Solanum eliagnifolium is an alien invasive weed that is also called "Satansbos", meaning "Satan's bush" in a local language which gives you some idea of how much of a pest it is. Solanum species are part of the nightshade family, which includes tomatoes, green peppers and potatoes. Alien invasive species are plants that don't occur naturally in this part of the world and this particular pest comes from North America.
The weed featured here is regarded as a serious agricultural pest because it has substantial underground roots which are capable of forming new plants and pulling it up is not enough - if any of the root remains underground - it will simply form new plants and this plant can quickly take over an entire tract of land. There aren't any effective weedkillers registered for this plant but biological controlis being looked at.
Biocontrol is becoming a more popular method of dealing with alien invasive plants because it is kinder to the environment than weedkillers are and if the insect predator that is introduced is host-specific, meaning that it only attacks that particular plant, it doesn't pose a threat to other naturally-occurring plants. A large part of the reason why introduced plants become invasive elsewhere is because their natural predators, which normally keep the plants under control, are not found here so these plants are able to flourish.
The english name "bitter apple" refers to the berries that this plant makes are inedible to humans although they aren't poisonous, as many other of the Solanum species are.
References:
Problem Plants of South Africa - Clive Bromilow. Briza Publications 2001
Poisonous Plants in South African Gardens and Parks:a Field Guide - Joan Munday. Delta Books 1988