Journal of Industrial Microbiology & Biotechnology (1997) 19, 77–82  1997 Society for Industrial Microbiology 1367-5435/97/$12.00 Are classification and phytopathological diversity compatible in Xanthomonas? L Vauterin and J Swings Laboratorium voor Microbiologie, Universiteit Gent, Ledeganckstraat 35, B-9000 Ghent, Belgium The genus Xanthomonas is characterized by its phytopathogenic diversity and the host specificity of its members. In the past, the classification of the members of this genus has been based primarily on the criterion of host speci- ficity. This has led to a classification system which focused only on naming phytopathogenic variants on different hosts. Extensive taxonomic examination of Xanthomonas has shown that the phytopathogenic specialization of the bacteria is not correlated with the actual relationships within the genus. Based upon total genomic DNA homology, the genus has been reclassified into 20 species. At present, non-pathogenic xanthomonads are frequently isolated from plant material. As these strains often cannot be classified to existing species, it becomes clear that the diversity of the genus is much greater than expected from the phytopathogenic subpopulation, which has been the primary subject in the past. The example of Xanthomonas also illustrates that attempts to divide bacterial populations into discrete taxa conflict with the actual continuous nature of biodiversity. Keywords: Xanthomonas; diversity; classification; pathogenicity Xanthomonas is a typical genus of plant pathogenic bac- pathogens, the genus has its beneficial aspects as well. Most xanthomonads produce an extracellular polysaccharide teria. Its representatives occur in many climatic regions and especially subtropical and tropical areas all over the world. called xanthan. This characteristic polymer of pentasacchar- ides, which is responsible for the typical mucous appear- The pathogens cause a variety of diseases including wilt, necrosis, gummosis and vascular or parenchymatous dis- ance of colonies and cultures of xanthomonads, has a num- ber of attractive physico-chemical features (for a recent eases on leaves, fruits or stems on diverse monocoty- ledonous and dicotyledonous plant families [2]. According review, see [31]). It renders solutions a high degree of vis- cosity and is resistant to high temperatures and salt concen- to the most thorough study in this domain [18], the host range of Xanthomonas includes at least 268 dicotyl and 124 trations, as well as to acid pH. Xanthan gum is produced industrially on a large scale as a stabilizing, emulsifying, monocotyl plant species. However, since most known plant pathogens are associated with crops and other cultivated and gelling agent in numerous commercial products, parti- cularly in the food industry [31]. plants, the real number of plant species that is susceptible to xanthomonad pathogens might be far greater. Members of the genus Xanthomonas infect many econ- A classification based on phytopathogenicity omically important crops. Among the most devastating of them are those pathogens affecting primary food crops in One of the most remarkable characteristics of Xanthomonas is the phytopathogenic diversity and the apparent host third world regions, such as X. oryzae on rice [22] and X. axonopodis (X. campestris) pv manihotis on cassava [21]. specificity of the members. Originally, each variant show- ing a different host range or producing different disease Other important pathogens include X. axonopodis pv phase- oli causing bacterial blight of bean, X. axonopodis (X. symptoms was classified as a separate species. This prac- tice, denounced as the ‘new host – new species’ concept campestris) pvs glycines causing bacterial pustule of soybean, citri, responsible for citrus canker, X. vesicatoria [29], led to a complex genus, finally containing more than 100 species. In contrast to the phytopathogenic diversity of (X. campestris pv vesicatoria), the causal agent of bacterial spot of pepper and tomato, X. campestris pv campestris Xanthomonas, the general phenotypic characteristics of the bacteria are remarkably uniform, at least as determined by which causes black rot of crucifers, X. translucens (X. cam- pestris pv translucens), causing leaf streak and black chaff available tests. Several comprehensive phenotypic studies have been performed in attempts to differentiate the phyto- of small grains. This list is not complete as many other Xanthomonas species and pathovars are highly specialized pathological groups by means other than the host from which isolated [3,6,34], but these have only illustrated the pathogens for various crops, trees and ornamental plants. A recent comprehensive survey is given by Hayward et phenotypic homogeneity of the genus. This knowledge, and also the fact that insufficient information was available al [12]. In spite of the importance of Xanthomonas as plant about the actual phytopathogenic specialization of the taxa, was the major motive for merging almost all Xanthomonas species into the single species X. campestris by Dye and Lelliott [7]. Later, Young et al [47] have proposed to Correspondence: Dr L Vauterin, Laboratorium voor Microbiologie, Univ- reclassify the former nomenspecies (ie, species only dis- ersiteit Gent, Ledeganckstraat 35, B-9000 Ghent, Belgium Received 16 April 1996; accepted 27 September 1996 tinguished by their name) into pathovars. A pathovar is an