The name Conyza sumatrensis (S.F. Blake) Pruski & Sancho does not exist. It is an erroneous transcription of  Conyza sumatrensis (Retz.) E.Walker var. leiotheca (S.F.Blake) Pruski & G.Sancho -- Novon 16(1): 98. 2006 [25 May 2006]. The only C. sumatrensis is based on Erigeron sumatrensis Retz. 1788 (see BHL)

According to PLANT LIST, it is clearly mentioned that Conyza sumatrensis (S.F.Blake) Pruski & G.Sancho is an accepted name and Conyza sumatrensis (Retz.) E.Walker is a synonym of Erigeron floribundus (Kunth) Sch.Bip. So, there is no doubt about there different identity. If you want to clarify more about that, you can compare the protolouges and type matterials of both plants.

I have checked the protologues and other relevant literaure, and (perhaps unbelievably for some people) there is a mistake in Plant List

Dear Jan, I'm sure there are some mistakes in TPL. That's the reason I prefer to ask these questions in this community. Anyway, I think that TPL is a good reference tool.

Thank you both. Your answers are very useful, but Jan response is more convincing for me.

Dear Carlos, according to Tropicos (http://www.tropicos.org/) and International Plant Name Index (http://www.ipni.org) the Jan's answer is correct. In my opinion these data bases are more reliable than Plant List.

Carlos, Jan Kirschner is absolutely right. Cheers!

Thank you, Juan and manuel, for the confirmation.

I would like to make another comment related to Erigeron sumatrensis Retz .:

That name appears in The Plant List as accepted name (1), but Conyza sumatrensis (Retz.) E.Walker (2) is considered synonymous with Erigeron floribundus (Kunth) Sch.Bip. TPL's argument is based on Global Compositae Checklist. In GCC Erigeron sumatrensis Retz is accepted (3), and Conyza sumatrensis (Retz.) E.Walker is one of the synonims of Erigeron floribundus (Kunth) Sch.Bip. (4)

(1) http://www.theplantlist.org/tpl1.1/record/gcc-43678

(2) http://www.theplantlist.org/tpl1.1/record/gcc-41097

(3) http://compositae.landcareresearch.co.nz/default.aspx?Page=NameDetails&TabNum=0&NameId=d903dfdc-7a39-46e4-9997-50f1cc3a90ff

(4) http://compositae.landcareresearch.co.nz/?Page=NameDetails&NameId=0FA96857-B31B-494B-9732-4C2C4E1E7195

However, in The IPNI, Erigeron sumatrensis Retz. is the basionym of Conyza sumatrensis (Retz.) E.Walker (5). This is also related to Euro Med Plantdatabase where Erigeron sumatrensis Retz appears as accepted name and Conyza sumatrensis (Retz.) E.Walker is the homotypic synonym (6).

(5) http://www.ipni.org/ipni/idPlantNameSearch.do?id=204572-1&back_page=%2Fipni%2FeditAdvPlantNameSearch.do%3Bjsessionid%3D6CE89EE945F9D46EEAFCFDBB9BB30213%3Ffind_infragenus%3D%26find_isAPNIRecord%3Dtrue%26find_geoUnit%3D%26find_includePublicationAuthors%3Dtrue%26find_addedSince%3D%26find_family%3D%26find_genus%3Derigeron%26find_sortByFamily%3Dtrue%26find_isGCIRecord%3Dtrue%26find_infrafamily%3D%26find_rankToReturn%3Dall%26find_publicationTitle%3D%26find_authorAbbrev%3D%26find_infraspecies%3D%26find_includeBasionymAuthors%3Dtrue%26find_modifiedSince%3D%26find_isIKRecord%3Dtrue%26find_species%3Dsumatrensis%26output_format%3Dnormal

(6) http://ww2.bgbm.org/EuroPlusMed/PTaxonDetail.asp?NameCache=Erigeron%20sumatrensis&PTRefFk=7000000

In conclusion, I think we would make a mistake if we apply

the name of Erigeron floribundus (Kunth) Sch.Bip. when we really want to make reference to Erigeron sumatrensis Retz., as shown in TPL.

What do you think?

General remark: This is not the first and only case, that TPL entries based on the Global Compositae Checklist are incorrect.

The treatment of Conyza floribunda in Flora of North America (http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=242421539 ) mentions, that some authors have treated Conyza floribunda and Erigeron sumatrensis as conspecific. However, FNA apparently does not join this line.

Erigeron sumatrensis Retz. was described 1788 (see http://www.ipni.org/ipni/idPlantNameSearch.do?id=204572-1 ), whereas Conyza floribunda Kunth is from 1818/1820 (see http://www.ipni.org/ipni/idPlantNameSearch.do?id=197350-1 ). So, if conspecific, sumatrensis should have priority anyway - see Art. 11.4 of the Melbourne Code (http://www.iapt-taxon.org/nomen/main.php?page=art11 )

According Tropicos (http://www.tropicos.org/Name/50294135), which follows to Pruski & Sancho (2006), Conyza floribunda is synonym of Conyza sumatrensis var. leiotheca (S.F. Blake) Pruski & G. Sancho. These last authors write that this nontypical variety is restricted to the Americas, while Conyza sumatrensis var. sumatrensis also is found widely elsewhere in the Americas and is the sole variety occurring in the Old World.

Pruski, J.F. & G. Sancho (2006) "Conyza sumatrensis var. leiotheca (Compositae: Astereae),a new combination for a common neotropical weed". Novon 16(1): 96–101.

http://www.bioone.org/doi/abs/10.3417/1055-3177%282006%2916%5B96%3ACSVLCA%5D2.0.CO%3B2

I also agree with the explanation of Dr. Kirschner