S. viarum is a fast-growing herb and a very aggressive invader. It produces thousands of small seeds (40,000 to 50,000 seeds per plant) that can be easily dispersed by birds, raccoons, cattle and by human activity (seed-contaminated grass, manure, mud, and farming machinery; Medal et al., 2012). Once established in new areas, S. viraum is able to grow forming monocultures that occur in patches of 4 ha or more (Waggy, 2009). S. viarum is classified as a weed by the Global Compendium of Weeds (Randall, 2012) and it also was declared a noxious weed by the US Department of Agriculture (USDA-Aphis) for the US territories of Alabama, Arizona, California, Florida, Georgia, Massachusetts, Minnesota, Mississippi, North and South Carolina, Oregon, Tennessee, Texas, Vermont and Puerto Rico (USDA-NRCS, 2012).

S. viarum thrives in overgrazed or drought-affected pastures, and invades plantation crops and natural habitats including forests and river banks. Although animals do not eat the foliage, the fruits are readily eaten and distributed by cattle and other mammals.

This species has previously been known as S. khasianum, and there is some continuing confusion over the status of  the two subspecies S. khasianum subsp. chatterjeeanum an accepted synonym for S. viarum, and S. khasianum subsp. khasianum, also known as S. aculeatissimum Jacquin). The latter differs in having glabrous rather than pubescent prickles. The name Solanum reflexum Schrank according to GBIF (2012) and Catalogue of Life (2012), equates with S. aculatissimum. According to The Plant List (2012) it is a synonym for the ‘accepted’ name S. viarum, while Missouri Botanical Garden (2012) indicates that S. reflexum is the ‘accepted’ name for S. viarum. For purposes of this datasheet, the name S. viarum is retained. Some relevant data for ‘S. reflexum’ and S. aculatissimum may have been overlooked.

S. viarum is an erect perennial, 50-150 cm high, with shortly pubescent stems and branches with recurved prickles up to 5 mm long, pubescent at their base. There are also longer, straight spines up to 2 cm long on the petioles and the veins of upper and lower surfaces of the leaves. The leaves are broadly ovate up to 20 cm long and 15 cm wide, bluntly lobed with markedly undulate edges, generally dark green, glossy above, duller below. The flowers are white, 1.5 cm across in clusters of 1-5 on pedicles about 1 cm long, the more distal flowers are often male only. Sepals about 3-5 mm long, corolla of white, somewhat narrow, reflexed petals, anthers pale yellow. The fruit is a globose berry, mottled green when young, maturing yellow, 2-3 cm across, containing up to 400 brown, flattened, discoid seeds, 2-3 mm in diameter (from Mill, 2001; Weber, 2003). Roots have buds which will regenerate new shoots. The root system can be extensive, with feeder roots 1-2 cm in diameter located a few cm below ground extending 1-2 m from the crown of the plant (Ferrell and Mullahey, 2006).

S. viarum is native to Argentina, southern Brazil, Paraguay, and Uruguay (Nee, 1999). It has been introduced into the southeastern United States, Mexico, Honduras, Puerto Rico, India, Nepal, South Africa and some other parts of Asia (Mullahey et al., 1993; 1996), where it has become a major concern to agriculture and cattle production (Bryson et al., 2009; Medal et al,. 2012). It is recorded as occurring ‘throughout India’ (GISIN, 2008) or ‘widely distributed from the Himalayan foot-hills in the North to the Nilgiris in the South of India’ (Singh et al., 1998), though it is confirmed in only a few of the states. In Bhutan, it occurs mainly below 2000m (Parker, 1993). Considering invasiveness-related traits of this species, it is expected to spread to other tropical and subtropical areas outside its current range (Nee, 1991). 

Originating in South America, S. viarum has found its way into northern America and into Africa and Asia. It has apparently been in India for a long time, as it has been widely cultivated and specially bred for medicinal purposes. In the Americas it has spread from its native range into other parts of South and Central America (including Mexico and Honduras) probably by seeds adhering to peoples shoes, mud, and contaminated grass seeds (Medal et al., 2008).

In Florida, S. viarum was first collected in 1988. The species was most likely introduced accidentally to Florida in cattle carrying undigested seeds that were imported from Brazil (ca. 1985) and from here it has spread to other southeastern and mid-Atlantic states (Medal et al., 2012). It is now also found in a small area of California (EDDMapS, 2013), and is on quarantine or noxious weed lists for a number of other states in the USA other than in its recorded range (USDA-NRCS, 2012). It has become a major concern of agriculture and a serious threat to natural plant communities (Cuda et al., 2002; Medal et al., 2008; ISSG, 2012) with over 400,000 ha of pasture land infested by this species in Florida alone (Medal et al., 2012).

S. viarum was first detected in Puerto Rico around 2006 in western areas (at Mayagüez). More recently, the species has been collected in various localities, including Puerto Rico’s east coast, suggesting a rapid spread throughout the island (Más and Lugo, 2013). In Australia, S. viarum was first recorded in the Kempsey area on the coast of New South Wales in August 2010; however it is thought to have been present there for some time. More recently it has been found in Southern Queensland and it is likely that this species is already present in other parts of eastern Australia (Australian Weeds Committee, 2012). In South Africa it was first reported in the early 2000s. 

According to different risk assessments performed on this species (Lehtonen, 1994; Gordon et al., 2008) the risk of introduction of S. viarum is high. This species is an aggressive invasive and each plant is able to produce thousands of seeds that can be dispersed by birds, cattle, and by human activities. It is also able to regenerate from its root system (Medal et al., 2008, 2012).

The risk of deliberate introduction is relatively high, given the popularity of the species as a medicinal herb and its importance as a source of pharmaceutical compounds, especially solasodine.

S. viarum is listed on the USA Federal Noxious Weed List, and hence its possession, movement and release is prohibited in the USA (USDA-NRCS, 2008). It is not yet recorded in Pacific or Indian Ocean islands (PIER, 2008), and it may appear imperative that introduction to these or any other new areas is prevented, noting its ability to spread very rapidly as has recently occurred in the south-eastern USA. 

S. viarum can be dispersed by seeds and by shoots from its extensive root system. This species is able to produce from 40,000 to 50,000 seeds per plant (Mullahey et al. 1993). Seeds can be dispersed by birds and other animals, including cattle, deer, feral pigs, and raccoons (Akanda et al. 1996; Waggy 2009; ISSG, 2012); also the tegu lizard (Tupinambis merianae) (Castro and Galetti, 2004). Animals do not eat the foliage but consume the fruits and spread the seeds in their faeces (Mullahey et al., 1998; Bryson and Byrd, 2007).

Sale of cattle that have recently been feeding on the fruits of S. viarum over long distances has resulted in spread of the plant from state to state in the USA, and the movement of hay and manure have also been implicated in accidental introductions (Mullahey et al., 2006). Other mechanisms of movement reported include transfer in turf, water, and grass seeds from contaminated pastures (Bryson and Byrd, 2007).

S. viarum is planted as a commercial medicinal crop especially in India, and thus intentional introduction is likely to have occurred at least in South Asia.

S. viarum has become a major concern of agriculture and cattle farming. It invades improved pastures where it reduces livestock carrying capacity and it is also a reservoir for at least six crop viruses (potato leaf-roll virus, potato virus Y, tomato mosaic virus, tomato mottle virus, tobacco etch virus, and cucumber mosaic virus) and the potato fungus Alternaria solani (McGovern et al., 1994a, 1994b, 1996; Medal et al., 2012). Several insect pests also utilize this species as an alternate host.

S. viarum can be distinguished from other Solanum species by its straight prickles, mixture of stellate and simple hairs with and without glands, clearly petioled leaves with a velvety sheen, terminal flower clusters, and yellow berries that are dark-veined when young (Florida Exotic Pest Plant Council, 2011). This species looks very similar to Solanum capsicoides and both have white flowers, but in S. viarum fruits are yellow when ripe while in S. capsicoides fruits are bright orange (Australian Weeds Committee, 2012).

Among other close relatives of S. viarum,S. anguivi differs in having mauve flowers and tomentose underside to the leaves, while S. aculeatissimum (= S. khasianum var. khasianum) and S. torvum both have white flowers but strictly glabrous prickles on the stems. In the USA, S. carolinense differs in being smaller, and having smaller fruits and leaves but larger flowers up to 3 cm across and a deeper root system, while S. tampicense has much narrower, more deeply lobed leaves.

Where native, S. viarum can be found growing in grassland, thickets, and disturbed places such as roadsides and river banks. Outside its native range, this species grows as a common weed in agricultural fields, pastures, and natural areas. In the southeastern USA it has been observed in pastures, ditch banks, citrus plantations, sugarcane fields, vegetable fields, and native areas like oak hammocks, pine forests, riparian habitat, and native grasslands (Medal et al., 2012). S. viarum thrives in disturbed areas associated with human activities.

S. viarum has been shown to be an alternate host of a wide range of viruses, nematodes, pathogens and insects, most quite unspecific, but surveys in South America revealed some insects with potential as biocontrol agents, especially the leaf-eating coleopteran Chrysomelidae Metriona elatior, Gratiana boliviana, G. graminea and a Platyphora sp.; also a flower bud weevil, Anthonomus tenebrosus from Argentina and Brazil (Medal et al., 2000; 2002) and a leaf-root feeder Epitrix parvula (Medal et al., 2012). G. boliviana has been used for biological control in Florida (Medal et al., 2010 and references therein). Two bacteria, Ralstonia solanacearum and Erwinia carotovora subsp. carotovora have also been shown to damage S. viarum and are of interest for inundative biocontrol in the USA. Tobacco mild green mosaic virus, which has no known vector, elicits a severe hypersensitive response in S. viarum and has been developed as a bioherbicide (Charudattan and Hiebert, 2007).

In Florida, USA, S. viarum occurs in pastures, citrus plantations, sugarcane and vegetable fields, turf fields and roadsides (Mullahey, 1996). It has spread extremely rapidly, especially in bahiagrass (Paspalum notatum) pastures, and especially in years of drought when its effect on available grazing is particularly serious. Foliage and stems are unpalatable to cattle (Medal et al., 2012). In addition, S. viarum grows forming dense monospecific stands that prevent cattle access to shaded areas and resulting in summer heat stress (Mullahey et al., 1998). For Florida ranchers, the control costs of S. viarum were estimated at $6.5 to 16 million annually (Thomas, 2007), and economic losses from cattle heat stress alone have been estimated at $2 million (Mullahey et al., 1998). It has also caused poisoning of goats in Florida (Porter et al., 2003).

It may act as an alternate host of a range of crop pathogens, including Cucumber mosaic virus, Potato leaf roll virus, Potato Y virus, Tomato mosaic virus and the fungus Alternaria solani (Cooke, 1997), though there are no reports of direct economic loss from this. It is also an alternate host of Meloidogyne javanica, providing a potential reservoir of this nematode in the USA (Inserra, 1994); also of Bemisia tabaci in Brazil (Lourenção and Nagai, 1994). Other insect pests detected in S. viarum (Sudbrink et al., 2000; Medal et al., 2012) include:

In Florida, management practices principally involve herbicide applications and mowing in invaded areas which provide temporary weed suppression at an estimated cost of US $61 and $47 per ha, respectively (Thomas, 2007). Based on survey of Florida cattle producers in 2006, S. viarum control costs resulted in economic losses throughout the state of $15 million annually (Salaudeen et al., 2013).

S. viarum has been identified as a noxious weed that can smother native plant communities where it has been introduced. It out-competes native plant species by crowding or shading them out (Waggy, 2009; Medal et al., 2012). This species also reduces biodiversity in natural forests because plants are able to dominate large areas in the understory affecting the germination and establishment of native species. In the southeastern USA it infests natural areas including state parks, nature preserves and hammocks (raised woodland above swamp land) (Mullahey, 1996). Plant prickles can also restrict wildlife grazing and create a physical barrier to animals, preventing movement through infested areas (USDA-FS, 2005). 

In India, S. viarum is being cultivated in many states, and collected from the wild in others, for a variety of traditional and modern uses.

S. viarum yields solasodine, a nitrogen analogue of diosgenin, whose hormonal derivatives are used in corticosteroid industry as an active ingredient of the contraceptive pill (Dasar etal., 2005; Maiti et al., 2000; Bhaskar et al., 2000; 2002). Varieties with very high yield potential for alkaloids have been developed (e.g. Glaxo, BARC, IIHR-2N, Arka-sanjivani, Arka Mahima and NBRI sel.) (Singh et al., 1998). Among these, Arka Mahima is one of the autotetrapoids which have been created, having higher levels of solasodine. Comparative histochemical studies of exotestas in diploid (2n), autotetraploid (4n) and tertiary trisomic (2n + 1) suggest that increased cell size in this layer in the autotetraploid plants probably accounts for the higher steroid content reported (Srinivas et al., 1998). However, inheritance of content is not correlated with yield. High alkaloid content is associated with low berry yield while high berry yield was correlated with medium levels of alkaloids. The weight of berries is dependent upon the number of seeds, the larger the number of seed, the higher the berry weight. Since glyco-alkaloids are located in the gelatinous layer around the seeds, increase in seed number per berry increases the alkaloid content.

The agronomy of S. viarum as a crop, grown in fallow land after rice, has been studied, especially in Karnataka, India. Solasodine content in the fruits is increased with increasing N rates as well as the fruit yield (Dasar et al., 2005). The best intercrop was found to be cluster bean (Bhaskar et al., 2000).

Stravato and Cappelli (2000) found individuals of S. viarum resistant to Fusarium oxysporum f.sp. melongenae, the causal agent of Fusarium wilt of aubergine or eggplant, and suggest the possibility of using these as a source for transferring disease resistance. Similarly, Matsui et al. (1995) found resistance to Thrips palmi in S. viarum.

Another potential use for S. viarum has been explored by Srinavasan et al.(2005), who showed that the plant contains potent attractants for Helicoverpa armigera.

S. viarum is also valued as a medicinal herb. In India, the fruit is used to treat asthma, bronchitis, coughs, colds, wounds, toothache, tooth decay, and as an abortifacient. The seeds are used as a contraceptive and for menstrual complaints (Mill, 2001). It is also given to dogs infected with Dirofilaria immitis (Chakraborty et al., 1994).