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Taxonomy of species in Rhododendron subsection Ledum
Kristian Theqvist, 25-Mar-2013, rev. 005

The inclusion of the ledum taxa in genus Rhododendron has led to some controversial naming issues. I have been somewhat confused on what names I should use in my documentation for the species and subspecies in subsection Ledum. I'll try now to clarify the situation by introducing a cross-reference table with Ledum names that are or have been in use. I made the table to guide in using valid names and to more easily match previously used names to the new ones.

Valid name in subsection Ledum of section Rhododendron subgenus Rhododendron genus Rhododendron
Other names that have been in use in various publications in subsection Ledum of genus Rhododendron
Old names in genus Ledum
Rhododendron tomentosum Harmaja, 1990.
R. tomentosum subsp. tomentosum
•  Used in the classification of genus Rhododendron by Chamberlain, D. et al., 1996.
R. tomentosum (Stokes) Harmaja, 1990.
•  An erroneous form.
R. palustre (L.) Kron & Judd, 1990.
•  An illegitimate later homonym.
Ledum tomentosum Stokes, 1812.
•  An illegitimate name.
L. palustre L. var. dilatatum Wahlenb., 1812.
L. palustre L., 1753.
R. subarcticum Harmaja, 1990.
R. tomentosum subsp. decumbens (Aiton) Elven & D.F. Murray, 2008.
R. tomentosum Harmaja subsp. subarcticum (Harmaja) Wallace, 1992.
•  An illegitimate name that has been used in the classification of genus Rhododendron by Chamberlain, D. et al., 1996.
L. palustre L. subsp. decumbens (Ait.) Hultén, 1930.
L. decumbens (Aiton) Lodd. ex Steud, 1841.
L. palustre L. var. decumbens Aiton, 1789.
R. diversipilosum (Nakai) Harmaja, 1999.  
L. palustre L. subsp. diversipilosum (Nakai) H. Hara, 1956.
L. palustre L. var. diversipilosum Nakai, 1917. [basionym]
L. palustre L. var. nipponicum Nakai ?
L. nipponicum (Nakai) Tolm.
•  An illegitimate name.
R. groenlandicum (Oeder) Kron & Judd, 1990.  
L. palustre L. subsp. groenlandicum (Oeder) Hultén, 1948.
L. latifolium Jacq., 1789.
L. groenlandicum Oeder, 1771. [basionym]
R. hypoleucum (Kom.) Harmaja, 1990.  
L. hypoleucum Kom., 1916. [basionym]
R. neoglandulosum Harmaja, 1990.   L. glandulosum Nutt., 1843.
R. subulatum (Nakai) Harmaja, 2002.  
L. subulatum (Nakai) A.P.Khokhr. & Mazurenko, 1991.
L. palustre L. subsp. angustissimum Vorosch., 1978.
L. palustre L. var. angustum E. Busch. ?
L. palustre L. var. subulatum Nakai, 1917.
R. tolmachevii Harmaja, 1990.  
L. macrophyllum Tolm., 1953.
L. palustre L. var. yesoënse Nakai, 1922.?
L. maximum (Nakai) A.P.Khokhr. & Mazurenko, 1991. ?
L. palustre var. maximum Nakai, 1917. ?
R. ×columbianum (Piper) Harmaja, 1990.
•  Most probably R. groenlandicum × R. neoglandulosum.

Note:
Based on flow cytometry tests the species is not a cross of two Ledum species and the name of the species should be changed to R. columbianum.
Kristian Theqvist 20-Dec-2011.
See information below the table. *
 
 
L. columbianum Piper, 1906. [basionym]
L. glandulosum Nutt var. columbianum (Piper) Hitchc.
 
 
L. palustriforme A.P.Khokhr. & Mazurenko.

* Additional information on R. columbianum
The Ploidy Levels of Ledum species were tested at the University of Coimbra in Portugal by a team led by Dr. João Loureiro. The collection of samples from the Ledum species was coordinated by John Perkins, American Rhododendron Society, Chapter 6, Salem, New Hampshire, USA.
In the tests R. groenlandicum and R. neoglandulosum were found to be tetraploids and R. ×columbianum was found to be a diploid. This means that R. ×columbianum cannot be a cross of the two Ledum species. It is a true species, not a cross, and should accordingly be called R. columbianum without the multiplication sign ×.
Written by Kristian Theqvist, 20-Dec-2011
Link to the poster on Ploidy level estimations in decidious and elepidote hybrids of Rhododendron.
Link to the results of tested ploidy levels of Ledum species: http://rosebayblog.blogspot.com/search/label/ledum.

References
1. Kron, K. A. & Judd, W. S . 1990. Phylogenetic relationships within the Rhodoreae (Ericaceae) with specific comments on the placement of Ledum. Syst. Bot. 15: 57–68.
2. Harmaja, H. 1991. Taxonomic notes on Rhododendron subsection Ledum (Ledum, Ericaceae), with a key to its species. Ann. Bot. Fennici 28: 171-173.
3. Harmaja, H. 1999. Rhododendron diversipilosum, comb. nov. (Ericaceae). Ann. Bot. Fennici 35: 263-264.
4. Harmaja, H. 2002. Rhododendron subulatum, comb. nova (Ericaceae). Ann. Bot. Fennici 39: 183-184.
5. Kihlman, B. 2004. Hybrids Between Ledums and Lepidote Rhododendrons. J. Amer. Rhod Soc. 58(2):74-81
6. Elven & D.F. Murray. 2008. Rhododendron tomentosum subsp. decumbens (Aiton) Elven & D.F. Murray. J. Bot. Res. Inst. Texas 2: 441. (Annotated Checklist of the Panarctic Flora (PAF), http://nhm2.uio.no/paf/740105b)
7. Voss, D.H. 2011. A Key for Ledum in Rhododendron. Journal of the American Rhododendron Society, 65(2):99-101.