Cuscuta is a genus with 194 species (Costea & al. 2015), distributed in warm-temperate, subtropical and tropical regions of the world. Only two species are native in Belgium: Cuscuta epithymum (L.) L. and C. europaea L. (Lambinon & al. 2004). A third species, Cuscuta epilinum Weihe, an obligate weed of linseed fields, is an extinct archaeophyte in Belgium. It was last seen as such in 1938. In 2011 it was rediscovered on Guizotia abyssinica and Linum usitatissimum on the gravelly banks of river Maas near Maaswinkel (see: http://waarnemingen.be/waarneming/view/61235961 and Barendse 2012).
Some authors tend to segregate Cuscuta in several units (Hadac & Chrtek 1970, 1973; Chrtek 2000) but this is not supported by recent molecular research (see for instance Stefanovic & Costea 2008). Costea & al. (2015) proposed a new infrageneric classification of the genus that is congruent with molecular data. They accept 194 species in 4 subgenera and 18 sections.
All current information on the genus Cuscuta is stored at the 'Cuscuta Digital Atlas' at: https://www.wlu.ca/page.php?grp_id=2147&p=8968.
1. Style (apparently) 1. Corolla lobes short, ca. 1/3 as long as the cylindrical corolla tube === Cuscuta lupuliformis
1. Styles 2. Corolla lobes longer, usually longer than or ca. as long as the campanulate corolla tube (rarely only ½ as long) === 2
2. Stigmas elongate. Stem pinkish, reddish or greenish white. Capsule regularly dehiscent at base === 3
2. Stigmas capitate. Stem yellow or orange. Capsule indehiscent or irregularly dehiscent === 5
3. Styles (incl. stigmas) longer than ovary (native) === C. epithymum
3. Styles at most as long as ovary === 4
4. Flowers all 5-merous. Calyx lobes acute === C. epilinum
4. Flowers mostly 4-merous (rarely some 5-merous). Calyx lobes obtuse (native) === C. europaea
5. Calyx much shorter than corolla tube (corolla tube 1,5-3x as long as calyx). Pedicels mostly at least as long as corolla (inflorescence lax) === 6
5. Calyx as long as corolla tube. Pedicels short, mostly shorter than corolla (inflorescence dense) === C. campestris
6. Corolla lobes deflexed. Corolla scales narrow, long fimbriate === C. gronovii
6. Corolla lobes erect. Corolla scales oblong-ovate, with short fimbriae === C. suaveolens
Additional alien: Cuscuta planiflora Ten.) (Medit., grain alien).
Literature
Barendse R. (2012) Cuscuta epilinum (vlaswarkruid) voor het eerst sinds 1938 weer waargenomen in België, nu als neofyt. Dumortiera 101: 48-49. [available online at: http://www.br.fgov.be/DUMORTIERA/DUM_101/Dum%20101_48-49_Cuscuta_epilinum_Barendse.pdf
Chrtek J. (2000) Cuscutaceae. In: Slavík B. (ed.), Kvetena Ceské Republiky, vol. 6. Academia, Praha: 169-178.
Costea M., García M. & Stefanović S. (2015) A phylogenetically based infrageneric classification of the parasitic plant genus Cuscuta (dodders, Convolvulaceae). Syst Bot. 40: 269-285. [available online at: https://legacy.wlu.ca/documents/60664/Phylogenetic_classification_Cuscuta.pdf]
Feinbrun N. (1972) Cuscuta. In: Tutin T.G. & al. (eds.), Flora Europaea, vol. 3. Cambridge University Press, Cambridge: 74-77.
García M.A., Costea M., Kuzmina M. & Stefanović S. (2014) Phylogeny, character evolution, and biogeography of Cuscuta (dodders; Convolvulaceae) inferred from coding plastid and nuclear sequences. Am. J. Bot. 101(4): 670-690.
Hadač E. & Chrtek J. (1970) Notes on the taxonomy of Cuscutaceae. Folia Geobot. Phytotax. 5: 443-445.
Hadač E. & Chrtek J. (1973) Some further notes on the taxonomy and nomenclature of Cuscutaceae. Folia Geobot. Phytotax. 8: 219-221.
Jauzein P. (1995) Flore des champs cultivés. INRA, Paris: 898 p.
Lambinon J., Delvosalle L., Duvigneaud J. (avec coll. Geerinck D., Lebeau J., Schumacker R. & Vannerom H. (2004) Nouvelle Flore de la Belgique, du Grand-Duché de Luxembourg, du Nord de la France et des Régions voisines (Ptéridophytes et Spermatophytes). Cinquième édition. Jardin botanique national de Belgique, Meise: CXXX + 1167 p.
Stefanović S. & Costea M. (2008) Reticulate evolution in the parasitic genus Cuscuta (Convolvulaceae): over and over again. Botany 86(8): 791-808.
Van Ooststroom S.J. (1942) Het geslacht Cuscuta in Nederland. Ned. Kruidk. Arch. 52: 159-210.
Yuncker T.G. (1932) The genus Cuscuta. Mem. Torrey Bot. Club 18: 113-331.