Fruit growers information  

Clonal rootstocks have the advantage over seedling rootstocks of uniformity, tree size control, precocity and resistance to soil borne pests and diseases. In some cases they also offer advantages in terms of improved fruit quality.

The choice of rootstock in any situation depends upon the choice of training system, spacing, site vigour, scion variety and soil e.g. incidence of soil borne diseases or replant disease. Rootstock choice, therefore, is part of the interrelated management considerations prior to planting the orchard. To move therefore to a more dwarfing apple rootstock from a more vigorous one may involve changes to tree training, spacing, tree support, machinery and bin size.

Apple rootstocks

The most widely used source of apple rootstocks throughout the world is the Malling and Malling Merton series. Hatton at East Malling in the early 1920s collected the available rootstocks and classified them into distinct clones. These were given the numbers M.1 to M.16. Unfortunately the number did not bear any relation to the dwarfing characteristics of the rootstock. A few years later a crossing program was started between these Malling rootstocks and the woolly apple aphid resistant variety Northern Spy to produce woolly aphid resistant rootstocks, particularly for parts of the British Empire in the Southern Hemisphere. This program, initiated at the John Innes Research Station, then at Merton, gave rise first to the Merton series (M778-793), from which only Merton 793 has survived in common use, and later to the Malling Merton series (MM.101-115). Later crossing at East Malling between M.9 and the other Malling series rootstocks produced M.26 and M.27.

The Merton and Malling Merton series are resistant to woolly aphid, deriving their resistance from Northern Spy, but unfortunately these two series do not cover the same vigour range as the Malling series. There are no dwarfing rootstocks resistant to woolly apple aphid. The most dwarfing (M.27-M.26) need staking or a trellis support throughout their orchard lives. In general, the more dwarfing the stock the earlier the tree comes into bearing.

All these Malling and Malling Merton series rootstocks were selected in the cool maritime environment of NW Europe and suffer from being susceptible to winter injury in the cold winters of central Europe and North America. There is, however, considerable spread in their sensitivity to winter cold with M.26 and M.7 being the most hardy. Consequently in these colder climes much effort has gone into producing rootstocks with a full range of vigour control and resistant to winter cold damage. From this breeding work have come rootstocks from USA (Geneva and Michigan), Canada (the Ottawa clones), Poland (the P series) and the Soviet Union (the Budogovsky series).

The following is a list of the common rootstocks available in order of tree size control, beginning with the most dwarf and going to the more vigorous.

• M.27 (3431)   It is finding an increasing use in areas of NW Europe where M.9 is too strong or with triploid cultivars. It is best to plant with the union close to the ground to avoid problems of burr knots. Tends to produce smaller fruit size than other dwarfing rootstocks. It is also used as an interstem. 
• M.9 the most widely planted rootstock in W. Europe, and used for spindlebush and axe trees. There are numerous clones exhibiting differences chiefly in nursery behaviour and to a limited extent differences in tree size control. More resistant to Phytophthora than MM.106 but not resistant to woolly apple aphid. There will be available shortly material on other clones of virus free M.9 e.g. EMLA 9 from England and NAKB 337 from the Netherlands. Experience of trees on virus free M.9 in New Zealand is rather limited to date, but some orchards have been planted in the last three years.
• Mark from the apple rootstock breeding program of Michigan State University, originally known as MAC 9. Tree size control is between M.9 and M.26; early reports indicated it was closer to M.26 but more recent work suggests trees on Mark in some growing regions are closer in vigour to M.9. It was originally thought to be self supporting but is now frequently staked. Similar or possibly better resistance to Phytophthora than M.9, but again not resistant to woolly apple aphid. Trees on Mark need care in the early years to prevent overcropping and subsequent lack of growth. Growth can often be severely limited by water stress. There are also reports of incompatibility and soil line swellings - a proliferation of tissue at or just below ground level.
• M.26 not as dwarfing as M.9 but more winter hardy. Best planted with the union close to the ground to avoid problems of burr knots and reverse fluting. 
• Northern Spy an apple cultivar from the United States resistant to woolly apple aphid, still used to a limited extent as a rootstock in NZ. Woolly apple aphid resistance in the MM series was derived from this source.
• MM.106 widely used in NZ and in older orchards in the UK, resistant to woolly apple aphid. Semi-vigorous rootstock showing high productivity and good precocity. Becoming less popular than M.9 in the UK, as fruit size is larger on M.9 and tree size is too large for modern requirements.
• M793 often used in replant situations or on clay soils in NZ. Less precocious and less productive than MM.106 with about 30% larger tree size.
• Aotea bred by Don MacKenzie and Bruce Taylor largely for resistance to Peniophora and Phytophthora . This rootstock produces a large tree, significantly larger than M793 in all trials on Moutere Hill orchards. Its resistance to woolly aphid is derived from M. sieboldii , rather than Northern Spy. It shows good tolerance to Peniophora and Phytophthora . Difficult to propagate by conventional stoolbeds. There are other Aotea rootstocks but most produce vigorous or very vigorous trees.

M.26 and MM.106 have both been successfully grown in the UK without stakes when budded high and deep planted, where there were adequate windbreaks. Deep planting can, however, slightly delay early growth and cropping. This practice may not be appropriate for the windier conditions of NZ.

Despite the efforts of rootstock breeders we do not yet possess a complete range of size controlling rootstocks that also exhibit resistance to woolly apple aphid and soil borne diseases. Interstem trees, although more expensive to produce than normal trees, do offer the advantage of linking a root system that is resistant to soil borne diseases with an interstem of a dwarfing rootstock to produce a composite tree with the required combination of characters. Interstem trees also offer a relatively simple way of using difficult to root rootstocks, that have otherwise useful characteristics. Some researchers have, however, noticed an increase in root suckers when a dwarfing interstem is used with a root system of a vigorous rootstock.

Other sources of apple rootstocks are as follows:-

• MAC series - raised by Dr Robert Carlson at Michigan State, with MAC9 or Mark receiving considerable interest; the others in the series are more vigorous, some are in test at the Appleby Research Orchard.
• Canada - Robusta 5, the Ottawa and Vineland series. Robusta 5 is vigorous, tolerant of winter cold and shows good resistance to fire blight and woolly aphid. Ottawa 3 gives similar tree size control to M.26 but is difficult to propagate. Ottawa 3 has proved a very useful parent in breeding programmes in the USA and UK. The Vineland series shows a useful range in tree vigour but there is some uncertainty about their resistance to woolly apple aphid.
• Poland - again winter hardiness is a priority, and the Polish series P.1-22 shows a wide range of tree size control, are said to be more winter hardy than the M and MM series, but some of the more dwarfing ones are difficult to root. P22 has been described as Poland's answer to England's M.27; however "woolly apple aphids love it".
• Russia - the Budagovsky series shows a wide range of vigour control, are winter hardy, but susceptible to woolly aphid and nearly all susceptible to fireblight.
• Geneva (USA) - six rootstocks recently released by Dr Jim Cummins cover a wide range of tree size control with improved winter hardiness. Although the aims of the programme were to produce fire blight, woolly aphid and Phytophthora resistant rootstocks, the two most dwarfing releases (size control similar to M.27 and M.9) are not woolly aphid resistant. Nevertheless there is considerable interest in this new series of rootstocks e.g. CG202 and CG210 are both resistant to fire blight, woolly apple aphid and Phytophthora and early work suggests that CG202 may produce a tree in the M.26 size range.
• East Malling - breeding of apple rootstocks is continuing. AR 86.1.20 and 86.1.25 are similar in tree size control to MM.106 . These two rootstocks are in trial at Appleby and Havelock North and are showing considerable promise. The present emphasis on apple rootstock breeding at East Malling is in the M.27 to M.9 size range and there is some interesting material coming through there. 

Pear rootstocks

There are three main sources of pear rootstocks - quince (widely used in Western Europe), pear Pyrus communis , (widely used in the United States) and other Pyrus species e.g. Pyrus calleryana .

Quince Cydonia oblonga

The major advantages of quince rootstocks are good precocity and good tree size control. They are tolerant of heavy clay soils and tolerant of collar rot. Quinces are also very resistant to crown gall and said by some to be very resistant to pear decline. (Pear decline, a disease caused by a mycoplasma transmitted by pear Psylla, is a serious problem in several parts of the world but fortunately NZ lacks the vector.)

Quinces suffer from being sensitive to winter cold, to lime induced chlorosis, to fire blight (via suckers) and from poor anchorage and poor compatibility. Poor graft incompatibility is usually overcome by using an compatible interstock e.g. Beurre Hardy, for cultivars such as Beurre Bosc. Cultivars such as Comice and Taylor's Gold are, however, fully compatible with quince rootstocks.

Pear Pyrus communis

Pear rootstocks are generally slow to come into cropping and tree size is much larger than with trees on quince rootstocks. They display good anchorage and compatibility, are resistant to pear decline and fire blight (if Old Home is in the parentage) and are tolerant of winter cold, clay soils and lime induced chlorosis. Clonal material e.g. BP 1 and BP 3, is often difficult to propagate. Seedlings of cultivars such as 'Williams' have been widely used particularly in the USA.

Other Pyrus species

P. betulifolia is vigorous - more than twice QA size - and shows slightly poorer precocity than QA. It shows very good anchorage and compatibility. Some sources are sensitive to fire blight, but others are resistant. It is sensitive to lime induced chlorosis. It is tolerant of winter cold and of both wet and dry soils. It is also resistant to pear decline.

P. calleryana shows good anchorage and compatibility, is very resistant to fire blight and resistant to pear decline. It is sensitive to winter cold injury and lime induced chlorosis. Some clonal material is available from Oregon.

The following is a list of the available rootstocks in New Zealand from dwarfing to vigorous:

• Quince C (QC) in common use in NW Europe, very precocious, but tends to produce smaller fruit size than QA, which is not a problem with large fruited cultivars such as Comice.
• The next four are of very similar tree size:
• Quince Sydo originating from Angers in France
• BP 1 a Pyrus communis clonal stock from South Africa, having the advantage of full compatibility with all pear cultivars but difficult to propagate.
• Quince A (QA) in common use in NW Europe.
• Quince Adams a number of clones of quince Adams are documented, with one giving tree size similar to that on quince C and the others showing vigour between QC and QA.
• Quince BA 29 a Provence quince from France and the most common quince in New Zealand
• OH x F51 the most dwarfing of the Old Home x Farmingdale crosses, originating from the Pacific North West, a Pyrus communis rootstock, but again difficult to propagate. There are a number of other Old Home x Farmingdale crosses e.g. OHxF333 (Brokmal), OHxF87 (Daytor), but all tend to be more invigorating and less precocious than trees on quince. They are finding an increasing use in the Pacific North West where they are replacing seedlings as rootstocks for pears. Quinces suffer from poor winter hardiness in this environment.
• BP 3 a Pyrus communis clonal stock from South Africa, difficult to propagate.
• Pyrus calleryana available as seedlings
• Pyrus communis seedlings, most commonly 'Williams'.
• Pyrus betulifolia available as seedlings

Other rootstocks are in the pipeline. QR 193/16 is a very promising quince from East Malling, giving about the same tree size control as QC but larger fruit size. It is an excellent stock in the nursery and the first trials were planted in 1995 in Hawkes Bay and Nelson.

For full list of available rootstocks from Tomagro International B.V. click here.