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Letter to the Editor

Difference and Diversity of Plant Pathogen Populations: A New Approach for Measuring. E. Kosman, School of Mathematical Sciences, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel-Aviv University, Ramat Aviv, Tel-Aviv 69978, Israel. Accepted for Publication 31 July 1996. Phytopathology 86:1152-1156. Copyright 1996 The American Phytopathological Society. doi:10.1094/Phyto-86-1152.

The diversity of pathogen populations is one of their important intrinsic characteristics. According to Groth and Roelfs (3), an optimal diversity index should satisfy several conditions. A pathogen population is more diverse (diversity index is higher) if (i) it consists of a larger number of phenotypes for a given number of isolates; (ii) it is characterized by an even distribution of phenotypes; and (iii) the number of differences in virulence between phenotypes is larger. Shannon's entropy and Simpson's index are two diversity indices that are commonly used in plant pathology studies. They are based on the relative frequencies of different races. However, they consider nonidentical phenotypes as equally distinct and ignore similarities of different races. Nevertheless, some isolates exhibit distinct virulence patterns, while others are quite similar. Therefore, a diversity index that also reflects this relationship between isolates would provide more information. The hierarchical version of Shannon's index (3,8) and the "weighted mean proportion" (6) were proposed to incorporate a degree of phenotypic similarity into a measurement of diversity of pathogen populations. However, grouping of phenotypes in the case of the hierarchical index of Shannon is rather subjective, which makes comparative analysis of results difficult. On the other hand, the "weighted mean proportion" is based on the "all possible comparisons" of all isolates and, in fact, it is calculated without regard to a phenotypic structure of population.