Rootstocks

Rootstocks in Tree Fruit Production by Dr. Hao Xu

Rootstocks usually originate from the wild relatives of the domesticated tree fruit species. They are widely used to control tree size, confer precocity, achieve high yield efficiency and enhance crop resilience. Root water transport capacity, regulated by root volume and xylem hydraulic traits, is among the main determinants of scion vigor and stress resilience. The water transport limitations in dwarfing rootstocks can effectively restrict tree growth therefore increase fruit production per acre in high density plantings [1,2]. In addition, production systems based on dwarfing rootstocks are easier to spray, prune and harvest, and are more compatible with future mechanical and robotic operations as well as protective cultivation facilities. According to the mature tree size from small to large, dwarfing rootstocks are classified into small dwarfing, standard dwarfing, large dwarfing and semi-dwarfing [3]. In general, smaller rootstocks are used to achieve higher planting density. However, their shallower root system, smaller trunk and lower foliage density can be drawbacks when tree survival is challenged by environmental extremes. Larger rootstocks require more spacing, but they possess some hydraulic advantages. Under sufficient irrigation, more effective shading and transpiration cooling in the canopy on larger rootstocks can help to reduce fruit surface temperature and UV exposure in the summer. In the winter, more vascular tissues in larger trunks can help mitigate cold damage.

To identify the most suitable rootstocks for a fruit variety, long-term trials are conducted to evaluate tree vigor, stress resilience traits (drought, heat, cold, pests, etc.), yield potential and fruit attributes (ripening time, color, size, soluble solids content, etc.), under specific climatic, edaphic and horticultural conditions. In recent years, a few new rootstocks of apple and sweet cherry have been assessed in the sandy-loam soil in irrigated orchards under the semi-arid Okanagan climate. Several innovative peach rootstocks which have been used to increase yield efficiency in high density plantings and to enhance disease resistance in the United States, have great potential for adaptation in BC. These rootstocks will play a key role in the industry’s transition into innovative production systems to attain higher yield efficiency, better resilience and more economic return. This section intends to highlight promising rootstock options from global rootstock breeding programs and from research projects conducted at Summerland Research and Development Centre, Agriculture and Agri-Food Canada. The relevant in-depth information can be found in Further Readings.

Table 1. Key Traits Evaluated in Rootstock Trials

Category	Traits
Tree Vigor	Scion vigor, trunk size (trunk cross section circumference at 30 cm above graft union), root volume
Stress Resilience	Drought tolerance, heat and cold resilience, pest and disease resistance or tolerance
Yield Potential	Cumulative yield, yield efficiency (yield of tree per unit of trunk cross section area)
Fruit Quality	Ripening time, color, size, soluble solids content, physiological disorders

Apple

Most of the commercially available apple rootstocks in North America originated from the breeding programs of Malling® (the M. series) and Budagovsky® (the B. series), including the standard dwarfing M.9 and M.26 and the small dwarfing B.9 which are commonly used in the apple orchards in BC. The standard dwarfing rootstocks have successfully transformed apple production into the trellised high-density planting; however, they showed susceptibility to extreme temperature events. For example, vascular tissue damage on M.9 due to cold injury was linked to the incidents of rapid apple decline in North America. In recent years, Geneva® rootstocks (the G. series) have been gaining recognition [4,5]. Compared to M.9, large dwarfing Geneva rootstocks can confer higher cumulative yield. In a ‘Honeycrisp’ trial at Summerland Research and Development Centre, semi-dwarfing G.202 and G.4004, and large-dwarfing G.935 and G.4814, led to 40%-70% increase in the 5-year cumulative yield in the 3’ × 12’ spacing trial, compared to M.9 [6]; in a ‘Buckeye Gala’ trial, G.935, G.4814, G.969 and G.41 exceeded M.26 and M.9 [7]; in an ‘Ambrosia’ trial, G.935 and G.202 surpassed M.26, M.9 and B.9 [8]. In addition, ‘Buckeye Gala’ and ‘Ambrosia’ on G.935 had larger average fruit size, and had lower ratio of sunburn-damaged fruits after the 2021 heat waves [7,8]. The Geneva rootstocks also confer resistance to fire blight and Phytophthora crown and root rot. Most of the Geneva rootstocks are more tolerant to replant disease complexes than Malling rootstocks, although trials at the Summerland Research and Development Centre did not demonstrate differences in susceptibility to root-lesion nematodes among M.9, M.26, G.935 and G.41 rootstocks.

Rootstocks in B. series are bred for winter hardiness. On the small dwarfing B.9, yield and fruit quality was lower than on G. series rootstocks in the 3’ × 12’ spacing in the Summerland ‘Ambrosia’ trial; its cumulative yield was similar to M.9 but lower than M.26. In the ‘Buckeye Gala’ trial, the yield on the moderate dwarfing B.10 was comparable to G.935 and G.969 and higher than M.9 and M.26; it also had the least yield variation across years.

Table 2. Performance of Apple Rootstocks in Summerland Trials

Series	Trialed Rootstocks	Performance in Summerland Trials
Malling (M. series)	M.9, M.26	- Industry standard rootstocks used for comparison in trials. - Lower cumulative yield compared to Geneva rootstocks (Honeycrisp, Buckeye Gala and Ambrosia trials).
Budagovsky (B. series)	B.9 B.10	- Cold hardy B.9: - Yield and fruit quality lower than Geneva rootstocks (Ambrosia trial). - Produced similar cumulative yield as M.9 but lower than M.26 (Ambrosia trial). B.10: - Yield comparable to G.935 and G.969 and higher than M.9 and M.26 (Buckeye Gala trial). - The least yield variation across years (Buckeye Gala trial).
Geneva (G. series)	G.202, G.4004, G.935, G.4814, G.969, G.41	Honeycrisp Trial: 40%-70% higher 5-year cumulative yield with G.202, G.4004, G.935, and G.4814 compared to M.9.
		Buckeye Gala Trial: G.935, G.4814, G.969 and G.41 outperformed M.26 and M.9 in yield and fruit size.
		Ambrosia Trial: G.935 produced higher yield and larger fruit compared to M.26, M.9, and B.9.
		Reduced sunburn damage observed in Buckeye Gala and Ambrosia on G.935 during the 2021 heatwaves compared to M.9, M.26 and B.9. G.935 is susceptible to latent virus, so using clean budwood is critical.

Please see Geneva Rootstock Comparison Chart at this link: GENEVA-Apple-Rootstocks-Comparison-Chart.pdf

Sweet cherry

Using vigorous rootstocks, mainly Mazzard and sporadically Mahaleb and Colt, BC sweet cherry industry has achieved tremendous success. Semi-dwarfing (Semivigorous) and fully-dwarfing rootstocks can help the industry achieve higher density planting for higher yield efficiency and easier orchard management [9]. Some rootstocks in the Gisela® series, the Krymsk® series, the MaxMa® series, the WeiGi® series and the Corette® series are great alternatives for Mazzard in terms of conferring precocity, controlling vigor and implementing innovative canopy training systems, for higher yield efficiency, higher horticultural management efficiency and better economic return [10,11].

At Summerland Research and Development Centre, ‘Skeena’ on Gisela 3, Gisela 5 and Gisela 6 has had 100% survival rate since 2010 in a rootstock and canopy training system trial. In the 5’ ×12’ spacing, average scion trunk cross section area at 30 cm above the graft union was 72.71 cm², 99.56 cm² and 111.94 cm² in Gisela 3, Gisela 5 and Gisela 6, respectively, at the 10^th leaf [12]. Gisela 6 led to higher accumulative yield than Gisela 3 and Gisela 5, and had sufficient vigor to support central leader TSA (Tall Spindle Axe) as well as multi-leader systems UFO (Upright Fruiting Offshoot) [13]; KGB (Kym Green Bush) training system can lead to heavy crop load and small fruits. The average cumulative yield during 2014‒2019 was higher than 61 tons / acre in both UFO/Gisela 6 and TSA/Gisela 6 systems [14]. Krymsk 5 was effective in controlling tree vigor for ‘Lapins’, however it showed susceptibility to cold damage. In a trial located in a frost pocket, about ½ of ‘Lapins’/Krymsk 5 trees have been in decline and never reached full production since the trial initiation in 2015.

Figure 1. Relative cherry rootstock size. Credit: Good Fruit Grower/Washington Tree Fruit Research Commission

Peach

Rootstocks used in peach production are selected for graft compatibility, resistance to abiotic and soil-borne biotic stresses, tree vigor control, high planting density, improved yield and fruit quality, and ease of horticultural management [15,16]. The most widely used rootstocks for commercial peach production are open-pollinated seedlings of the wild relatives of peach Prunus persica, including the vigorous rootstocks Guardian®, Bailey® and Lovell® [17], and the cold hardy rootstocks Siberian C, Chui Lum Tao and Tzim Pee Tao. Several Peach × Plum hybrid rootstocks such as the Controller® series, MP-29®, Krymsk 1® and Krymsk 86® [16,17], and Peach × Almond hybrids such as the Rootpac® series [18], demonstrated great potential in vigor control, variety compatibility, soil adaptation and disease resistance in the southern climates. Guardian® was selected in the southeastern US for tolerance to ring nematodes and the associated Peach Tree Short Life canker complex which is triggered by ring nematodes. Ring nematodes have recently become recognized to be widespread in BC Prunus orchards. The performance of these rootstocks with popular peach varieties should be evaluated in Canadian peach production regions.

Table 4. Peach Rootstocks

Rootstock Type	Examples	Key Traits	Notes
Seedling Rootstocks	Guardian®, Bailey®, Lovell®	- Vigorous - Graft compatibility	Widely used in commercial peach production.
Cold Hardy Seedling Rootstocks	Siberian C, Chui Lum Tao, Tzim Pee Tao	- High cold tolerance	Effective in cold climates
Peach × Plum Hybrids	Controller® series, MP-29®, Krymsk 1®, Krymsk 86®	- Adapted to Southern climates - Disease resistance - Variety compatibility - Vigor control	Evaluated in southern climates, potential in BC Requires trials in B.C.
Peach × Almond Hybrids	Rootpac® series	- Adapted to Southern climates - Disease resistance - Variety compatibility - Vigor control	Evaluated in southern climates, potential in BC Requires trials in B.C.

Pear

A few quince rootstocks and the OHxF series can effectively control pear tree vigor [19,20]. Their winter hardiness and water demand require further investigation.

Acknowledgement

The innovative rootstocks in the ‘Honeycrisp’ apple trial, the ‘Buckeye Gala’ apple trial and the ‘Skeena’ cherry trial at Summerland Research and development Centre were obtained through the NC-140 Regional Research Project “ Improving economic and environmental sustainability in tree fruit production through changes in rootstock use” (nc140.org). The ‘Ambrosia’ apple rootstock trial is supported by the research funds provided by New Tree Fruit Varieties Development Council.

The above-mentioned ‘Honeycrisp’ trial at Summerland Research and Development Centre was established by Dr. Cheryl Hampson. The ‘Skeena’ sweet cherry trial was established by Dr. Denise Neilson and Dr. Gerry Neilson. The ‘Lapins’ cherry trial was established by Dr. Tom Forge.

Dr. Gregory Reighard, Dr. Ioannis Minas, Dr. John Cline, Kathryn Carter, David Nield, Graham Karner and Nick Ibuki provided invaluable insights to the discussion about peach rootstocks.

Further Readings (To the guide reviewers: the references used in this draft are listed below in the grower-friendly format. The web links are embedded.)

Apple rootstocks

[1] Rootstock–scion hydraulic balance influenced scion vigor and yield efficiency of Malus domestica cv. Honeycrisp on eight rootstocks.

[2] Rootstocks with different vigor influenced scion–water relations and stress responses in AmbrosiaTM apple trees (Malus Domestica var. Ambrosia).

[3] Budagovsky, Geneva, Pillnitz, and Malling apple rootstocks affect ‘Honeycrisp’ performance over eight years in the 2010 NC-140 ‘Honeycrisp’ apple rootstock trial.

[4] The Geneva series of apple rootstocks from Cornell: performance, disease resistance, and commercialization.

[5] Geneva apple rootstocks comparison chart v.4

[6] Apple rootstock vigor and production: Effects of rootstock on Honeycrisp. Agriculture and Agri-Food Canada, Summerland, BC, Canada. 11p.

[7] Tools for climate resilience in tree fruit I: large-dwarfing rootstocks can alleviate sunburn damage in “Buckeye Gala” apple.

[8] Characteristics of sunburn browning fruit and rootstock-dependent damage-free yield of Ambrosia™ apple after sustained summer heat events.

Sweet cherry

[9] Precocious, dwarfing, and productive—how will new cherry rootstocks impact the sweet cherry industry?

[10] Cherry Rootstocks for the Modern Orchard.