. Groth, Professor, Department of Plant Pathology, Cereal Rust Laboratory, USDA, University of Minnesota, St. Paul 55108; A. P. Roelfs, Research plant pathologist and Professor, Cereal Rust Laboratory, USDA, University of Minnesota, St. Paul 55108. Phytopathology 77:1395-1399. Accepted for publication 31 March 1987. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1987. DOI: 10.1094/Phyto-77-1395.

Four different diversity indexes were applied to race survey samples of wheat stem rust (Puccinia graminis f. sp. tritici) made annually during 19181982 rate survey samples of wheat and screened on the Stakman differential wheat line set for the years 19181982. Several kinds of diversity were measured by these indexes, including phenotype number, evenness of frequency, and temporal, summed change in frequency of each phenotype. The trend for all four indexes was downward through the years. Multiple regression analysis established that the contributions of two aspects of diversity, phenotype number and evenness, and a third variable, sample size, to the Gleason, Shannon, and Simpson indexes were different, so that, depending on the nature of the population being sampled, one or other of the diversity indexes should prove to be more appropriate. The ease of computation of these indexes and the complementary nature of the Rogers index to the other three suggest that more than one of them can be applied to more fully describe diversity of a plant pathogen.