Erigeron L.
(incl. Conyza Less.)
Literature: Cronquist* (1947), Danin (1973), Marshall (1973), Danin (1976), Rivière (1987), Nesom* (1989), Wurzell (1994), Thébaud & Abbot (1995), Milović (2004), Nesom* (2006), Noyes (2000), Greuter (2003), Pruski & Sancho (2006), Sell & Murrell (2006), Strother (2006), Rand (2008).
(only references with an asterisk focus on Erigeron s.str.).
Conyza was traditionally included in Erigeron but Cronquist (1943) defended its separation from Erigeron. However, recent molecular phylogenetic research showed that Conyza is in fact polyphyletic and nested within Erigeron (Noyes 2000). See also Greuter (2003). Nesom (2008), on the contrary, maintains Conyza as a genus separate from Erigeron.
Erigeron is a taxonomically difficult genus and counts – in its wide sense – at least 400 species. It is almost cosmopolitan but predominates in temperate regions. Most representatives of the former genus Conyza are originally native to subtropical and warm-temperate areas but several are notorious weeds and have spread far beyond their original distribution range. One species, Erigeron acer L., is native in Belgium.
The identification of species formerly included in Conyza is often critical. The number of lobes of the tubular florets is a valuable character (for instance to distinguish between Erigeron canadensis and E. floribundus) but is best investigated on fresh material. Ligular (peripheral) florets are always present in Erigeron (incl. Conyza). They are female and filiform and the laminae are sometimes inconspicuous. Central florets are tubular, yellow and bisexual.
The systematics of the species formerly included in Conyza are still insufficiently understood and treatments in the New World are not in accordance with those in Europe (and elsewhere in the Old World). Especially the application of the binomials Erigeron floribundus and E. sumatrensis (incl. numerous putative synonyms) is still confusing (see for instance Pruski & Sancho 2006, Strother 2006).
1 Laminae of ligular florets absent or inconspicuous, at most 1 mm long (if present, capitulae less than 4 mm wide at anthesis). Ligular florets usually more numerous than tubular florets (“Conyza”) === 2
1 Laminae of ligular florets always developed, at least 2 mm long. Capitulae at least 6 mm wide at anthesis. Ligular florets usually less numerous than tubular florets (Erigeron s.str.) === 5
2 Leaves nearly glabrous above or with scattered hairs along midrib only, margins distinctly ciliate (at least in lower third, ciliae often 1 mm long). Involucral bracts nearly glabrous. Capitulae ca. 2-4 mm wide at anthesis === 3
2 Leaves densely shortly pubescent above, margins hardly ciliate (ciliae, if present, very short). Involucral bracts softly hairy. Capitulae ca. 4-10 mm wide at anthesis === 4
3 Inner (tubular) florets mostly 4-lobbed, ca. 10-15 per capitulum. Ligules always present, white, distinctly exceeding involucre. Inflorescence often cylindric, much longer than wide. Plant annual, usually yellowish-green, stem not hirsute === 3. Erigeron canadensis
3 Inner (tubular) florets mostly 5-lobed, ca. 4-6 per capitulum, rarely more. Ligules absent or rudimentary, not exceeding involucre. Inflorescence much broader, usually only slightly longer than wide. Plant annual or short-lived perennial, dull greyish-green, stem hirsute === 4. E. floribundus
4 Leaves narrow, less than 5 mm wide, the uppermost linear. Inflorescence often with greatly enlarged side branches overtopping the main axis. Apex of involucral bracts often purplish. Capitulae ca. 6-10 mm at anthesis. Pappus brownish === 2. E. bonariensis
4 Most leaves wider, 3-20 mm wide, never linear. Side branches of the inflorescence not overtopping the main axis. Apex of involucral bracts not purplish. Capitulae ca. 4-6 mm at anthesis. Pappus whitish === 6. E. sumatrensis
5 Stem procumbent to ascending. Lower leaves often with a pair of lateral lobes. Short-lived perennial === 5. E. karvinskianus
5 Stem erect. Leaves sometimes coarsely dentate but never lobed. Annual to perennial === 6
6 Laminae of ligular florets inconspicuous, lilac, 2-4 mm long and hardly exceeding involucre. Lower cauline leaves narrowly elliptical, entire. Basal leaves present at anthesis (native) === E. acer
6 Laminae of ligular florets usually conspicuous, white to pale blue, ca. 6-8 mm long (rarely less). Lower cauline leaves ovate-lanceolate, usually coarsely dentate (rarely nearly entire). Basal leaves withered at anthesis === 1. E. annuus
Additional aliens: Erigeron pampeanus Parodi (syn.: Conyza pampeana (Parodi) Cabrera) (S-Am., wool alien), E. philadelphicus L. (syn.: Stenactis philadelphica (L.) Hayek) (N-Am., garden escape) and E. speciosus (Lindl.) DC. (syn.: Stenactis speciosa Lindl.) (N-Am., garden escape). Records of Erigeron lechleri Schultz-Bip. (syn.: Conyza lechleri (Schultz-Bip.) Cabrera) (see Lambinon & al. 2004) were all erroneous and referable to Dittrichia graveolens (Verloove & Lambinon 2008).
Literature:
Cronquist A. (1947) Revision of the North American species of Erigeron, north of Mexico. Brittonia 6: 121-300.
Danin A. (1973) On three adventive species of Conyza (Compositae) in Greece. Candollea 31: 107-109.
Danin A. (1976) Notes on four adventive Composites in Israel. Notes Roy. Bot. Gard. Edinburgh 34: 403-410.
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Greuter W. (2003) The Euro+Med treatment of Astereae (Compositae) – generic concepts and required new names. Willdenowia 33: 45-47.
Marshall J.B. (1973) Conyza-taxa found in Britain. Watsonia 9: 372-373.
Milović M. (2004) Naturalised species of the genus Conyza Less. (Asteraceae) in Croatia. Acta Bot. Croat. 63(2): 147-170.
Nesom G.L. (1989) Infrageneric taxonomy of New World Erigeron (Compositae: Astereae). Phytologia 67: 67-93.
Nesom G.L. (2006) Erigeron. In: Flora of North America Editorial Committee (eds.), Flora of North America, vol. 20. Oxford University Press, New York-Oxford: 256-348.
Nesom G.L. (2018) Erigeron floribundus and E. sumatrensis (Asteraceae) in the USA and Mexico. Phytoneuron 2018-27: 1–19. [available online at: http://www.phytoneuron.net/2018Phytoneuron/27PhytoN-Conyzasumatrensis.pdf]
Noyes R.D. (2000) Biogeographical and evolutionary insights on Erigeron and allies (Asteraceae) from ITS sequence data. Plant Systematics and Evolution 220: 93-114
Pruski J.F. & Sancho G. (2006) Conyza sumatrensis var. leiotheca (Compositae: Astereae), a new combination for a common neotropical weed. Novon 16: 96-101.
Pyke S. (2020) Conyza (Asteraceae): una valoración crítica basada en las poblaciones de Cataluña, España. Collect. Bot. 39: e005. [available online at: http://collectaneabotanica.revistas.csic.es/index.php/collectaneabotanic...
Rand M. (2008) Difficulties with Conyza (Fleabanes). BSBI News 108: 40-43.
Rivière G. (1987) Sur quelques Composées adventices de Bretagne. Le Monde des Plantes 427-428: 1-5.
Sell P. & Murrell G. (2006) Flora of Great Britain and Ireland. Vol. 4 Campanulaceae – Asteraceae. Cambridge University Press, Cambridge: XXVIII + 624 p.
Strother J.L. (2006) Conyza. In: Flora of North America Editorial Committee (eds.), Flora of North America, vol. 20. Oxford University Press, New York-Oxford: 348-350.
Thébaud Ch. & Abbot R.J. (1995) Characterization of invasive Conyza species (Asteraceae) in Europe: quantitative trait and isozyme analysis. Amer. J. Bot. 82: 360-368.
Wurzell B. (1994) A history of Conyza in London. BSBI News 65: 34-38.