Micropropagation of plum cultivars: A first for South Africa
Micropropagation in tissue culture is widely recognised as an effective method for the mass production of disease-free plants. It’s especially valuable for plants that are difficult to grow by traditional methods, such as seeds or cuttings. Another advantage of micropropagation is that it enables rapid multiplication of plants throughout the year under controlled conditions.
In the case of deciduous fruit, micropropagation of rootstocks is becoming increasingly common because many of the newer rootstocks can’t be rooted conventionally. “About 80% of the rootstocks for all the major tree-fruit and nut crops in California are now micropropagated,” says John Driver. “And it’s probably going to end up being around 90%.”
Driver is a nurseryman and tissue culture expert who was instrumental in the widespread adoption of tissue culture in the California fruit industry.
He also helped to set up a South African industry-supported tissue culture facility (TCF) at Bernheim Farm in Paarl. The initial focus of TCF was on the propagation of disease-free rootstocks, including the stone-fruit rootstocks Atlas, Flordaguard, GF677, and Viking.
In 2025, Culdevco contracted TCF to begin micropropagation of plum cultivars. The first young plants were transferred from the laboratory into tunnels in July 2025. The micropropagation of plum cultivars in tissue culture is a first for South Africa.
How does micropropagation work?
Conventional fruit-tree propagation multiplies rootstocks from seeds or cuttings. Scion material for grafting or budding onto the rootstock is cut from mature trees. In contrast, micropropagation of fruit trees involves growing tiny cuttings in tissue culture.
Marizanne Horn is the laboratory manager at TCF. She explains that the first step in micropropagating the Culdevco cultivars is called initiation. It involves cutting 1 – 2 cm nodal segments from soft shoots and placing these in sterile initiation medium.
“We add hormones like cytokinins and auxins to the initiation medium to stimulate shoot formation,” she says. “Once the nodal segment forms a shoot and grows bigger, we cut it into more shoots and place those on multiplication medium. So, the shoots multiply exponentially into more shoots that are then rooted to become plantlets.”
Initiation is the most challenging step, as cultivars differ in how readily they adapt to tissue culture. It generally takes 2–3 months, but once the micro-shoots have settled in, the process accelerates. A shoot can be divided into new micro-cuttings every 4 – 6 weeks, and this process can continue for months or even years.
Once there are enough micro-shoots, they can be rooted in preparation for the real world. Rooting can be achieved by adding hormones to the tissue culture medium or, if the micro-shoots are strong enough, they can be rooted in Ellepots in a tunnel.
“The advantage of tissue culture is that every plantlet is identical to the parent – we’re cloning the plants,” says Horn. “Mutations can occur over time, but we initiate new material every 3 – 4 years to minimise the risk.”
Why propagate cultivars in tissue culture?
For Culdevco, the rationale for this project is clear. “We want to provide growers with cleaner plant material,” says Mish Anderson, General Manager of Culdevco.
Culdevco manages more than 150 fruit cultivars, all bred by the Agricultural Research Council and well-suited to South African climatic conditions. In the 2023-24 season, only six plum cultivars generated more than a million export cartons each. Three of these – African Delight, Ruby Star, and Ruby Sun – are in the Culdevco stable.
Although South Africa has a plant improvement scheme that should enable growers to obtain healthy nursery trees, it has only been partially successful. In 2024, only 28% of stone-fruit trees were certified under this scheme.
“We don’t know what we don’t know,” says Anderson. “And as an industry, we haven’t addressed the question of how to provide virus-free material.”
For example, plum marbling was already widely established when the cause, a viroid, was discovered in 2020. The viroid had not been part of the plant improvement scheme because no one knew it existed.
Micropropagated rootstocks are a step in the right direction, but not sufficient to protect growers. In the case of plum marbling, research has demonstrated that the infection is graft transmissible. Therefore, if the viroid is present in either the rootstock or the scion, the tree is infected, and symptomatic fruit may appear.
“If you’re using tested, clean tissue-cultured rootstocks, your biggest problem becomes getting clean budwood,” says Hugh Campbell, former Plant Material Manager at Hortgro Science.
During a technical visit to European plant improvement institutions and nurseries in 2022, Campbell went to commercial nurseries that produce both the scion and rootstock in tissue culture.
“They basically made the trees in the laboratory,” he recalls. “There are all sorts of advantages, but if you’re talking about risk management, then tissue culture in combination with pots reduces your risk dramatically.”
The many benefits of micropropagation
To ensure that their micropropagated budwood is clean, Culdevco provided TCF with starting material from their nucleus unit. Nucleus units hold the original material of new imports and newly discovered or developed cultivars or improved clones. The nucleus plants are isolated in containers within a greenhouse and tested annually for specific viruses.
“With tissue culture, if you start with clean material, you end with clean material of high quality,” says Horn.
The importance of establishing orchards with clean trees cannot be overstated, but this isn’t the only benefit of micropropagation. The process is also more efficient and can produce better trees than conventional propagation.
Campbell describes seeing the top or green grafting of micropropagated material at a commercial nursery in Europe. “It’s a simple process,” he says. “And it has a lower risk of contamination than conventional grafting.”
Green grafting is done when the scion and rootstock are about 3 – 4 mm in diameter. Unlike conventional stone-fruit grafting, the graft union is the only wound, and that’s covered, significantly reducing the risk of stem cankers.
Additionally, green grafting has an excellent success rate. The resulting graft unions are usually stronger and develop less callus than conventional graft unions.
As micropropagation is conducted indoors under environmentally controlled conditions, and young trees are maintained in glasshouses, trees can be produced year-round. It takes approximately 8 months to grow a 60–80 cm tree. The containerised trees can be planted at any time.
Despite the strong case for micropropagation, Anderson is not afraid to admit that this is uncharted territory for South African plum cultivars. Culdevco will be carefully evaluating the progress and performance of the first trees produced at TCF over the coming months.
“The industry has valid concerns about infected plant material,” says Anderson. “Culdevco has commercialised cultivars of real economic value. As the cultivar manager, it is our responsibility to ensure that we supply clean material so growers can achieve higher yields and improved fruit quality.”
