Tropical soda apple (Solanum viarum)

Tropical soda apple is an aggressively invasive, prickly, thicket forming perennial shrub 1–2 m high.

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How does this weed affect you?

Tropical soda apple (Solanum viarum), is an aggressive, prickly, perennial shrub 1–2 m high. It invades open to semi-shaded areas, particularly pastures and riparian zones, but also forests, roadsides, recreational areas, and horticultural and cropping areas. It reduces biodiversity by displacing native plants and disrupting ecological processes. Its foliage is unpalatable to livestock, thus reducing carrying capacities, however cattle eat the fruit and spread viable seeds in manure. Thorny thickets of this plant create a physical barrier for animals preventing access to shade and water. The plant is a host for many diseases and pests of cultivated crops, and it contains solasodine which is poisonous to humans.

Tropical soda apple is spread when cattle eat the fruit or the fruit float and move in water. If not controlled a few plants will form a hectare sized thicket in 6 months, with each plant producing 150 fruit containing 45 000 seeds each year. Herbicides kill the plants, but do not kill the seeds inside the fruit. In the USA, this plant infested over half a million hectares in 5 years. In NSW it is critical to achieve site-based eradication of this plant before it becomes widespread. 

What does it look like?

Tropical soda apple is an upright, branching, perennial shrub growing to 2 m in height. It has broad-based, straight, cream-coloured prickles to 12 mm long scattered on most plant parts.

Key identification features

• Leaves are mostly 10–20 cm long and 6–15 cm wide. The upper and lower leaf surfaces are densely covered in short hairs; mid-veins and primary lateral-veins are cream-coloured on both sides of the leaves.
• Flowers are white, 1.5-2 cm wide, with 5 petals. They occur in clusters of 3–6 off a short stem.
• Mature fruit are yellow and golf ball-size (2–3 cm in diameter). When immature they are pale green with dark green veins, like immature water melons. In the USA plants produce an average of 45 000 seeds.

Where is it found?

Tropical soda apple is a native of north eastern Argentina, south eastern Brazil, Paraguay and Uruguay. It was first recorded in Florida in 1987 and was known to infest 10 000 ha by 1990 and half a million hectares by 1995. By 2007 it had spread to nine other south eastern states in the USA, where it is now a Federal Noxious Weed. Tropical soda apple has also naturalised in Africa, India, Nepal, West Indies, Honduras and Mexico and outside its native range in South America.

It was first identified in Australia in the upper Macleay Valley in New South Wales (NSW) in August 2010, however it is believed to have been present in this area for a number of years. Subsequent surveys found infestations at Wingham, Grafton, Bellingen, Coffs Harbour, Bonalbo, Casino, Murwillumbah and Wauchope. The smaller infestations have been eradicated and the larger infestations are subject to active control programs. In 2011 infestations were discovered in the Namoi and Border Rivers-Gwydir catchments associated with the movement of cattle from infested coastal areas. Infestations were traced to properties near Tamworth, Attunga and Inverell and are currently the focus of an eradication campaign.

Tropical soda apple has the potential to spread in coastal regions of NSW and Queensland, and inland through cattle movements.

Maps and records

• Recorded presence of Tropical soda apple during property inspections (Map: Biosecurity Information System - Weeds, 2017-2023)
  These records are made by authorised officers during property inspections under the Biosecurity Act 2015. Officers record the presence of priority weeds in their council area and provide this to the NSW Department of Primary Industries. Records reflect the presence of the weed on the date of inspection.
  
  

How does it spread?

Tropical soda apple reproduces via seed and can regenerate from root and stem material. The fruit are sweet and cattle will smell and seek them out, spreading viable seed in their manure for up to 6 days after consuming the fruit. After 6 days any consumed seeds that are passed are no longer viable. In NSW cattle movements are currently the major vector of spread and infestations have been found by tracing cattle movements from infested properties using the National Livestock Identification System database. Horses have also been observed to eat the fruit and seedlings have germinated in horse manure. 

Seed is also moved when the pithy fruits float in water, and infestations along waterways and floodzones have occured.

The sticky seeds can also be spread by feral animals and birds that feed on the fruit; and via contaminated fodder, produce, soil and equipment.

Hold new cattle for 6 days

To mitigate the very high risk of introducting this plant, any new cattle coming onto any property must be held for 6 days, in an area that can be closely inspected for seedlings. Tropical soda apple seeds become ready to sprout when they are eaten by cattle and passed within 6 days. After 6 days, any seeds that are passed won't sprout. Studies of seed viability after being consumed by horses have not yet been done, and any new horses coming onto a property should also be held for 6 days in an area that can be checked for seedlings. 

Rotational paddocks, holding paddocks, quarantine paddocks or electric-fenced areas are all suitable, and must be checked regularly for the presence of seedlings. Plants can produce fruit within 2 months of germinating.  

Livestock movements and the law

It is illegal to knowingly transport the seeds of this plant inside an animal, or to knowingly buy or sell an animal that contains seeds. The movement of seed in cattle is traceable, and knowingly moving seeds in animals, or buying or selling animals containing seeds can result in prosecution and fines. 

 

References

Akanda R.U., Mullahey J.J., Shilling D.G., (1996). Environmental factors affecting germination of tropical soda apple (Solanum viarum). Weed Science, 44(3), 570-574.

Bryson, C. T., & Byrd Jr, J. D. (2007). Biology, reproductive potential, and winter survival of tropical soda apple (Solanum viarum). Weed Technology, 21(3), 791-795.

Bryson, C. T., Reddy, K. N., & Byrd Jr, J. D. (2012). Growth, development, and morphological differences among native and non-native prickly nightshades (Solanum spp.) of the south-eastern United States. Invasive Plant Science and Management, 5(3), 341-352.

Call, N. M., & Coble, H. D. (1998). Phosphorus effects on tropical soda apple (Solanum viarum Dunal) growth and development. Southern Weed Science Society (USA).

Mullahey, J. J. (2012). Biology Ecology and Control of Tropical Soda Apple (Solanum viarum). Pakistan Journal of Weed Science Research, 18: 447-456, Special Issue.

PlantNET (The NSW Plant Information Network System). Royal Botanic Gardens and Domain Trust, Sydney. Retrieved 20 January 2021 from: https://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=sp&name=Solanum~viarum 

Trenholm, L. E.; Sturgis, A. K.; Ninaji, A.; Gallaher, R. N.; Akanda, R. U.; Mullahey, J. J. (1995). Growth and nutrient accumulation in tropical soda apple (Solanum viarum Dunal). Agronomy Research Report AY-95-04. Gainesville, FL: University of Florida, Department of Agronomy. 27 p

van Driesche R et al. (2002). Biological control of invasive plants in the eastern United States, USDA Forest Service.

Welman, W. G. (2003). The genus Solanum (Solanaceae) in southern Africa: subgenus Leptostemonum, the introduced sections Acanthophora and Torva. Bothalia, 33(1), 1-18.

More information

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Control

Successful weed control requires follow up after the initial efforts. This means looking for and killing regrowth or new seedlings. Using a combination of control methods is usually more successful.

If you find tropical soda apple, contact your local council weeds officer as soon as possible for advice and assistance to eradicate it from your property. Tropical soda apple grows quickly and seeds prolifically. A few plants can form a hectare sized thicket in six months. Inappropriate control efforts can accidentally spread this weed.

To tackle tropical soda apple:

• seek advice on the control of mature plants as soon as you become aware of them
• check stock quarantine areas for new seedlings emerging from dung 1 to 2 weeks after the quarantine, and kill any seedlings
• check for new plants in spring and summer
• kill plants before they are 2 months old to prevent fruit and seed set
• collect and dispose of fruit to reduce seedling numbers.

Prevention

Hold new stock

Hold new cattle and horses for at least six days in a paddock that can be inspected for weeds. After 6 days, any excreted seeds won't sprout. Inspect the holding area for seedlings for several months as the seeds are viable for a long time.

Hold stock before sale or movement

Hold livestock in a paddock free of tropical soda apple for at least 6 days before selling them or moving them off your property.

Check your property regularly

Check your land as often as possible, particularly during spring and summer. Check:

• cattle camps, stock yards, feed-out areas and holding paddocks
• waterways, drains, gullies, floodplains, flats and areas with flood debris
• fence lines, tracks and roads
• forested areas and areas where feral animals may have been.

Check all control sites every 2 months.

Physical Control

By Hand

Small plants and plants in soft soil can be pulled or dug out with a hoe or mattock. Plants need to be disposed of so that they do not reshoot from seeds or stems. Wear gloves or use pliers to grip the stems to avoid injury from prickles.

Disposal

Plants can be burnt in a hot fire. Check the burnsite regularly for any seedling growth. If storing fruit, keep it in a container away from soil, flood zones livestock and vermin.

Contact your local council for other appropriate disposal methods.

Chemical control

Spraying

Spraying is effective and especially suitable for dense infestations. Ensure that all the leaves are covered in the herbicide. Use appropriate herbicides if plants are near waterways. Ensure that a wetter is added to the herbicide mix.

Herbicides kill the plants, but do not kill the seeds inside the fruit. All fruit must be removed by hand, even small green fruit can have viable seeds.

Cut stump

Large scattered plants can be controlled by the cut stump method. Cut the stem close to the ground and then apply the herbicide within 10 seconds to the stem left in the ground.

If using gel type herbicide, apply a 3-5 mm layer of gel for stems less than 20 mm. Apply a 5 mm layer on stems above 20 mm. Liquid herbicides can be painted onto the stem.

Dispose of the cut plant and any fruit after treating the stump.

Herbicide options

See Using herbicides for more information.

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PERMIT 9907 Expires 31/03/2025
Glyphosate 360 g/L (Various products)
Rate: 200 mL in 10 L of water
Comments: Spot spray application
Withholding period: Nil.
Herbicide group: M, Inhibitors of EPSP synthase
Resistance risk: Moderate

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PERMIT 9907 Expires 31/03/2025
Glyphosate 360 g/L (Various products)
Rate: 1 part per 1.5 parts of water
Comments: Cut stump application
Withholding period: Nil.
Herbicide group: M, Inhibitors of EPSP synthase
Resistance risk: Moderate

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PERMIT 12942 Expires 30/08/2025
Glyphosate 360 g/L with Metsulfuron-methyl 600 g/kg (Only products registered for aquatic use)
Rate: 2 L glyphosate plus 10g of metsulfuron in 100 L of water
Comments: A wetter must be used at a rate of 500 mL per 100 L. Apply a maximum of 2 times per year at a minimal interval of 60 days. Ensure spray covers all foliage and stems as incomplete application will result in regrowth.
Withholding period: Nil.
Herbicide group: M, Inhibitors of EPSP synthase
Resistance risk: Moderate

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PERMIT 12942 Expires 30/08/2025
Picloram 100 g/L + Triclopyr 300 g/L + Aminopyralid 8 g/L (Grazon Extra®)
Rate: 350 to 500 mL in 100 L of water
Comments: A wetter must be used at a rate of 500 mL per 100 L. Apply a maximum of 2 times per year at a minimal interval of 60 days. DO NOT use products containing picloram and triclopyr within 5 m of a waterway. Ensure spray covers all foliage and stems as incomplete application will result in regrowth.
Withholding period: Where product is used to control woody weeds in pastures there is a restriction of 12 weeks for use of treated pastures for making hay and silage; using hay or other plant material for compost, mulch or mushroom substrate; or using animal waste from animals grazing on treated pastures for compost, mulching, or spreading on pasture/crops.
Herbicide group: I, Disruptors of plant cell growth (synthetic auxins)
Resistance risk: Moderate

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PERMIT 12942 Expires 30/08/2025
Triclopyr 300 g/L + Picloram 100 g/L with Metsulfuron-methyl 600 g/kg (Various products)
Rate: 350 to 500 mL plus 10g metsulfuron in 100 L of water
Comments: A wetter must be used at a rate of 500 mL per 100 L. Apply a maximum of 2 times per year at a minimal interval of 60 days. DO NOT use products containing picloram and triclopyr within 5 m of a waterway. Ensure spray covers all foliage and stems as incomplete application will result in regrowth.
Withholding period: Nil.
Herbicide group: I, Disruptors of plant cell growth (synthetic auxins)
Resistance risk: Moderate

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Picloram 44.7 g/L + Aminopyralid 4.47 g/L (Vigilant II ®)
Rate: Undiluted
Comments: Cut stump application. Apply a 3-5 mm layer of gel for stems less than 20 mm. Apply a 5 mm layer on stems above 20 mm.
Withholding period: Nil.
Herbicide group: I, Disruptors of plant cell growth (synthetic auxins)
Resistance risk: Moderate

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Biosecurity duty

The content provided here is for information purposes only and is taken from the Biosecurity Act 2015 and its subordinate legislation, and the Regional Strategic Weed Management Plans (published by each Local Land Services region in NSW). It describes the state and regional priorities for weeds in New South Wales, Australia.

Area	Duty
All of NSW	General Biosecurity Duty All pest plants are regulated with a general biosecurity duty to prevent, eliminate or minimise any biosecurity risk they may pose. Any person who deals with any plant, who knows (or ought to know) of any biosecurity risk, has a duty to ensure the risk is prevented, eliminated or minimised, so far as is reasonably practicable.
All of NSW Tropical Soda Apple Control Zone: Whole of NSW	Control Order Tropical Soda Apple Control Zone (Whole of NSW): Owners and occupiers of land on which there is tropical soda apple must notify the local control authority of new infestations; destroy the plants including the fruit; ensure subsequent generations are destroyed; and ensure the land is kept free of the plant. A person who deals with a carrier of tropical soda apple must ensure the plant (and any seed and propagules) is not moved from the land; and immediately notify the local control authority of the presence of the plant on the land, or on or in a carrier.

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Reviewed 2018