Solanum viarum
(tropical soda apple)

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.

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). 

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. 

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 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.

Genetics

The normal chromosome number is 2n=24 (Kumaraswamy and Krishnan, 1987; Chiarini, 2006; Missouri Botanical Garden, 2008) but autotetraploids have been deliberately created for cultivation in India (Srinivasappa et al., 1999). The variety Arka Sanjeevini is diploid whereas the variety Arka Mahima is tetraploid with a lower overall yield but higher content of solasodine (Paturde et al., 2002). 

Reproductive Biology

All Solanum species have poricidally dehiscent anthers that make this genus an example of the buzz pollination syndrome, found in about 200 plant genera (Buchmann, 1983). Solanum flowers are mainly hermaphroditic, nectar is absent and pollen is the exclusive floral reward. Pollination in these flowers is performed by insects (mainly bees). In Florida, the orchid bee Euglossa viridissima was observed collecting pollen from S. viarum and may serve as a pollinator (Pemberton and Wheeler, 2006).

The plant can recover from the rootstock after the shoots are killed by frost, and there is some degree of vegetative reproduction with shoots developing from spreading roots. Seed production is high, and seeds survive in the soil for up to one year (Mullahey et al., 1998). 

Physiology and Phenology

Flowering and fruit production occur throughout the year in S. viarum, but in Florida, reproductive activity is concentrated from September through May (Lehtonen, 1994). Throughout the year, plant of S. viarum may have both immature and mature fruit present, ensuring production of large numbers of viable seeds (approximately 40,000 or more per plant; Akanda et al., 1996). Seedling emergence primarily occurs from August through March. New plants can emerge from seed or from roots. The species has an extensive root system which may extend 1-2 metres horizontally from the crown of the plant (Medal et al., 2012).

Germination of S. viarum is moderately photoblastic with 30% germination occurring in the dark. Germination increases in response to green (545 nm) and red light (650 nm) to 75 and 66%, respectively, indicating phytochrome regulation. No germination occurs in response to blue (450 nm) or far-red light (750 nm). Germination increases from 4 to 64% between 10°C and 30°C, but no germination is found at 5°C and 40°C. Maximum germination occurs at 30°C. Seedling emergence is maximum from a depth of 3-6 cm, but no seedling emergence occurs when seeds are deeper than 12 cm. Optimum germination occurs at a depth of 5.6 cm. Mechanical and sulfuric acid scarification increases the rate of germination but not the overall percentage. Tap water or hot water pretreatments increase the rate of germination by 26%, and KNO₃, GA₃ or ethephon increase germination by 53%. Suryawanshi et al. (2001) recommend treatment with 12.5% nitric acid for 15 minutes followed by 1000 ppm gibberellic acid for 24 h with alternate temperatures of 20-30°C. In conclusion, seed germinates in response to variable environmental and edaphic conditions which could allow its establishment in diverse ecosystems (Akanda et al., 1996).        

Longevity

In favourable temperatures, plants flower within 60-100 days after emergence in photoperiods of 8-16 h, with flowering delayed by photoperiods less than 10 h or temperatures lower than 24/20°C (Patterson et al., 1997). Green stems persist in mild winter temperatures (Coile, 1993). Plants become less productive or may die in summer when standing in water (Mullahey and Colvin, 1993) or in winter when leaves may be severely damaged by frost (Lehtonen, 1994).

Although normally perennial, S. viarum can produce fruit in the first season of growth and thus behave as an annual. 

Associations

Within its native range, S. viarum grows in the understory of rainforest, in grasslands and in shrubby-thicket dry forests (Nee, 1991). In invaded areas, S. viarum often occurs in cultivated pastures, primarily pastures planted with bahia grass (Paspalum notatum), citrus plantations, sugarcane plantations, and vegetable fields (Medal et al., 2008; 2012; ISSG, 2012). This species is also common as a weed in sites associated with anthropogenic disturbance (waste grounds and along roadsides) and in native plant communities (Medal et al., 2012). 

Environmental Requirements

Growth and development is enhanced under sunny conditions with temperatures ranging from 20°C to 35°C and average annual rainfall from 700 mm to 2000 mm. Plants survived in 8°C nights with day temperatures of 18-36°C, but biomass and leaf area were only 3-10% of the maximum (Patterson et al., 1997).

This species does not tolerate water-logging or frosty conditions for extended periods. S. viarum prefers to grow in well drained sandy loam soils with high organic matter content (Lehtonen, 1994).

The rapid spread of S. viarum is apparently favoured by moderate drought conditions in Florida, USA, though this may be partly due to the overgrazing which occurs under those conditions (Hogue et al., 2006).

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).

Genetic importance

• Gene source

Materials

• Chemicals
• Pesticide

Medicinal, pharmaceutical

• Source of medicine/pharmaceutical
• Traditional/folklore
• Veterinary

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.

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Prevention

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). 

To avoid the risk of introducing the weed when purchasing cattle from an infested area, the livestock should be kept in a restricted area for at least 6 days by which time any ingested seeds should have been voided (Mullahey et al., 2006). Correspondingly, those selling cattle should ensure that they are not fed on contaminated pastures prior to sale, and that any hay sold off the farm is free of contamination. 

Control

S. viarum is difficult to eradicate and the best management strategy varies according to the population size. Individual plants and small populations should be pulled up and burned completely along with all fruit. Roots have to be removed entirely because this species can regenerate shoots and re-grow from the root fragments (Medal et al., 2012). Larger populations require repeated mowing and/or repeated applications of herbicides or bioherbicides. It is important that plants are not allowed to fruit in order to prevent seed dispersal (Waggy, 2009); mowing below 10 cm plant height every 60 days will prevent fruit production and result in some plant mortality (Mullahey et al., 2006). Gloves are needed when handling the cut plants. 

Biological control

A petition for release of Gratiana boliviana in the USA was approved in 2002 (Medal et al., 2007; USDA/TAG, 2008). The beetle was introduced from Argentina and Paraguay: releases began in Florida in 2003 and more than 100,000 beetles have been released in Florida, Georgia, Alabama, South Carolina and Texas. Establishment has been good, with spread of 1-10 miles (1.5-16.0 km) per year from the release sites, with 20-100% defoliation and no non-target damage has been observed (Medal et al., 2006; 2007; Medal, 2008). Studies have shown that G. boliviana is better suited for control of small infestations of S. viarum than large or remote infestations (Waggy, 2009). 

Field tests in Brazil and Argentina confirmed the specificity of Metriona elatior to S. viarum and lack of attack on related solanaceous crops (Bredow et al., 2007; Gandolfo et al., 2007), and “these data suggest that a host range expansion of M. elatior to include eggplant, potato, tomato, or bell-pepper is highly unlikely” (Bredowet al., 2007). However, other laboratory tests had shown some feeding on eggplant (Medal et al., 2002) and the release of M. elatior in the USA was rejected in 2008 (USDA/TAG, 2008). Release of Gratiana graminea was also refused at this time. A petition to release Anthonomus tenebrosus was submitted in 2007 (USDA/TAG, 2008), and work with the flower-bud weevil, Platyphora sp. is apparently still in progress. 

The bacterial pathogen Ralstonia solanacearum is effective in causing plants to wilt and die (USDA-FS, 2005) and has been applied with a wet-blade mower to control S. viarum in field trials (DeValerio et al., 2011). Another bacterium, Erwinia carotovora subsp. carotovora, has been used in conjunction with reduced doses of herbicide to increase the level of control (Roberts et al., 2002). 

A bioherbicide containing Tobacco mild green mosaic virus strain U2 has been recently registered for use against S. viarum in the USA (Charudattan and Hiebert, 2007; Charudattan, 2015).

Chemical control

S. viarum can be effectively controlled using any of several herbicides such as glyphosate, imazapyr or triclopyr, though it is suggested that fruits are also collected and destroyed to prevent reestablishment (USDA-FS, 2005). Other recommended herbicides include aminopyralid (Akanda et al., 1997; Ferrell et al., 2006), hexazinone (Mislevy and Martin, 1999), glufosinate-ammonium, picloram, clopyralid, fluroxypyr and dicamba (Dowler, 1995). Effectiveness of triclopyr and hexazinone was enhanced when they were applied 60 days after frost had damaged the foliage (Mislevy and Martin, 1999). Effectiveness of triclopyr was similarly enhanced when applied after one or two prior mowings (Miselvy et al., 1999). However, aminopyralid can be applied at any time of year and will control existing plants and germinating seedlings for over 6 months after application (Hogue et al., 2006).

Treatments containing picloram or triclopyr controlled eight-leaf, 16-leaf, and 1-yr-old S. viarum greater than 90%, 8 weeks after treatment (Call et al., 2000). Acifluorfen, clopyralid, dicamba, fluroxypyr, picloram, triclopyr, glyphosate and imazapyr all resulted in >90% weed control after 145 days, though the latter two also caused >90% damage to Paspalum notatum (Akanda et al., 1997).

Treatment of an entire pasture should consist of mowing adult plants in April-May, allowing 50-60 days for regrowth, followed by single or repeated doses of triclopyr. Follow-up treatments will be necessary to control escaped adult plants and seedlings 90-120 days after the initial treatment (Mullahey et al., 1996). 

Integrated management

Integrated weed management strategies include prevention (avoidance of contaminated hay or grass seed, control of movement of cattle), control (mechanical, chemical, biological) and monitoring (Mullahey et al., 1998). 

Integrated control combining bioherbicides with chemical herbicides have been shown to be more effective than the chemical treatments alone (Roberts et al., 2002; Ferrell et al., 2008).

06/05/08 Original text by:

Chris Parker, Consultant, UK

01/02/13 Updated by:

Julissa Rojas-Sandoval, Department of Botany-Smithsonian NMNH, Washington DC, USA

Pedro Acevedo-Rodríguez, Department of Botany-Smithsonian NMNH, Washington DC, USA