Research rundown

A summary of Hortgro-funded apple-rootstock research projects. By Anna Mouton.

Rootstock research at Hortgro Science includes evaluation trials and horticultural and disease-related projects.

Rootstock evaluation

The first Hortgro-funded trials of Geneva rootstocks, led by Carlo Costa of the ARC Infruitec-Nietvoorbij, were planted in 2000 at Bo-Radyn in Villiersdorp.

Various Geneva rootstocks, including G.222, G.228 and G.778, were compared to M.9 Cepiland, M.7 EMLA and industry standard M.793. All were grafted to Golden Delicious Reinders.

Costa reported that among the Geneva rootstocks G.222, G.228 and G.778 were more yield efficient than M.793 relative to tree size. M.9 Cepiland was the most yield efficient overall, followed by G.222. Pot trials showed G.222 to be the most tolerant of replant disease and woolly apple aphids.

More trials comparing Geneva rootstocks with industry standards, such as M.793, followed, with the first planted in 2010 at Paardekloof in the Witzenberg Valley. The trees were grafted to Rosy Glow (Table 1).

Table 1: Apple rootstocks evaluated from 2010–2016 in the Witzenberg Valley. G.222 and M.793 were included as controls in both groups.

Dwarfing to semi-vigorous	Semi-vigorous to vigorous
M.9 Lancep	G.222
M.9 Cepiland	M.793
M.9 Nic29	G.228
MM.109 | M.9 EMLA	Marubakaido
G.222	CG.934
M.7	MM.109
CG.3007	G.778
MM.106	M.25
M.793

The project, initiated by Dutoit Agri and completed by Dr Xolani Siboza of Hortgro, found that M.9 Nic29 and MM.109 with an M.9 EMLA interstem were the more precocious, productive, and yield efficient rootstocks in the more dwarfing planting. At the same time, G.778 performed best in these respects in the more vigorous planting. Siboza concluded that G.788 could potentially replace M.793 and MM.109.

Three projects established by Willie Kotze, currently of Dutoit Agri, and subsequently led by Siboza, were conducted from 2013 at Breëvlei and Oak Valley in Elgin and Helderwater in the Langkloof.

The same rootstocks and interstems — assigned to a more or a less vigorous planting — were assessed at all three sites (Table 2). The dwarfing G.202 was erroneously included in the more vigorous planting. Everything was grafted to Fuji.

The research team measured vegetative and reproductive performance until the conclusion of the trials in 2022. The results are included in the Apple rootstock tables. The final report is due later this year.

Table 2: Apple rootstock and interstem combinations evaluated from 2013–2022 in Elgin and the Langkloof.

More dwarfing planting	More vigorous planting
M.9 EMLA	G.202
M.9 Nic29	M.7 EMLA
G.222	M.793
M.7 EMLA	G.228
M.793 | M.9 EMLA	MM.109
M.793 | G.222	G.778
MM.109 | M.9 EMLA
MM.109 | G.222
G.778 | M.9 EMLA
G.778 | G.222

Trials of rootstocks in three vigour classes (Table 3) were established in 2017 at Môrester in the Koue Bokkeveld and Oak Valley in Elgin. Everything was grafted to Bigbucks.

Independent cultivar-evaluation company Provar manages the project under the leadership of Tristan Dorfling. It is set to run until 2024.

Table 3: Apple rootstocks under evaluation since 2017 in Elgin and the Koue Bokkeveld.

Môrester		Oak Valley
Dwarfing	Semi-dwarfing to semi-vigorous	Vigorous
M.9 EMLA	G.757	M.793
M.9 Nic29	G.222	G.228
G.202	Supporter 4	MM.109
	M.7 EMLA	G.778
	G.890

Provar commenced another trial of rootstocks in two vigour classes in 2018 at Molteno in Elgin and Bokveldskloof in the Koue Bokkeveld. Due to apple replant disease, the Molteno site was replaced by one at Oewerzicht in Greyton in 2019 (Table 4).

The trees were also grafted to Bigbucks.

Table 4: Apple rootstocks under evaluation since 2018 in the Koue Bokkeveld and Greyton.

Bokveldskloof		Oewerzicht
More dwarfing	More vigorous
CG.13	G.222	MM.109 | M.9 EMLA
M.9 T337	M.7 EMLA	G.757
M.9 EMLA	G.969	G.41
G.757	MM.116	G.202
G.41	G.778	G.222
G.202		M.7 EMLA
		G.969
		G.228
		G.778

The preliminary results of the two Provar-managed trials described above are included in the Apple rootstock tables.

Provar-managed rootstock trials will be planted at three locations in the Highveld region in 2023 to assess rootstock performance in fertile soils and under fixed nets. The dwarfing rootstocks M.9 T337, G.757, G.213, G.202 and Bay|Oz CM1 will be grafted to Lady in Red and evaluated until 2031.

New Provar-managed rootstock trials of three vigour classes (Table 5) grafted to RDS, a Cripps Red mutation, are planned for 2024. The scion was chosen because it is considered to be less vigorous. Evaluations will take place in the EGVV, Koue Bokkeveld and Langkloof.

Table 5: Apple rootstocks planned for inclusion in new trials with RDS as scion to be planted in 2024.

Dwarfing to semi-dwarfing	Semi-vigorous	Vigorous
M.9 T337	G.757	G.890
G.757	G.222	G.228
G.213	M.7 EMLA	G.778
G.202	G.890	Selection 2 + 3 New Zealand

Watch Dorfling on YouTube discussing current and new apple-rootstock evaluation projects at the recent Hortgro Science Research Showcase.

Rootstock physiology and management

A 2012–2014 project led by Laura Allderman of Hortgro investigated dormancy progression and chill requirements of M.9, G.202, G.222, M.7, CG.3007, M.793, G.228, MM.109, G.778 and M.25. She collected shoots at different times from Suikerbosrand in the Koue Bokkeveld and Tygerhoek in Riviersonderend and forced them in the laboratory.

The Geneva rootstocks in this trial entered dormancy rapidly, reached maximum dormancy early, and exited dormancy slowly. Malling and Merton rootstocks entered dormancy more slowly and reached maximum dormancy at different times but exited dormancy at similar rates. Allderman found that some Geneva rootstocks achieved deep dormancy before the usual time that Granny Smith and Cripps Pink would be harvested.

M.9 and M.7 reached maximum dormancy later than M.793, MM.109 and M.25. M.9 and M.7 had shallow dormancy, whereas the depth of maximum dormancy varied for the other rootstocks. M.9 and M.7 appeared to have similar dormancy progressions to Granny Smith, whereas M.793, MM.109 and M.25 had similar progressions to Royal Gala.

Allderman reported that chill requirements differed for shoots collected from the two sites — the chill requirement calculated for the colder site tended to be higher than for the warmer site.

Although the study produced a suggested ranking of rootstocks according to dormancy progression and chill requirements, Allderman cautioned that absolute chill requirements calculated using forced shoots were likely to be inaccurate and should be assessed based on data from many sites and seasons.

New research is planned to assess the effect of different rootstocks on bud break. Brazilian observations and research suggest that some Geneva rootstocks can increase bud break in the upper part of trees. However, this may relate to differences in apical dominance rather than chill requirements.

Dr Michael Schmeisser of the Department of Horticultural Science at Stellenbosch University investigated the effect of different rootstocks on net photo-assimilation, water-use efficiency, and water status.

He worked on Rosy Glow trees that were established for a rootstock trial at Paardekloof in the Witzenberg Valley. His measurements included net photosynthesis, stomatal conductance, leaf area index, leaf mineral content, and stem water potential.

Trunk diameter was positively correlated to the leaf area index. Water-use efficiency, stem water potential, and net photosynthesis were similar for all rootstocks.

He recommended that ecophysiological studies of rootstock performance should be done on full-bearing trees. He also advocated further research on the drought tolerance of various rootstocks.

The 2015–2017 drought created concerns among growers about the water-stress tolerance of less vigorous rootstocks. Prof. Stephanie Midgley, Climate Change and Risk Assessment Scientist at the Western Cape Department of Agriculture, led a 2017–2019 study to examine the drought response of full-bearing and young Rosy Glow trees on different rootstocks.

This project also used the existing rootstock-trial planting at Paardekloof mentioned above.

The full-bearing trees were grafted on M.9 Nic29, G.222, M.7, MM.109 with an M.9 EMLA interstem, and M.793. The young trees were grafted on M.9 Nic29, G.202, G.222, M.7 and M.793 and grown in pots. Water stress was created through deficit irrigation. The research team collected data on environmental conditions and tree physiology.

In this trial, more dwarfing rootstocks were not more sensitive than more vigorous rootstocks to drought stress. However, the results might not apply to all dwarfing rootstocks or areas — the mature trees were in the relatively mild Witzenberg Valley.

Calculations of water use per hectare at standard industry tree spacings showed that M.9 used less water than more vigorous rootstocks. More dwarfing rootstocks had better water productivity in these trials, suggesting that they could help growers to optimise water utilisation on water-limited sites.

This project is discussed fully in the June 2023 Fresh Quarterly. Midgley also presented this data at the recent Hortgro Science Research Showcase — watch on YouTube.

Uniform dark green skin colour drives premium prices for Granny Smith. A new trial managed by Provar and led by Dorfling will evaluate the scion growth, productivity, and green fruit colour of Granny Smith under nets. The trees were planted in the Langkloof in 2022 (Table 6).

A second trial will examine the role of rootstocks on the growth synchronisation of Louterwater Granny Smith planted in low-chill areas. Louterwater Granny Smith trees were established in Greyton and Villiersdorp in 2023 (Table 6).

Trees grafted on dwarfing to semi-dwarfing rootstocks are being trained as single-leader spindles, whereas trees on semi-dwarfing to vigorous rootstocks are being trained as dual-leader spindles.

Table 6: New rootstock trials with Granny Smith as scion.

Granny Smith under nets		Louterwater Granny Smith
Dwarfing	Semi-vigorous	Dwarfing to semi-dwarfing Single-leader spindles	Semi-dwarfing to vigorous Dual-leader spindles
CG.13	G.757	M.9 Nic29	G.222
M.9 T337	G.222	G.757	M.7 EMLA
G.757	G.890	G.41	G.969
G.213	M.7 EMLA	G.213	G.210
G.202		G.202	G.890
G.222		G.222	G.228
G.890		G.189	G.778
		G.969
		G.210

The advantages of tissue-cultured rootstocks are much debated in the industry, but research on the topic still needs to be done. A new project managed by Provar and led by Dorfling compares the performance of rootstocks derived from tissue culture and layer beds.

Dorfling will evaluate trees on G.202, M.7 EMLA and G.778 at three locations. The project is set to run until 2027.

Soil-borne pests and diseases

Disease susceptibility is a focus of Hortgro-funded rootstock research because it is a crucial determinant of rootstock performance. Soil-borne pests and diseases of interest include apple replant disease, various root rots, plant-parasitic nematodes, and woolly apple aphids.

A project led by Abraham Vermeulen of the ARC Infruitec-Nietvoorbij assessed the tolerance of M.9, G.202, G.222, M.7, CG.3007, M.793, G.228, MM.109 and G.778 to apple replant disease. Various field and pot trials from 2010–2017 compared tree growth in fumigated and unfumigated soil.

The results showed that rootstock susceptibility was site-specific. Pot trials did not accurately reflect results from the field trials. Note that the pot trials in this project involved rooted cuttings evaluated over two years, by which time the trees were suffering root restriction.

Two subsequent trials of susceptibility to apple replant disease are ongoing. Both are managed by Provar and led by Dorfling. Both trials will assess susceptibility to replant disease by comparing tree performance in fumigated and unfumigated soils on a replant site.

In the first trial, planted in 2021, ungrafted M.9 T337, G.757, G.41, G.202, M.7 EMLA, G.969, G.210, G.890, G.228, MM.109 and G.778 will be followed for two years.

In the second trial, rootstocks (Table 7) grafted to Bigbucks were planted in 2022 and will be followed for several years.

Watch Dorfling on YouTube presenting this data at the recent Hortgro Science Research Showcase.

Table 7: Rootstocks currently under evaluation for susceptibility to replant disease.

Dwarfing to semi-dwarfing	Semi-vigorous	Vigorous
M.9 EMLA	M.7 EMLA	G.890
G.213	G.969	G.228
G.202	G.210	G.778
G.222	G.890	M.793
		MM.109

Lesion nematodes — Pratylenchus hippeastri, P. penetrans and P. vulnus — can damage apple roots, leading to poor growth and declining yields. Dr Rinus Knoetze of the ARC Infruitec-Nietvoorbij screened rootstocks for lesion nematode susceptibility during three growing seasons in 2019–2022.

The rootstocks were planted in pots and inoculated with laboratory-grown lesion nematodes. Knoetze quantified the nematodes 180 days after inoculation and calculated each rootstock’s nematode burden and reproductive potential.

Knoetze also determined the nematode burden of rootstocks from five existing rootstock-trial sites in the EGVV, Koue Bokkeveld and Langkloof.

None of the rootstocks tested were resistant to lesion nematodes, but some were moderately susceptible or tolerant. G.228 showed reduced susceptibility at the trial sites but was not included in the pot tests due to lack of tissue-cultured material.

The results of these trials are included in the Apple rootstock tables.

Watch Knoetze on YouTube presenting this data at the recent Hortgro Science Research Showcase.

Woolly apple aphids damage apple roots and buds. Resistance to this pest is a common goal among rootstock breeders. However, resistance achieved in one part of the world may not transfer to a different part due to the evolution of region-specific aphid strains.

Matthew Addison, Crop Protection Programme Manager at Hortgro, is surveying woolly apple aphids in rootstock-evaluation blocks in Elgin and Ceres. The aim is to compare the woolly apple aphid susceptibility of the various rootstocks.

Watch Addison on YouTube presenting an update at the recent Hortgro Science Research Showcase.

White root rot — Rosellinia necatrix — appears to be an increasing cause of tree losses in South African apple orchards. A project led by Prof. Adéle McLeod of the Department of Plant Pathology at Stellenbosch University is assessing the sensitivity of different rootstocks to Rosellinia.

Her team conducted replicated glasshouse trials with M.9, G.757, G.41, G.202, G.222, M.7, G.969, G.210, G.890, M.793, G.228, MM.109 and G.778. All proved to be susceptible. Although G.757 and G.202 developed the disease slightly more slowly and survived slightly longer than the others, the difference is too small to be useful in the orchard.

McLeod is also leading a project on rootstock tolerance to Phytophthora cactorum, which causes tree losses due to root and crown rot.

Watch Anika Keuck, the student working on these projects, on YouTube presenting her latest results at the recent Hortgro Science Research Showcase.

A new project led by Yolanda Petersen from the ARC Infruitec-Nietvoorbij will include crown-gall susceptibility assessment of different rootstocks.