Abstract

Parasitic plants are often associated with agricultural, forestry and grassland economic losses, but they are also keystone species in their natural ecosystems. Cuscuta (Convolvulaceae) and Cassytha (Lauraceae) are parasitic plants which have evolved similar stem habit and morphology, rendering them remarkably similar during the vegetative stage. Since both genera are common in the tropics, misidentifications are frequent, which is detrimental for understanding their geographical distribution, biology and ecology, as well as to the development of adequate control or conservation practices. We here present a practical identification guide for a clear and accurate distinction between Cuscuta and Cassytha, using stems and reproductive structures of both fresh plants and herbarium specimens, aimed at taxonomists and agricultural experts. An identification key, a comparative table, detailed descriptions and illustrations are included to facilitate genus recognition. The current practice of macroscopic observation of the filiform stems, on which many professionals rely, may not be enough to distinguish the two genera. The analysis of stem micromorphology, and/or of the flower or fruit morphology, are necessary for a conclusive identification.

Key words
dodders; invasive species; love vine; morphology; weeds

Resumo

Plantas parasitas são frequentemente associadas a perdas agrícolas e silvo pastoris. Entretanto, elas são espécies chave na dinâmica dos ecossistemas. Os gêneros Cuscuta (Convolvulaceae) e Cassytha (Lauraceae) são ambos parasitas filiformes que têm evoluído em habitats similares, sendo a morfologia dos seus ramos notavelmente similar durante sua fase vegetativa. Devido ao fato de ambos os gêneros serem comuns nos trópicos, erros de identificação são frequentes, o que prejudica a compreensão de suas distribuições geográficas, biologia, e ecologia, bem como o desenvolvimento de medidas de controle ou práticas de manejo e conservação adequadas. Esse estudo oferece um detalhado guia de identificação para taxonomistas e cientistas agrários, usando ramos vegetativos e estruturas reprodutivas de representantes dos dois gêneros, tanto de coleções vivas como herborizadas. Estão incluídas uma chave de identificação, uma tabela comparativa, descrições detalhadas, fotografias e ilustrações como subsídio para as identificações. A atual prática de observação macroscópica dos ramos filiformes, na qual muitos profissionais se apoiam, mostra-se insuficiente para distinguir os dois gêneros. A análise micro morfológica dos ramos e/ ou das flores ou frutos é necessária para uma identificação conclusiva.

Palavras-chave
cipó-chumbo; plantas invasoras; fios-de-ovos; morfologia; plantas daninhas

Parasitic plants are often overlooked in floristic studies although they are common in many natural ecosystems and may have a significant detrimental economic impact as agricultural pests or noxious weeds (Parker & Riches 1993Parker C & Riches CR (1993) Parasitic weeds of the world. Biology and control. CAB International, Wallingford. 332p.; Costea & Tardif 2006Costea M & Tardif FJ (2006) The biology of Canadian weeds. 133. Cuscuta campestris Yuncker, C. gronovii Willd. ex Schult., C. umbrosa Beyr. ex Hook., C. epithymum (L.) L. and C. epilinum Weihe. Canadian Journal of Plant Science 86: 293-316.; Teixeira-Costa 2016Teixeira-Costa L (2016) Plantas parasitas. In Peña EMH (org.) Botânica no inverno. Instituto de Biociências da Universidade de São Paulo, São Paulo. Pp. 131-137.). As agricultural pests, the infestation by some parasitic plants can lead to severe yield losses, increased harvesting costs and contamination of commercial seed lots (Parker & Riches 1993Parker C & Riches CR (1993) Parasitic weeds of the world. Biology and control. CAB International, Wallingford. 332p.; Dawson et al. 1994Dawson JH, Musselman LJ, Wolswinkel P & Dörr I (1994) Biology and control of Cuscuta. Reviews of Weed Science 6: 265-317., Costea & Tardif 2006Costea M & Tardif FJ (2006) The biology of Canadian weeds. 133. Cuscuta campestris Yuncker, C. gronovii Willd. ex Schult., C. umbrosa Beyr. ex Hook., C. epithymum (L.) L. and C. epilinum Weihe. Canadian Journal of Plant Science 86: 293-316.; Ashigh & Marquez 2010Ashigh J & Marquez E (2010) Dodder (Cuscuta spp.) biology and management. NM State University, Cooperative Extension Service, College of Agricultural, Consumer and Environmental Sciences, New Mexico. 4p.). At an organismal scale, they directly or indirectly affect the physiology of their hosts (Dawson et al. 1994Dawson JH, Musselman LJ, Wolswinkel P & Dörr I (1994) Biology and control of Cuscuta. Reviews of Weed Science 6: 265-317., Nelson 2008Nelson SC (2008) Cassytha filiformis. Plant Desease 42: 1-10.); cause behavioral modifications in herbivores (Gómez 1994Gómez JM (1994) Importance of direct and indirect effects in the interaction between a parasitic angiosperm (Cuscuta epithymum) and its host plant (Hormathophylla spinosa). Oikos 71: 97-106.) and animal poisoning (Barcellos 1990Barcellos JM (1990) A cultura da alfafa. Embrapa Cerrados-Comunicado Técnico (INFOTECA-E), Planaltina. 12p.); and are involved in pathogen transmission (Dawson et al. 1994Dawson JH, Musselman LJ, Wolswinkel P & Dörr I (1994) Biology and control of Cuscuta. Reviews of Weed Science 6: 265-317.; Nelson 2008Nelson SC (2008) Cassytha filiformis. Plant Desease 42: 1-10.). In natural plant communities, parasitic plants act as ecosystem engineers and keystone species, increasing the diversity of species in plant communities and modifying abiotic factors (e.g., Pennings & Callaway 1996Pennings SC & Callaway RM (1996) Impact of a parasitic plant on the structure and dynamics of Salt Marsh vegetation. Ecology 77: 1410-1419.; Watson 2009Watson DM (2009) Parasitic plants as facilitators: more Dryad than Dracula? Journal of Ecology 97: 1151-1159.). Another positive aspect of parasitic plants is their wide use in traditional medicine systems, for example in Asia (Visaka et al. 2010; Donnapee et al. 2014Donnapee S, Li J, Yang X, Ge AH, Donkor PO, Gao XMY & Chang YX (2014) Cuscuta chinensis Lam.: A systematic review on ethnopharmacology, phytochemistry and pharmacology of an important traditional herbal medicine. Journal of Ethnopharmacology 157: 292-308.), Bahamas, West Indies and Polynesia (Nelson 2008Nelson SC (2008) Cassytha filiformis. Plant Desease 42: 1-10.). In recent years, some parasitic plants have been proposed as biological control agents for green invasive plants (Nelson 2008Nelson SC (2008) Cassytha filiformis. Plant Desease 42: 1-10.; Yu et al. 2008Yu H, Yu FH, Miao SL & Dong M (2008) Holoparasitic Cuscuta campestris suppresses invasive Mikania micrantha and contributes to native community recovery. Biological Conservation 141: 3653-2661.; Cirocco et al. 2018Cirocco RM, Facelli JM & Watling JR (2018) A native parasitic plant affects the performance of an introduced host regardless of environmental variation across field sites. Functional Plant Biology 45: 1128-113.).

Cuscuta L. and Cassytha L. are the only parasitic lineages that evolved within Convolvulaceae and Lauraceae, respectively (Stefanović & Olmstead 2004Stefanović S & Olmstead RG (2004) Testing the phylogenetic position of a parasitic plant (Cuscuta, Convolvulaceae, Asteridae): Bayesian Inference and the Parametric Bootstrap on data drawn from three genomes. Systematic Biology 53: 384-399., APG IV 2016APG IV - Angiosperm Phylogeny Group IV (2016) An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG IV. Botanical Journal of the Linnean Society 181: 1-20., Nickrent 2020Nickrent DL (2020) Parasitic angiosperms: How often and how many? Taxon 69. Available at < https://onlinelibrary.wiley.com/doi/abs/10.1002/tax.12195>. Access on 24 May 2020. DOI: <https://doi.org/10.1002/tax.12195>.). Although the genera are very distantly related to one another (APG IV 2016APG IV - Angiosperm Phylogeny Group IV (2016) An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG IV. Botanical Journal of the Linnean Society 181: 1-20.; Nickrent 2020Nickrent DL (2020) Parasitic angiosperms: How often and how many? Taxon 69. Available at < https://onlinelibrary.wiley.com/doi/abs/10.1002/tax.12195>. Access on 24 May 2020. DOI: <https://doi.org/10.1002/tax.12195>.), they evolved a similar habit as stem parasitic vines and represent a remarkable example of convergent evolution (Heide-Jørgensen 2008Heide-Jørgensen H (2008) Parasitic flowering plants. Brill., Nickrent 2020Nickrent DL (2020) Parasitic angiosperms: How often and how many? Taxon 69. Available at < https://onlinelibrary.wiley.com/doi/abs/10.1002/tax.12195>. Access on 24 May 2020. DOI: <https://doi.org/10.1002/tax.12195>.): their leaves are reduced to minute scales, and stems are filiform, greenish, yellow, orange or reddish, dextrorsely twining around the stems of the hosts, and eventually covering them. Haustoria develop on the inner side of the stem coils (Dawson et al. 1994Dawson JH, Musselman LJ, Wolswinkel P & Dörr I (1994) Biology and control of Cuscuta. Reviews of Weed Science 6: 265-317.; Heide-Jørgensen 2008Heide-Jørgensen H (2008) Parasitic flowering plants. Brill.). The confusion between Cuscuta and Cassytha, due to their morphological similarities, is reflected in these two genera sharing in the tropics the vernacular name “love vine”.

The two genera overlap geographically in the southern hemisphere, namely in tropical regions, where numerous species of Cuscuta have diversified (Yuncker 1932Yuncker GT (1932) The genus Cuscuta. Memoirs of the Torrey Botanical Club 18: 109-331.; Costea et al. 2015Costea M, García MA & Stefanović S (2015) A phylogenetically based infrageneric classification of the parasitic plant genus Cuscuta (Dodders, Convolvulaceae). Systematic Botany 40: 269-285.) and one species of Cassytha in particular, C. filiformis L., is ubiquitous (Weber 1981Weber JZ (1981) A taxonomic revision of Cassytha (Lauraceae) in Australia. Journal of the Adelaide Botanic Garden 3: 187-262.). Especially during the vegetative stage, their morphologically similar stems, either fresh or dried, can make the genus separation challenging (Heide-Jørgensen 2008Heide-Jørgensen H (2008) Parasitic flowering plants. Brill.; Tennakoon et al. 2016Tennakoon KU, Rosli R & Le QV (2016) Biology of aerial parasitic vines in Brunei Darussalam: Cuscuta and Cassytha. Scientia Bruneiana 15: 58-64.). Indeed, we have often noted that in tropical herbaria more than half of the Cuscuta specimens are misidentified as Cassytha. The frequent confusion between the two genera has hindered the adequate delimitation of their geographical distribution ranges (e.g., Nelson 2008Nelson SC (2008) Cassytha filiformis. Plant Desease 42: 1-10.), understanding their biology and ecology (including host ranges), and ultimately developing appropriate pest control management protocols.

Guidelines to assist both taxonomists and agricultural or forestry experts to correctly identify these two genera are not available. Some taxonomic works focusing on one or both genera have briefly mentioned some diagnostic features (Schroeder 1967Schroeder CA (1967) The stem parasite Cassytha filiformis a botanical relative of avocado. California Avocado Society 51: 159-160.; Kuijt 1969Kuijt J (1969) The biology of parasitic flowering plants. The biology of parasitic flowering plants. University of California Press, Berkeley. 246p.; Weber 1981Weber JZ (1981) A taxonomic revision of Cassytha (Lauraceae) in Australia. Journal of the Adelaide Botanic Garden 3: 187-262.; Dawson et al. 1994Dawson JH, Musselman LJ, Wolswinkel P & Dörr I (1994) Biology and control of Cuscuta. Reviews of Weed Science 6: 265-317.; Baitello 2003Baitello JB (2003) Lauraceae. In: Wanderley MGL et al. (eds.) Flora Fanerogâmica do estado de São Paulo. Instituto de Botânica, São Paulo. Vol. 3, pp. 149-224.; Heide-Jørgensen 2008Heide-Jørgensen H (2008) Parasitic flowering plants. Brill.; Kropf et al. 2015Kropf MS, Quinet A & Andreata RHP (2015) Lauraceae das restingas do estado do Rio de Janeiro, Brasil. Iheringia Série Botânica 70: 287-308.). However, even in more comprehensive works (e.g., Dawson et al. 1994Dawson JH, Musselman LJ, Wolswinkel P & Dörr I (1994) Biology and control of Cuscuta. Reviews of Weed Science 6: 265-317.; Heide-Jørgensen 2008Heide-Jørgensen H (2008) Parasitic flowering plants. Brill.; Nelson 2008Nelson SC (2008) Cassytha filiformis. Plant Desease 42: 1-10.; Tennakoon at al. 2016), no comparative morphological details were provided for the separation of the two genera, especially when using (dried) herbarium specimens, which are routinely used in systematic, floristic and biogeographic studies. Therefore, our objective is to provide a detailed guide for the accurate identification of Cuscuta and Cassytha, supporting research studies and agroforestry management strategies.

Morphological data were generated from fieldwork, conducted between 2010 and 2020, in Brazil (Bahia, Minas Gerais, Piauí, Rio de Janeiro, and São Paulo) and Mexico (Nayarit, Colima, Jalisco, Michoacán, Puebla, Oaxaca, Veracruz). In addition, thousands of herbarium specimens were examined and annotated from the following institutions: AAU, ABH, ALTA, ARAN, ARIZ, ASU, B, BAB, BC, BCN, BM, BOL, BORD, BR, BRIT, CAL, CANB, CAS, CEN, CHR, CHSC, CIIDIR, CIMI, COI, CTES, DAO, DIAM, E, ESA, F, FT, G, GH, H, HB, HRCB, HUEFS, HUFU, HUSC, HUJ, IAC, IEB, IND, J, JACA, JE, JEPS, K, L, LAU, LD, LE, LIL, LL, LP, LPB, LPS, M, MA, MACB, MAF, MBM, MEL, MERL, MEXU, MGC, MICH, MO, MPU, MSTR, NAP, NBG, NMC, NY, OAC, OKLA, OSC, OXF, P, PACA, PMSP, PRE, QCNE, QFA, R, RB, RNG, RSA, S, SALA, SAM, S, SD, SEV, SGO, SI, SJRP, SP, SPF, SPSF, TEX, TRT, TRTE, UA, UB, UBC, UCR, UCT, UEC, UNM, UPCB, UPRRP, UPS, US, USAS, VAL, W, WTU, and XAL (Herbaria acronyms follow Thiers 2018-continuously updated).

Macromorphological characters were photographically documented during fieldwork. Micromorphological features were characterised by study of herbarium specimens, with structures analysed in both dried and rehydrated conditions, and imaged under Light and Scanning Electron Microscopy. For observations and photographic records under Light Microscopy, Zeiss Opticam Stemi SV6 and Nikon SMZ1500 stereomicroscopes were used, with application of Extended Depth of Focus (EDF). Scanning Electron Microscopy measurements and photographs were taken using a Hitachi SU1510 variable pressure scanning electron microscope, at 10 kV. Prior to examination, samples were sputter-coated with 30 nm of gold using an Emitech K550 (Emitech, Ltd. Ashfort, UK).

A comparison table was prepared which includes both vegetative and reproductive characters (Tab. 1), and identification keys that can be used for different ontogenetic stages were prepared. Full morphological descriptions of the genera are also provided, to assist in the preparation of floristic and taxonomic works.

Identification Key

Fresh sample in vegetative state (magnification 30× or more required). (For additional details see Tab. 1).

• 1. Stems greenish to orange with fine longitudinal rugae or ridges; trichomes present or absent ............................................................................ Cassytha

• 1’. Stems yellowish to orange or reddish-purple (greenish in seedlings), usually smooth (see Table 1); trichomes absent ................... Cuscuta

Dry or fresh sample at reproductive stage (magnification 30× or more required)

• 1. Inflorescences racemose (usually spike), few-flowered. Flowers white-creamy or greenish, 3-merous with perianth elements free. Stamens 9; staminodes may occur...................Cassytha

• 1’. Inflorescences monochazial cymes with numerous flowers, usually compound and forming dense aggregations of flowers. Flowers white, white-creamy, yellowish, pink to reddish, 4-5-merous (rarely 3-merous), with perianth elements fused. Stamens 4-5, all fertile ................... Cuscuta

Genus description

Cassytha L. Species Plantarum 1: 35. 1753. Type: Cassytha filiformis L. INDIA: Type: Osbeck s.n. (LINN n.v.). Figs. 1a-g; 2a-c; 3a-e.

Figure 1
Morphological characterization of Cassytha and Cuscuta, using living and dried specimens, imaged with stereomicroscopy. a-i. Cassytha filiformis - a. fragment of herbarium specimen; b. rehydrated stem from herbarium specimen, note longitudinal ridges (white arrows indicate lines of stomata); c. dried flower; d. dried fruit; e. inflorescence of living plant showing pubescent peduncle (also note the greenish color of stems); f. top overview of 3-merous flower; g. rehydrated flower viewed under stereomicroscope; h. side view of flower in the field. i. Fresh, immature fruits; j. Cuscuta racemosa var. miniata - fragment of herbarium specimen. k. rehydrated stem of Cuscuta gronovii (note the absence of hairs and scarce stomata indicated with asterisks); l. Cuscuta platyloba - dried inflorescence. m-p. Cuscuta racemosa var. miniata - m. dried flower; n. capsule; o. top view of 5-merous flower; p. habit. (a. A. Macedo 4166; b. O.J. Pereira 410; c. E.S.G. Guarino & B.M.T. Walter 579; d. B. Stannard & T. Silva 52807; j. I. Cordeiro et al. CFSC 8211; k. M. Costea s.n.; l. A.A. Conceição 893; m. H.F. Leitão-Filho 27718; n. E. Pereira 1517). Images: a, c, d. S.S. Silva 2018; b, g, k, M. Costea; e. R. Simão-Bianchini; f, g. S.E. Martins 2007; i. M.L. Brotto 2016; j, l-p. S.S. Silva 2018-2019. Scale bars- a, i. 1 cm; b. 0.5 mm; c, d, l. 2 mm; g. 1 mm; k. 0.5 mm; m, n. 1 mm.

Figure 2
Micromorphology of Cassytha and Cuscuta stems, using rehydrated herbarium specimens and imaging under scanning electron microscopy. a-c. Cassytha filiformis - a. stem at low magnification, showing longitudinal ridges and lines of stomata; b. detail of stomata rows; c. stoma and epicuticular wax. d-g. Cuscuta gronovii. D. general view, note the smooth epidermis and only one stoma present (indicated with arrow); e. epidermis surface and stoma detail; f-g. haustorial stems with multicellular protuberances bearing stomata; f. general view; g. multicellular protuberance viewed from the top (arrow indicates stoma). (a-c M.R.R. Vidal 325; d-g Costea s.n.). Scale bars - a. 0.5 mm; b. 200 mm; c. 15 mm; d. 0.4 mm; e, g. 100 mm; f. 0.5 mm.

Figure 3
Schematic representation of the main diagnostic reproductive characteristics of Cassytha and Cuscuta. a-e. Cassytha filiformis. a. inflorescence. b. longitudinal section through a flower, showing the disposition of androecium and gynoecium. c. ovary. d. fruit covered by the hypanthium. e. fruit. f-j. Cuscuta racemosa. f. dissected calyx. g. dissected corolla showing infra-staminal scales. g-h. part of an inflorescence. i. ovary. j. fruit with persistent corolla and calyx. (a-e E.S.G. Guarino & B.M.T. Walter 579; f-j G. Hashimoto 1950). Scale bars 1mm. Illustration by Stephanie Karoline de Oliveira (2020).

Perennial hemiparasitic; stems filiform, greenish, or orange, glabrous to tomentose. Leaves alternate, reduced to scales, more evident at stem apices. Inflorescences axillary, few-flowered, racemose, commonly spikes, sometimes racemes or panicles, often reduced to heads. Flowers bisexual, 3-meous, sessile or short-pedicellate; 1 bract and 2 ovate bracteoles with ciliate apex; perianth ovoid to urceolate, distally contracted after anthesis; perianth elements 6, in 2 whorls: outer 3 smaller, inner 3, larger. Stamens 9, arranged in 3 whorls, rarely those of the second whorl reduced to staminodes; filaments of 3rd whorl with 2 subsessile glands; anthers 2-locular; cells of 1st and 2nd whorls introrse, those of the 3rd whorl extrorse, dehiscing via flaps (apically hinged valves); staminodes 3, in a 4th whorl. Ovary globose-elliptic, glabrous. Fruit a drupe, enclosed in the persistent and dilated floral tube, presenting a small apical opening; seeds 1 per fruit, with 2 fleshy cotyledons (Weber 1981Weber JZ (1981) A taxonomic revision of Cassytha (Lauraceae) in Australia. Journal of the Adelaide Botanic Garden 3: 187-262.; Baitello 2003Baitello JB (2003) Lauraceae. In: Wanderley MGL et al. (eds.) Flora Fanerogâmica do estado de São Paulo. Instituto de Botânica, São Paulo. Vol. 3, pp. 149-224.; Nelson 2008Nelson SC (2008) Cassytha filiformis. Plant Desease 42: 1-10.; Tennakoon et al. 2016Tennakoon KU, Rosli R & Le QV (2016) Biology of aerial parasitic vines in Brunei Darussalam: Cuscuta and Cassytha. Scientia Bruneiana 15: 58-64., pers. obs.).

Distribution: The genus is predominantly distributed in the Southern Hemisphere (Chanderbali et al. 2001Chanderbali AS, Werff H & Renner SS (2001) Phylogeny and historical biogeography of Lauraceae: Evidence from the chloroplast and nuclear genomes. Annals of Missouri Botanical Garden 88: 104-134.). Most species are Australian, some also occurring in Africa and Asia. Cassytha filiformis has a pantropical distribution (Baitello 2003Baitello JB (2003) Lauraceae. In: Wanderley MGL et al. (eds.) Flora Fanerogâmica do estado de São Paulo. Instituto de Botânica, São Paulo. Vol. 3, pp. 149-224.).

Cuscuta L. in Species Plantarum 1: 124. 1753. Type: Cuscuta europaea L. FRANCE. PARIS. Dalib 53 (LT n.v.). Figs. 1i-n; 2d-g; 3f-j.

Annual to perennial, hemi-parasitic to holo-parasitic; stems filiform, yellowish, orange, purple or reddish, smooth and glabrous. The coiled stems producing haustoria (haustoria stems) of subg. Grammica exhibit multicellular protuberances with stomata. Leaves alternate, reduced to scales. Inflorescences axillary; the units are monochazial cymes which are further aggregated in thyrses or compound, dense cymes; bracts and bracteoles 1–15. Flowers bisexual 4–5-merous (rarely 3-merous); sepals and petals connate; infra-staminal scales usually present, variously dentate or fimbriate; stamens 4–5 on one whorl, alternating with the corolla lobes; filaments fused with the corolla tube; anthers opening via longitudinal slits. Ovary depressed-globose, ovoid to obovoid, glabrous to papillose, in species with two styles, an interstylar aperture is present; styles 1–2; stigmas elongate, capitate. Fruit a capsule, dehiscent, in which case circumscissile, indehiscent, or irregularly dehiscent. Seeds 1–4 per capsule; embryo filiform, 1–4-coiled, cotyledons absent (Yuncker 1932Yuncker GT (1932) The genus Cuscuta. Memoirs of the Torrey Botanical Club 18: 109-331., Riviere et al. 2013Riviere S, Clayson C, Dockstader K, Wright MA & Costea M (2013) To attract or to repel? Diversity, evolution and role of the “most peculiar organ” in the Cuscuta flower (dodder, Convolvulaceae)—the infrastaminal scales. Plant Systematics and Evolution 299: 529-552., Costea et al. 2015Costea M, García MA & Stefanović S (2015) A phylogenetically based infrageneric classification of the parasitic plant genus Cuscuta (Dodders, Convolvulaceae). Systematic Botany 40: 269-285., pers. obs.).

Distribution: A relatively reduced number of species (~15) are common agricultural invasive weeds (Costea & Stefanović 2009Costea M & Stefanović S (2009) Cuscuta jepsonii (Convolvulaceae): An invasive weed or an extinct endemic? American Journal of Botany 96: 1744-1750.) that have been dispersed world-wide as seed contaminants (Olszewski et al. 2020Olszewski M, Dilliott M, García-Ruiz I, Bendarvandi B & Costea M (2020) Cuscuta seeds: diversity and evolution, value for systematics/identification and exploration of allometric relationships. PLOS One (accepted).). The remaining ~185 species have more restricted geographical distributions but are encountered on all continents (Yuncker 1932Yuncker GT (1932) The genus Cuscuta. Memoirs of the Torrey Botanical Club 18: 109-331.; Costea et al. 2015Costea M, García MA & Stefanović S (2015) A phylogenetically based infrageneric classification of the parasitic plant genus Cuscuta (Dodders, Convolvulaceae). Systematic Botany 40: 269-285.). Subgenus Grammica accounts for over 75% of the genus diversity and has evolved in the New World (Stefanović et al. 2007Stefanović S, Kuzmina M & Costea M (2007) Delimitation of major lineages within Cuscuta Subgenus Grammica (Convolvulaceae) using plastid and nuclear DNA sequences. American Journal of Botany 94: 568-589.).

Even though Cuscuta and Cassytha are morphologically similar due to convergent evolution of their habit and life form, it is possible to accurately distinguish them during both their vegetative and reproductive stages, using either fresh or dry, herbarium specimens. The confusion between the two genera is likely to occur when only stems are available and examination is superficial, with the naked eye. However, stem micromorphological characters (Table 1) allow the reliable separation of the two genera even when only small stem fragments are available. The identification of Cuscuta and Cassytha using reproductive traits should pose no problem due to the markedly contrasting floral/fruit morphologies; however, the use of a magnifier is recommended for the flowers.

Acknowledgements

We would like to thank the staff of the SP herbarium for hosting our studies of Cuscuta in Brazil, as well as all the curators of the herbaria cited in the Material and Methods section. We are grateful to the photographers Marcelo Leandro Broto and Suzana Ehlin Martins, who kindly provided photos; Dra. Fátima Otavina de Souza Buturi, and Dr. João Batista Baitello, for useful scientific discussions and suggestions; as well as to the anonymous reviewers who contributed with suggestions and corrections to the manuscript. This research was funded by the following grants: CAPES (process n° 88882.4444239/2019-01, BJT 88881.067993/2014-01), CNPq (process n° 311738/2016-8), and NSERC Discovery (327013).

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