What have we learnt from rootstock trials and how should growers apply this information? By Anna Mouton.

Rootstocks can make or break a commercial orchard, according to Chad van Wyk, a stone-fruit rootstock evaluator at Provar. Van Wyk was addressing the audience at the 2026 Hortgro Stone-fruit Seminar and Orchard Walk.

“We have seen from trials that the rootstock can change tree vigour by 62% and fruit size by 32%, depending on the scion cultivar,” he said. “The rootstock can have a yield impact of 98%, essentially doubling the cumulative yield.”

In his presentation, Van Wyk highlighted the key results of the past 20 years of Hortgro-funded rootstock trials. He noted that many of these trials were started by the late Dr Piet Stassen. Van Wyk is continuing this work and managing new trials.

New options and recent results

Van Wyk walked the audience through a summary of trial results for the main rootstocks (Atlas, Cadaman, Flordaguard, GF 677, Guardian, Kakamas, Marianna, SAPO 778, and Viking) available to South African plum, peach, and nectarine growers. This information is collected in Table 1.

For more detailed rootstock tables, featuring 15 cultivars, including Barrier, OSM 99-3, and Rootpac-70, readers can refer to the June | July 2024 SAFJ.

“In terms of new rootstock cultivars, there’s roughly 20 that could be included in trials,” said Van Wyk. “The first of these is Barrier, from the University of Bologna in Italy. It was included in one trial at Slanghoek, where its yields were second only to Guardian, but Barrier is very vigorous.”

The Rootpac series from the Spanish Agromillora Group is represented in current trials by Rootpac 17, 19, and R, where the R stands for replant. Rootpac T, a low-chill clone of Flordaguard, will be included in new trials, as will rootstocks from the University of California’s Controller series.

Van Wyk also mentioned the OSM and OSK series, developed by the ARC at their Simondium breeding facility, and the brand-new Spanish Provedo series. So, growers can look forward to even more rootstock options in the coming years.

How to choose a rootstock

Van Wyk discussed four categories of factors to consider when choosing a rootstock: scion-rootstock compatibility, soil and environment, pest and disease resistance, and orchard management.

“Genetic compatibility between the scion and the rootstock will become very important over the next few years,” he said. “Going forward, this will be one of the major questions, especially as new cultivars come into play.”

Growers should also think about chill requirement and rootstock-scion synchronisation. For example, when a low-chill scion is grafted on a high-chill rootstock in a warm region, the scion may come into growth while the rootstock is still dormant and unable to supply water and nutrients to developing blossoms and leaves.

Hortgro-funded research has shown that correctly matching the cultivar and rootstock to the production region’s chill can more than double yields.

The preference of different rootstocks for particular soil types is well known. This enables growers to produce stone fruit across a range of environments. For example, Flordaguard prefers light, sandy soil while Marianna can cope with heavier, wetter soil.

“Something like GF 677 could thrive in soils with free lime,” said Van Wyk, “but to my knowledge, there’s no rootstock that can handle brackish soils.”

Ideally, rootstocks should be able to withstand pests and diseases, especially nematodes. Whereas some rootstocks are tolerant to root-knot nematodes, none are tolerant to ring nematodes. Rootstocks are likely to vary in their susceptibility to other soil-borne problems, notably replant disease, but we lack good data in this area.

Lastly, Van Wyk described the role of rootstocks in orchard design and management. By controlling tree vigour, rootstocks influence tree spacing, training systems, and even fruit size.

“We have also observed that rootstocks affect harvest timing,” he said. “For example, Flordaguard and Kakamas can advance the harvest by a few days, while something like Atlas or Viking delays it, even with the same scion cultivar.”

The focus of rootstock trials

As Van Wyk explained, the reason for new rootstocks is to solve industry problems. He listed eight areas for rootstock research, of which ring nematode tolerance enjoys the highest priority, as these pests occur in more than 95% of stone-fruit orchards.

“We planted a trial last year in Robertson, Tulbagh, and Wellington, with three plum cultivars, to test ring nematode tolerance,” he said. “We plan to expand this to peaches and nectarines next year.”

Once the trees in these new trials come into bearing, the trial sites will host open days where growers can view the results firsthand. In addition to field trials, ring nematode tolerance is currently being tested in greenhouses at the ARC.

The other seven research areas are chilling requirements, low-chill adaptation, scion-rootstock compatibility, water needs and drought resistance, dwarfing rootstocks for high-density planting systems, replant disease tolerance, and standardising rootstock assessments.

A technical committee, which includes industry representatives, decides the priorities for Hortgro-funded rootstock research and sets the direction for trials. Once trials are completed, the results are interpreted by technical advisers, who integrate this with their cultivar and regional knowledge.

“I think this is the most important part,” said Van Wyk. “They understand each environment, and use their expertise, often based on 20 or 30 years of experience, to put all the information together and provide growers with the appropriate recommendations.”

 

 

Table 1. Summary of the characteristics of the most common stone-fruit rootstocks.

	Atlas	Cadaman	Flordaguard	GF 677	Guardian	Kakamas	Marianna	SAPO 778	Viking
Parentage	Interspecific hybrid	Peach x wild peach	Peach x wild peach	Peach x almond	Peach x wild peach	Peach seedling	Plum hybrid	Peach x almond	Interspecific hybrid
Number of trials	Peach: 2 Plum 8	Peach: 1 Plum 4	Peach: 5 Plum 5	Peach: 2 Plum 9	Peach: 1 Plum 2	Peach: 5 Plum 2	Peach: 0 Plum 10	Peach: 2 Plum 9	Peach: 3 Plum 9
Vigour	High	Very high	Very high	Moderately high	Moderately high	Moderate	Moderately high	Moderately high	Moderately high
Yields: peach	Moderately high	–	High	–	High	Moderate to low	–	–	Moderately high
Yields: plum	High	Moderately high	Moderately high to high	Moderately high to high	Moderate	–	Moderately high	Moderately high	Moderately high
Fruit size	Good.	Good	Good.	Good with plums	Good unless stressed	Small on sandy soils.	Good unless stressed or ring nematodes present.	Very good	Good unless ring nematodes present.
Ripening	Late	–	Early	–	–	Early	Middle	–	Late
Chill	Medium	Low to medium	Low to medium	Medium	Medium	Medium	Medium	High	Medium.
Soil type	Wide range	Prefers sandy but adapted to less than 20% clay	Deep and well-drained sandy	80% sand or less than 20% clay. Lime tolerant.	Well-drained sand and clay	High potential soils	80%–90% sand or less than 20% clay. Tolerates wet conditions.	Sensitive to sandy soils and wet feet	Clay. Not sandy or stony. Drought sensitive.