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October 2007. A member alerted PestNet to changes in the nomenclature of citrus canker pathogen.
There have been recent changes in the naming of the xanthomonads affecting citrus. Most importantly there have been changes to the naming of the bacterium causing Asiatic Citrus Canker, canker A. The purpose of this message is to inform PestNet of the name changes and the key publications for reference.
The two key papers are:
1) Schaad et al. (2005) Reclassification of Xanthomonas campestris pv. citri (ex-Hasse 1915) Dye 1978 forms A, B/C/D, and E ……. Systematic and Applied Microbiology 28: 494-518
2) Schaad et al. (2006) Emended classification of xanthomonad pathogens on citrus. Systematic and Applied Microbiology 29: 690-695.
In the first paper, Schaad et al. proposed renaming of the Asiatic Citrus Canker pathogen as Xanthomonas smithii subsp. citri (synonyms: X campestris pv. citri and X axonopodis pv. citri). After publication, these authors became aware that this name was illegitimate. The name Xanthomonas citri (ex Hasse 1915) was reinstated by Gabriel et al. in 1989 and accordingly has priority.
In their second paper, Schaad et al. have accepted that Xanthomonas smithii subsp.citri is an illegitimate name and proposed that the Asiatic Citrus Canker agent should be a new subsp. of X citri, i.e., X citri subsp. citri. The second paper also contains recommendations for changes in naming of the other xanthomonad pathogens on Citrus.
To obtain information on name changes affecting other plant associated bacteria it is useful to refer to the service on the internet provided by JP Euzeby. His website can usually be accessed by including his name and the name of the generic or specific name of the bacterium of interest in Google. For further information please refer to the following publication:
JP Euzeby (1997) List of bacterial names with standing in nomenclature, a folder available on the internet. International Journal of Systematic Bacteriology 47: 590-592.
Bacterial nomenclature has undergone many changes in the past two decades largely because of the unsuspected phylogenetic relationships shown up by studies of conserved DNA sequences. It is necessary to keep pace with the consequent nomenclatural changes when they affect such major quarantine pathogens as the agent of Asiatic Citrus Canker.
In a second posting, after a question requesting an update on Citrus Canker races/species taxonomy worldwide, it was stated: The main reason for bringing this matter up comes from recent attendance at a conference where the name Xanthomonas smithii subsp. citri pathotype A was used for the Asiatic Citrus Canker pathogen. This was new to the author of this message and some others at the conference. Search of the literature provided the answer given in my previous contribution. It is likely that in time the name Xanthomonas citri subsp. citri will gain currency, not Xanthomonas smithii subsp. citri, because the name is illegitimate.
Whether names become accepted depends on the quality of the taxonomy as judged by the scientific community. Name changes at the level of subspecies or above are governed by the Code of Nomenclature for Bacteria; below subspecies there are informal classifications using terms such as “race”, biotype or biovar, serotype, etc, etc. These infrasubspecific terms have sometimes been used in an inconsistent way leading to confusion.
To give an example of a name change which met resistance at first, but which is now universally accepted. For 70 or 80 years the bacterium causing bacterial wilt of solanaceous plants was known as Pseudomonas solanacearum, then in the early 1990s there were two changes first to Burkholderia then to the new genus Ralstonia. The name Ralstonia solanacearum is now universally accepted. The establishment of the new genus Ralstonia was based on sound polyphasic taxonomy and the new names were validated by publication in what was then the International Journal of Systematic Bacteriology, now the International Journal of Systematic and Environmental Microbiology.
The nomenclature of the xanthomonad pathogens on citrus has changed and is/has been confused. This creates a difficult problem for national and international agencies. There should be uniformity in usage otherwise there will be a very confused situation.
Members are referred to the publications by Schaad et al. in 2005 and 2006 for further information.
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