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Fruit tree propagation
Perennial plants can be propagated either by sexual or vegetative means. Sexual reproduction occurs when male pollen from one tree fertilises the ovules (incipient seeds) of the flower of another, stimulating the development of fruit. In turn this fruit contains a seed or seeds which, when germinated, will become a new specimen. However, the new tree will inherit many of the characteristics of both its parents, and it will not grow 'true' to the variety from which it came. That is, it will be a fresh individual with many unpredictable characteristics of its own. Although this is desirable in terms of increasing biodiversity and the richness of the gene pool (such sexual recombination is the source of most new cultivars), only rarely will such fruit trees be directly useful or attractive to the tastes of humankind. A tendency to revert to a wild-like state is common.
Therefore, from the orchard grower or gardener's point of view, it is preferable to propagate fruit cultivars vegetatively in order to ensure reliability. This involves taking a cutting (or scion) of wood from a desirable parent tree which is then grown on to produce a new plant or 'clone' of the original. In effect this means that the original Bramley apple tree, for example, was a successful variety grown from a pip, but that every Bramley since then has been propagated by taking cuttings of living matter from that tree, or one of its descendants.
Additional recommended knowledge
The essentials of our present methods of propagating of fruit trees date from pre-Classical times. Grafting as a technique was first developed in China from where it was imported to Greece and Rome. Classical authors wrote extensively about the technical skills of fruit cultivation, including grafting techniques and rootstock selection. The oldest surviving named varieties of fruits date from classical times.
The simplest method of propagating a tree asexually is rooting. A cutting (a piece of the parent plant) is cut and stuck into soil. Artificial rooting hormones are sometimes used to assure success. If the cutting does not die of desiccation first, roots grow from the buried portion of the cutting and it become a complete plant. Though this works well for some plants (such as figs and olives), most fruit trees are unsuited to this method.
Root cuttings (pieces of root induced to grow a new trunk) are used with some kinds of plants. This method also is suitable only for some plants.
A refinement on rooting is layering. This is rooting a piece of a wood that is still attached to its parent and continues to receive nourishment from it. The new plant is severed only after it has successfully grown roots. Layering is the technique most used for propagation of clonal apple rootstocks.
The most common method of propagating fruit trees, suitable for nearly all species, is grafting onto rootstocks. These are varieties selected for characteristics such as their vigour of growth, hardiness, soil tolerance, and compatibility with the desired variety that will form the aerial part of the plant (called the scion). For example, grape rootstocks descended from North American grapes allow European grapes to be grown in areas infested with Phylloxera, a soil-dwelling insect that attacks and kills European grapes when grown on their own roots. Grafting is the process of joining these two varieties, ensuring maximum contact between the cambium tissue (that is, the layer of growing plant material just below the bark) of each so that they grow together successfully. Two of the most common grafting techniques are 'whip and tongue', carried out in spring as the sap rises, and 'budding', which is performed around July and August.
Whip and Tongue grafting
Another reason for grafting onto rootstocks is that this enables the grower to determine the tree's eventual size. Apple tree size classes number one to ten in increasing height and breadth.  A "1" is a dwarf which can be productive and as short as three(3) feet with proper pruning. A "10" is the standard sized tree with no dwarfing and will grow to twenty(20) or more feet tall and wide, 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.
Apple tree rootstocks are referred to by numbers prefixed by letters indicating the developer of the rootstock.
"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. 
That is only a sample of some of the more important current apple rootstocks that are available. There are at least a hundred more that have been developed to either provide enhancement or prevent potential damage from one kind of pest or another.
The problem with growing fruit trees, especially apple trees, is that they are subject to many different types of damage from bacteria, fungi and insects. The general approach of the commercial industry has been to use as many chemicals as necessary to insure attractive and marketable fruit. The attitude, still prevalent, has been "Who cares? Nobody eats a tree!"  but as environmental problems increase and the general public pushes for low or no-spray fruit, there has become a commercial need for fruit that does not require such intensive spray programs. This is being achieved, albeit slowly, by rootstocks and trees that are bred to have natural disease and pest resistance.
The Malling series and clones have been standard rootstocks for apples for many years and remain the standard "workhorses" for the commercial industry(in USA).  However, since most of them are susceptible to disease some Malling rootstocks are being replaced by new breeds, including the Cornell-Geneva series. One of the newest rootstocks, only released commercially in 2004, is CG5202(G.202) which adds resistance to the woolly apple aphid(WAA) for the "CG" series of stocks which already has resistance to the major problems preventing quality production of apples utilizing organic control systems. Combined with highly resistant trees such as "Liberty" it is showing great potential.  
That leads to another characteristic of rootstocks that is or can be bred into them: environmental adaptability. This may be tolerance to wet|dry soil conditions, acidity|alkalinity of soil or even hot|cold air temperature. 
Some new rootstocks based on Siberian Crab Apple are being used in colder areas for more cold tolerance.
The ability of rootstocks to modify or augment characteristics of fruit trees is limited and often disappointing in the final results. 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 (apple) 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 "Gala" apple(and possibly others) so that it is now recommended to be staked and wired.
To get a clear picture and push the industry forward, a consortium was founded and the so-called "NC-140" trials of rootstocks began.  These test many pome rootstocks in many different sites across the USA and thereby provide growers, be they backyard or commercial, 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. As one can imagine, this has the potential for a large economic benefit to both growers and consumers as well as going a long way to eliminating the need to spray pesticides as frequently as is currently required. 
Pears are usually grafted onto quince rootstocks, which produce small to medium sized trees. Some varieties however are not compatible with quince, and these require double working. This means that a piece of pear graft-work compatible with both the quince rootstock and the pear variety is used as an intermediate between the two. If this is not done the pear and the rootstock could eventually separate at the graft. Varieties that require double working include 'Bristol Cross', 'Dr Jules Guyot', 'Doyenné d' été' and 'Williams Bon Chrétien'.
Pear stock: Very vigorous- Pears grafted onto pear rootstocks make very large standard trees, not suitable for most gardens.
Until the 1970s, cherries were grown of the vigorous Malling F12/1, Mazzard (Prunus avium), or Maheleb (P. maheleb) rootstocks, which required much space and time before cropping began, thus the growing of cherries was not a realistic option on a garden scale. The introduction of the rootstock 'Colt' enables trees reaching a maximum height of 12 to 15 ft (4 to 5 m) to be grown, and if trained as a pyramid it is possible to restrict growth to about 10 ft (3 m). The popular sweet variety 'Stella' could even be grafted onto a 'Colt' rootstock and successfully grown in a pot on the patio.
Plum rootstocks include;
Own-Root Fruit Trees
Some species of fruit are commonly grown on their own roots; new plants are propagated by rooting, layering, or modern tissue-culture techniques. In these cases there are may be no great advantages to using a special rootstock or improved rootstocks are not available. Fig, filbert, olive, pomegranate, gooseberry, bramble, and other fruits are commonly grown without any special rootstock.
Though vegetative propagation of apple, pear, stone fruits, and many other species is a nearly universal practice, it does have some detractors. Here is an account of one advocate of own-root fruit trees.
Own-root apples in a Permaculture design
Permaculture designer and fruit nurseryman Phil Corbett has for a number of years been working on an innovative project involving growing fruit trees on their own roots. His work is based on research carried out by Hugh Ermen of the Brogdale Horticultural Research Station, home of the National Apple Collection. Corbett writes; "Hugh discovered that there are several advantages in growing apples on their own roots- that is, not grafted onto a rootstock. The graft union, which is a union between two genetically different individuals, always creates a degree of incompatibility. Not having this incompatibility, own-root trees were found to have better health and better fruit quality. The only disadvantage of own-root trees is that most varieties are more vigorous than is usually wanted. This means that trees may make a lot of wood at the expense of fruit bud production, giving big trees that take a long time to come into crop. Conventionally this vigour is controlled by grafting onto a dwarfing rootstock. With trees on their own roots, however, a number of traditional techniques can induce early cropping. Once cropping begins, the tree's energies are channelled into fruit production and growth slows down to a controllable level. The techniques that are usually sufficient to bring about cropping are:
Once cropping has begun, a normal feeding and watering regime can begin. The average cropping own-root tree can be maintained at a size very similar to the same variety on MM106 rootstock" (Phil Corbett, from The Common Ground Book Of Orchards, pub Common Ground, 2000).
He is also conducting research into the 'coppice-ability' of own-root fruit trees, including an experimental 'Coppice Orchard' project, wherein own-root trees are planted in north-south rows; "When the canopy of the orchard closes, a north-south row will be coppiced and the land in the row used for light demanding crops while the trees regrow. The trees on either side of the glade will have higher light levels on their sides and produce more fruit buds". Over time a series of parallel sheltered glades will be created which will be coppiced on a rotational basis, allowing for multifunctional use of land in order to produce not only fruit but also small wood products, soft fruit, vegetables and even possibly cereals, fungi and the more traditional bees and poultry. This is a project with much promising potential, and deserving of attention from the wider organic growing movement for the valuable lessons that will no doubt be provided over time.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Fruit_tree_propagation". A list of authors is available in Wikipedia.|