Tropical soda apple
Native to: southern Brazil, northeastern Argentina and neighboring parts of Paraguay and Uruguay
Tropical soda apple (TSA) is established in the West Indies, Mexico and Central America, as well as tropical regions of Africa, Asia and Australia. It has invaded most of the southeastern and south central US, from North Carolina and Tennessee west to Texas and Oklahoma. It was first reported in the US from Florida in 1988 and is now widespread and particularly problematic in disturbed sites such as pastures, citrus groves, vegetable fields and roadsides, but has also been observed in a variety of natural habitats including scrub, flatwoods, swamps, floodplain forests and on the edges of pinelands and hammocks. Because of its rapid population expansion in Florida and the concerns of livestock producers, Solanum viarum was placed on the Florida Noxious Weed List in 1994 and the Federal Noxious list in 1995.
Habit: Herbaceous perennial shrub that grows 3-6 feet high, with shortly pubescent stems and branches with recurved prickles up to .5 cm long, pubescent at their base. 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 3-6 feet from the crown of the plant.
Leaves: alternate, broadly ovate up to 4-8 inches long and 2-6 inches wide, bluntly lobed with markedly undulate edges, generally dark green, glossy above, duller below. There are long, straight spines up to 2 cm long on the petioles and the veins of upper and lower surfaces of the leaves.
Flowers: 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.
Fruit: globose berry, mottled green when young, maturing yellow, 2-3 cm across.
Seeds: fruits contain up to 400 brown, flattened, discoid seeds, 2-3 mm in diameter.
Distribution in Florida: reported from almost every county
TSA has had serious impacts on agriculture in Florida. It readily invades cattle pastures resulting in reduced forage production and lower stocking rates. In 2007, the cost of control to cattle ranchers was estimated at $6.5 to 16 million annually and it has caused the poisoning of goats. It can also act as host to a range of crop pathogens including Cucumber mosaic virus, Potato leaf roll virus, Potato Y virus, Tomato mosaic virus and the fungus Alternaria solani. In natural areas it displaces native plant species. Where is grows densely, the plant’s spines can restrict wildlife grazing and movement. The fruit contains solasodine, which is poisonous to humans if consumed.
It is important not to move TSA to uninfected areas. Thoroughly cleaning any equipment (mowers, tractors, etc.), clothing, and gear upon leaving infested areas is critical. When moving livestock from infested areas, they can be held in one area for up to 6 days in case there is viable seed in their system and then monitor that area for any germination.
Hand pull individual plants and small populations. Care must be taken to remove roots entirely.
Mowing below 10 cm plant height every 60 days will prevent fruit production and result in some plant mortality. A combination of mowing and herbicide application can be effective.
A successful biocontrol program involving the tropical soda apple leaf-eating beetle, Gratiana boliviana, has provided considerable control by causing extensive defoliation. The beetle, which attacks and damages TSA exclusively, was first released on a Polk County, Florida, ranch in 2003. Since then, beetles have been released in Florida, Georgia, Alabama and Texas, spreading from one to 10 miles per year from the initial release sites. The beetles are considered established in Florida and are not currently reared for additional releases.
Milestone and GrazonNext HL herbicides are the most effective for controlling dense stands of TSA. These herbicides possess postemergence control of existing plants and preemergence control of germinating seeds. Detailed herbicide recommendations can be found here.