Fruit Tree Propagation - Apple Rootstocks

Apple Rootstocks

One reason for grafting onto rootstocks is that this enables the grower to determine the tree's eventual size. Apple tree size classes number from one to ten in increasing height and breadth. A "1" is a dwarf which can be productive and as short as 3 feet (0.91 m) with proper pruning. A "10" is the standard sized tree with no dwarfing and will grow to 20 feet (6.1 m) tall and wide or more, dependent upon the variety chosen. In general the class range is (1) 10-20% of full size, (2) 20-30%, (3) 30-40% and so forth to size 10 which is 100% of full size.

The following are a selection of apple tree rootstocks. They are referred to by numbers prefixed by letters indicating the developer of the rootstock.

"Bud 118" A winter hardy early bearing replacement for MM 111 bred in the Soviet Union. Full sized tree unless allowed to bear young which will stunt its growth. Hardy to USDA zone 3.

"Bud 9" A winter- hardy, early- bearing replacement for M9 bred in the Soviet Union. Dwarf tree resistant to crown rot and less susceptible to drought than most other dwarfing stocks. Produces large fruit, is precocious and hardy to USDA zone 3.

"M" designates Malling series developed stocks. East Malling Research is a pioneer in the development of dwarfing rootstocks. East Malling Research Station in Kent, England collected clones of the Paradise stocks from France in 1912 from which 24 "M" were designated with no particular order to the rootstock characteristics other than where they were located in the garden at the time the numbers were assigned. In other words, M.2 is larger tree than M.9 while M.27 is smaller than M.26.

"MM" designates Malling-Merton stocks developed from joint breeding program by John Innes Institute, in Merton, England, & East Malling Research Station in the early 1950s. The "MM" series was developed primarily to provide resistance to Woolly Apple Aphid(Eriosomatinae) infestation.

"EMLA" designates East Malling / Long Ashton research stations who took the "M" stocks and developed virus free versions. E.g., EMLA 7 is M 7 with a guaranteed virus free stock. EMLA characteristics are often different from the parent "M" rootstock. Note that nearly all the apple rootstocks in the industry are now virus free.

"CG" or "G" designates Cornell-Geneva stocks which are those developed via the Cornell & USDA collaboration at the New York Agricultural Experiment Station in Geneva, NY. The "G" is the old designation. All newer stocks are "CG" followed by numbers that actually provide some information about the stock. As one might surmise, this is a huge improvement in the classical naming scheme which has no identification method at all.

• M.27 Malling 27: A very dwarfing rootstock. Unless the central leader is supported, the tree will be very small. Often only used as an intermediate stem piece on MM.106 or MM.111. If handled and spaced properly, it can be a very productive stock for a vertical axe system. Trees can be grown three to four feet tall and produce about 45 fruit, roughly 2 pecks, depending on fruit cultivar.
• M.9: Very dwarfing - Reaches a height of 8 to 10 ft (2.4 to 3.0 m), coming into fruit after 3–4 years, reaching full capacity of 50 to 65 pounds (23 to 29 kg) after 5 to 6 years. It will grow under average soil conditions, but needs a good rich soil to thrive. A good choice where space is limited and fertility is high. Permanent staking is required, as is routine feeding and watering. Trees on this rootstock always require leader support. The rootstock is very susceptible to fire blight and can develop burr knots.
• G.41 Geneva 41, released in 2005, produces trees the size of M.9. The rootstock was developed from a cross between M.27 and Robusta 5 made in 1975. Resistant to Crown|Collar|Root rot(Phytopthora) and fire blight.
• M.26: Dwarfing - Similar to M9 in effect, although somewhat more vigorous and generally stronger, with a higher expected eventual yield of 65–75 pounds (29–34 kg) and height of 8 to 10 ft (2.4 to 3.0 m). A good choice where soil quality is average and compact growth is required. Comes into fruit after 3–4 years, reaching full cropping capacity after 5 to 6 years. Staking needed for first five years of its life. It is susceptible to collar rot and fire blight and should not be planted in a wet site. Certain varieties when grafted onto this rootstock may exhibit signs of graft union incompatibility(i.e., the union breaks).
• G.11 Geneva 11 is the second release of the Cornell breeding program similar in size to M.26(Class 4) but more productive. Has the advantage of being resistant to fire blight and crown rot as well as only rarely producing suckers or burr knots.
• G.202 Geneva 202(CG 5202) is a semidwarfing rootstock that produces a tree in class 5 slightly larger than M.26(Size Class 4) and is more productive than M.26. It was developed from a cross of M.27(Size Class 1) and Robusta 5 to be fire blight and Phytopthora resistant as well as having resistance to woolly apple aphids. In a 9-year study with the scion cultivar of the "Liberty" apple, G.202 was about 50 percent smaller than M.7 but had much greater production efficiency.
• M.7 Malling 7 rootstock produces a semidwarf tree of Class 6 that is freestanding in deep well drained soils but in rocky, steep, or shallow soils, it tends to lean. The rootstock may sucker profusely and is susceptible to collar rot(Phytopthora).
• MM.106: Semi-dwarfing - Sometimes referred to as semi-vigorous, this is the most widely used of rootstocks. It is probably the best choice for the average garden under average conditions, being tolerant of a wide range of soils, and producing a tree with an eventual size of 14 to 18 ft (4.3 to 5.5 m). Trees on this stock begin producing fruit within three to four years, and yield 90 to 110 pounds (41 to 50 kg) after some seven or eight years. MM106 is very suitable for use with weaker varieties that would produce under sized bushes with more dwarfing rootstocks. Can be trained as a half standard tree, but is rather too vigorous for cordons unless the soil is poor. Requires staking for the first five years or so of its life. Trees on MM.106 are highly susceptible to collar rot especially when planted in soils that remain wet(poor percolation).

• M.111 : Vigorous - Not generally suitable for garden scale growing, being both too large and spreading (18-25'), and too slow to come into cropping. They are however suitable for growing as specimen standards in the large garden, or for producing medium sized bushes on poorer soils. Begins to fruit after six or seven years, reaching full capacity of 160 to 360 lb (73 to 160 kg) after eight to nine years. It is not winter hardy in US zone 3 unless you receive abundant snow cover. Bud 118 was developed in the Soviet Union to replace MM 111, Bud 118 is winter hardy in zone 3 and very precocious. (Early bearing). Planting depth of this rootstock is critical. The union should be no higher than 1 to 2 inches above the final soil line.
• M.25: Very vigorous - Suitable for a grassed orchard, and to grow on as a full standard. Plant 20 ft (6.1 m) apart, makes a tree of 15 to 20 ft (4.6 to 6.1 m) or more height and spread, eventually yielding 200 to 400 lb (91 to 180 kg) per tree. This rootstock is primarily used in UK and is rarely seen in USA where M.111(size Class 8) is used for this size tree.
• Seedling: Very vigorous trees produced on a rootstock grown from seed. There is greater variability than with the vegetatively propagated rootstocks. Apples used for production of seedling rootstocks include 'Dolgo' and 'Antonovka', which are both extremely hardy and vigorous.

The Malling series and clones have been standard rootstocks for apples for many years and remain the "workhorses" for the commercial industry in the USA and the UK. However, since most of them are susceptible to disease some Malling rootstocks are being replaced by new breeds, including the Cornell-Geneva series, which has resistance to the major problems preventing quality production of apples utilizing organic control systems. One of the newest rootstocks of the "CG" series, only released commercially in 2004, is CG5202(G.202) which adds resistance to the woolly apple aphid, and when combined with highly resistant cultivars such as "Liberty" it is showing great potential.

Another desirable characteristic of rootstocks is environmental adaptability. This may be tolerance to wet/dry soil conditions, acidity/alkalinity of soil or even hot/cold air temperature. Rootstocks based on Siberian Crab apple are being used in colder areas for more cold tolerance.

The ability of new rootstocks to modify or augment characteristics of fruit trees is limited and may disappoint in the long term. It takes ten years to get a full picture of the effects of any one rootstock, so a rootstock that appears promising in the first five years of a trial may fail in the last five years. The Mark rootstock was such a stock and has now fallen mostly into disfavor. Another, the G.30, has proved to be an excellent stock for production but it was only after a number of years of trials that it was found to be somewhat incompatible with the cultivar 'Gala', so that it is now recommended to be staked and wired.

An industry consortium undertakes trials of different rootstocks - called the "NC-140" trials. These test many pome rootstocks in many different sites across the USA and thereby provide growers a clearer picture of what to expect when growing fruit trees on specific stock, in specific planting methods in their specific area of the USA. This information has the potential to create economic benefits to both growers and consumers, as well as helping to reduce the need to spray pesticides as frequently as is currently required.