Rootstocks for grafting apples are chosen for their influence on tree growth and fruiting, tolerance of prevailing soil conditions, and resistance to disease. While indigenous Malus species have traditionally been used in parts of China (Gu et al. 2003) and probably in Europe (Juniper & Mabberley 2019), modern commercial rootstocks are cultivars of M. domestica selected for the purpose.
As early as the 15th century, dwarfing (‘Paradise’) and semi-dwarfing (‘Doucin’) rootstocks were in use in Europe, perhaps introduced from the Caucasus or Iran. 20th century practice has been dominated by a series of stocks selected at East Malling research station, Kent, UK, largely out of the great range of existing Paradise and Doucin stocks (Juniper & Mabberley 2019, NIAB EMR 2020); they are usually denoted by the prefixes ‘M’ (Malling) and ‘MM’ (Malling-Merton, raised in partnership with the John Innes Institute). The dwarfing stock ‘Malling 9’ or M9 has been dominant in commercial orchards across the world. Given the rapid cycles of vegetative propagation of rootstocks, it is no surprise that somatic mutation and accumulation of viruses has given rise to numerous sub-strains of M9 in particular. An important range of virus-free heat treated versions of Malling stocks is prefixed ‘EMLA’, although others are available; EMLA9 denotes heat treated M9, and so on. The following Malling rootstocks are commonly used in Britain today, listed from least to most vigorous: M27, M9, M26, MM116, MM106, MM111, M25 (Matthews 2018).
More recently, stocks developed by the Cornell University/USDA program at Geneva, NY, have come to the fore in North America, their names usually prefixed by ‘G’; we list a selection of the most widely available. Budagovsky stocks (prefixed ‘B’ or ‘Bud’), raised initially by V.I. Budagovsky at the Minchurin Institute, Russia, are exceptionally cold hardy due to genetic input from M. baccata (Jackson 2003). They are also gaining popularity in North America. Rootstock breeding is a slow business, Geneva rootstocks having on average taken over 30 years to bring to market (Fazio et al. 2015).
Rootstocks are traditionally propagated by stooling, the practice of cutting back a first-year rootstock to near ground level when dormant, then earthing up the shoots which regrow. This encourages them to root independently, before they are separated for use or further stooling. Hardwood and softwood cuttings are also used, and increasingly micropropagation (Webster 1995).
In the cultivar descriptions, comments relating to vigour and fruiting relate to scions grafted onto that stock. Cultivar information from Matthews (2018); Ashridge Nurseries (2020); Dininny (2016); Crassweller & Schupp (2018); Washington State University (2020).
Very vigorous, adaptable and tolerant of dry sandy soils but tends to encourage biennial cropping; very cold hardy; moderate resistance to fireblight and collar rot.
Very dwarf, some support needed; very cold hardy; high resistance to collar rot and fireblight; susceptible to wooly aphid.
Dwarfing, slightly more vigorous than M9, and more productive; frost tolerant; resistant to collar rot and some strains of fireblight.
Dwarfing, less vigorous than M9; high overall disease resistance; graft union can be physically weak, so support recommended.
Semi-dwarfing, vigour comparable to M26; high disease resistance; good for replanting, especially on difficult soils.
Semi-dwarfing, slightly more vigorous than M26; cold hardy; good disease resistance but susceptible to wooly aphid; sensitive to viruses, so suitable only for virus-free scions.
Very vigorous, suitable for half to full standards, for example in traditional cider orchards; useful on poor soils which tend to slow growth; heavy cropping from an early age; susceptible to wooly aphid.
Semi-dwarfing and usually requiring some support; suitable for some varieties as a bush or cordon; widely susceptible to pests and diseases, largely superseded in North America.
Very dwarf, to 2 m or less with reduced fruit size, suiting the smallest sites, fertile soils, and vigorous triploid scions.
Semidwarfing, but very prone to suckering; very hardy; resistant to fireblight. Descended from a French Doucin variety. Until recently popular in eastern North America.
Dwarfing, productive with good fruit size; tends to provide poor anchorage and may need permanent staking; susceptible to fireblight; selected from European Paradise varieties; a mainstay of commercial orchards in many parts of the world, existing in many strains, some of which are virus-free.
Semi-vigorous, for half-standards, bushes and cordons, a popular general-purpose rootstock often used for ornamental crabs; very productive from an early age but prone to collar rot on wet soils.
Vigorous, making a standard on good soils, a half-standard in poorer sites, popular as a stock for ornamental crabs; good resistance to pests and diseases.
A more recent East Malling variety, semi-dwarfing, between M26 and MM106 in vigour; as productive as MM106 but more disease-resistant, particularly to phytophthora.
Malus Supporter 4™
Semi-dwarfing, and usually needing some support; similar vigour to M26 but better productivity; cold tolerant; resistant to wooly aphid but susceptible to scab. Selected by O. Schindler, Germany, 1921 (Fischer 1997); increasingly used in Europe and North America.