Specific rootstock benefits have been taken advantage of agriculturally at both the species level and at the clonal level, depending on species and extent of horticultural domestication. Rootstock benefits at the species level include the dwarfing effect of Poncris trifoliata rootstocks on citrus, although this has been shown to be due to an asymptotic virus ( Davis & Albrigo, 1994), and the dwarfing effect of quince (Cydonia oblonga) on pear. One of the earliest reports of a specific seedling rootstock effect is found in 6th century Chinese writings of Chia Shi-yi in the book Tsee Ming Yau Su. He reported that grafting of pear onto seedlings of Pyrus betulifolia rootstock gave better fruit than on P. phaeocarpa rootstocks. With a few crops, most notably apple, pear and grape, deliberate selection of superior rootstock genotypes maintained by clonal propagation (see Propagation of clonal rootstocks, below) has allowed for further refinement.
1. Variability associated with seedling rootstocks
Most species for which grafting is a common mode of propagation, are worked onto understocks grown from seed. A range of examples can be seen in table 1. Since outcrossing is the prominent breeding system among tree species, most of these seedling rootstocks would be highly genetically heterozygous for most characteristics. Hence, they would exhibit a great deal of phenotypic variability, i.e. lack of trueness-to-type . This would be true not only for obvious above ground characteristics like fruit size and color but also for root-system associated and other characteristics, which would influence performance of a grafted tree. Hence, for seedling rootstocks, genetic variability would exist with respect to such root system-specific characters as root architecture (depth and distribution), tolerance of edaphic environmental stresses (soil acidity, drought, salinity, etc.) and biotic stresses (root-specific diseases and pests). The genotype of a given rootstock affects not only these root-system associated characteristics, but also a number of effects by which a rootstock more directly influences the growth and performance of a scion worked onto it. These would include scion vigor (tree size), precocity, etc. In many species of trees that are commonly grafted, this range of genetic and ultimately phenotypic rootstock-associated variability of seedlings is considered acceptable horticulturally for use as rootstock material. This would include most of the seed propagated rootstocks in table 1.
2. Apomictic (clonal) seedling rootstocks
Not all clonal rootstocks require deliberate use of conventional vegetative propagation techniques (cuttage, layering, micropropagation). In the case of species that exhibit apomixis, clonal reproduction is the natural strategy by which the species reproduces. Apomixis refers to seed embryo development entirely from maternal, flower-associated tissues, without fertilization from a male (pollen) parent. Since the apomictic embryo develops entirely from maternal cells, it is genetically identical to the maternal parent, i.e. a clone of the parent tree. Apomixis is an interesting exception to genetic heterozygosity and lack of true-to-type phenotype associated with seed propagated rootstocks. Apomixis occurs in only a small minority of all plant species. In some species that exhibit apomixis, both an apomictic (clonal) embryo as well as a "normal" zygotic (sexual) embryo is produced. These species exhibit polyembryony, i.e. more than one embryo per seed. Citrus and mango are examples of apomictic, polyembryonic species. Some apple species (e.g. Malus hupahensis) produce apomictic, but monoembryonic embryos.
In the section on Reasons for Grafting and Budding, the use of grafting to clonally propagate a selected, genetically superior individual of a species that is otherwise difficult to clonally propagate is discussed. Most deliberate clonal selection is made for one or more above ground (i.e. scion) characteristics such as flower color or fruit size, branching habit, etc. However, in a few species, including apple and grape, not only have scion varieties been selected for above ground characteristics, but also rootstock varieties have been selected for specific root system-associated characteristics such as adaptation to soil pH, abiotic (e.g. drought) and biotic stresses (diseases and insects). In addition, an important selection / breeding criterion may be the effects of rootstock on scion performance. Apple rootstocks, for example, have been selected for their ability to cause dwarfing of the scion (fruiting) variety, by inducing the above ground portion of the tree (shoot system) to go dormant sooner and hence put on less total seasonal vegetative growth.
- These specific rootstock effects, whether they are characteristics of the root system per se, or influences of the rootstock on the scion, are discussed in more detail in the section on Reasons for Grafting and Budding (Section F, Grafting to achieve independent optimization of component genotypes - Specific Rootstock / Interstock Benefits.), and in the section on Size control, below.
Although apple may not have been the first crop for which clonal rootstock selection began, no other crop has been so profoundly influenced by this practice. This is due, at least in part, to the fact that apple is economically the second most important fruit crop in the world. It is superceded only by grapes. Nonetheless, apple has received more attention in terms of clonal rootstock selection and breeding than grapes or any other woody crop. For this reason, apple will be given special consideration below (Section D).
Scions are clonally propagated by grafting, but the clonal rootstock they are grafted onto must be propagated by clonal methods other than grafting, including cuttage, layering, micropropagation, and apomictic seed. In fact, difficulty of clonal propagation of fruit tree rootstocks is one of the major limitations to rootstock selection in some cases. According to the federally funded multi-state NC-140Fruit Tree Rootstock evaluation program,"Knowledge of the propagation characteristics of newer rootstocks and reasons for incompatibility between cultivars and rootstocks is a continuing need. Some promising pome- and stone-fruit rootstocks presently cannot be considered for commercial use because of difficulty in rooting clonal material using existing techniques. Alternative methods of propagation only recently have begun to offer solutions to these problems. Using different propagation methods such as hardwood or softwood cuttings, which are not commonly used, may be very effective for mass production of some of the rootstocks that do not propagate well by conventional means. Expanded research with micropropagated plant material needs to be done to anticipate potential problems before widespread adoption by the fruit industry occurs. These techniques may also allow early screening of plant material for desirable characteristics such as disease and insect resistance." -- NC-140 Web site.
In relatively easy-to-clone species, such as grapes and some roses, rootstock cultivars are rooted from cuttings either directly in the field (rose hardwood cuttings in Kenya), where grafting will subsequently occur, or cuttings are rooted in a greenhouse, as is the case for non-hardy, florists' roses such as these grown in Colombia. Other crops, for which clonal rootstocks are sometimes propagated from cuttings, include plum, peach, and cherry.
For species that will not root easily from cutting, layering is often practiced, because roots are able to form before the propagule (branch, shoot, etc.) is detached (cut) from the parent plant. Apple is relatively difficult to root from cuttings, and layering is by far the most important method for clonal propagation of this crop.
3. Apomictic Seed
Many citrus species and mango varieties, used as rootstocks, are apomictic and polyembryonic, as described above. Hence clonal rootstock propagation in this case is simply a matter of seed germination.
Micropropagation (tissue culture) for clonal propagation of some fruit tree rootstocks has been practiced for years by breeders and/or for research applications, but it has proven to be too expensive to be widely commercially viable.
1. History of Clonal Apple Rootstocks
a. Ancient history
The domestication of apple began thousands of years BC, chiefly from wild Malus pumila, indigenous to the Caucasus Mountains which lie between the Black and Caspian Seas (sometimes said to form the eastern boundary of Europe). These were carried by Caucasians south to the Eastern Mediterranean, where they were eventually used by both the Greeks and Romans, who grafted selected clones onto seedlings or clonal rootstocks from cuttings ( Westwood, 1993). The Greek philosopher Theophrastis (370 BC) wrote of a dwarf apple variety called "spring apple" (Malus pumila), brought by Alexander the Great from Asia Minor to Greece. He also wrote of the practice of grafting apples. Given the fact that these early-domesticated apples were relatively easy to root, it is possible that "Spring Apple" may have been the earliest dwarfing clonal rootstock. Some of the early apple varieties survived the breakup of the Roman Empire, and eventually found their way all over Europe.
b. European history
(1). Paradise apple.
Eventually apple varieties derived from the spring apple became known, at least by the 15th century in France, as the Paradise apple ( Tukey, 1964), and it is likely that it was from these that some of the early dwarfing rootstock selections were made. Most of these variants would have been selected by observant plantsmen, from seedlings, and easily vegetatively propagated from cuttings or layering. Numerous selections were made of the Paradise apple, and since no systematic naming conventions were in effect, a great deal of confusion arose. It is not entirely clear when the transition was made from Paradise apples grown on their own roots to the use of Paradise as a dwarfing rootstock for other apples. The famous gardens at Versailles in France, during the reign of Louis XIV (1643-1715) included both apple and pear trees worked onto the dwarfing Paradise rootstocks. In 1879, in France, a seedling was selected for its dwarfing effect on fruit varieties grafted onto it, and given the name Jaune de Metz (Metz's Yellow), and clonally propagated. Over a century later, this clone and many other pre-existing European genotypes served as the starting point for modern apple rootstock selection and breeding.
(2). Doucin apple
A second group of dwarf apple rootstocks that became common in Europe is the Doucin apple, first mentioned in the European literature during the early 16th century. Doucin apple differs from Paradise in two important ways. First, although somewhat dwarf, it was significantly more vigorous than Paradise, and second, it was not so easily clonally propagated as Paradise.
2. Modern Era of Apple Rootstock Improvement
The modern era of apple production may be said to have begun in England around the turn of the century. The impetus was a need to bring order to the confusing state of affairs with respect to the many rootstock clones descended from the original Paradise and Doucin apples. With the name Paradise, for example, being applied by nurserymen to what were obviously more than one clonal rootstock genotype, consumers simply could not know what they were getting when they purchased "Paradise". Early 20th century clonal rootstock selection and breeding is described below in the sections on the East Malling and the Malling Merton rootstock improvement programs.
a. Selection criteria for clonal apple rootstocks (see NC-140 Web site for a tabular comparison of these characteristics among the currently most common clonal apple rootstocks)
(1). Size Control
Clonal apple rootstock selection has been directed at and is perhaps best known (but not limited to) size control, i.e. the ability of a rootstock genotype to reduce vegetative growth and hence tree size of the scion (fruiting) variety. Indeed, more or less "dwarf" apple trees are a prominent feature of modern apple production. The earliest modern systematic attempt to categorize apple rootstocks with respect to their effect on the vigor and overall size of fruiting varieties grafted onto them was the East Malling program described below.
- Size control is relative, not absolute
- The relative effects of several commonly used apple rootstocks are shown graphically in the associated figure, and numerically, along with other characteristics in the Apple Rootstock Table
- It is important to note that the actual size of any given apple tree depends not only on the rootstock genotype but also on the genotype of the scion variety, and also on soils and other environmental characteristics. In fact even the relative sizes shown in the figure and table are not inviolate: in some cases even the relative size may depend on environmental conditions. Hence the size classes shown in the figure and table should be used only as a guide not a blueprint.
- Mechanism of dwarfing by selected rootstocks. Although the hormonal regulation of the dwarfing process is not clearly resolved in the scientific literature, it is understood that dwarfing rootstocks act by inducing the scion portion of the tree to stop growing (enter the early stages of dormancy) earlier in the summer than the same fruit variety on a non dwarfing rootstock. Hence, total seasonal growth is reduced.
(2). Other important selection criteria for clonal apple rootstocks include, but are not necessarily limited to, the elimination of some of the "problems" discussed below. Some of these include:
- Resistance to insect pests such as wooly aphid.
- Resistance to bacterial diseases such as fire blight caused by Erwinia amylovora.
- Resistance to fungal diseases such as root rot (Phytophthora sp).
- Absence of burr knots (see b.(1), below)
- Absence of suckering.
- Tolerance of adverse soil conditions such as drought, acidity, waterlogging, etc.
Many of these selection criteria are discussed in more detail in the section on Reasons for grafting and budding (Section F. Grafting to achieve independent optimization of component genotypes - Specific Rootstock / Interstock Benefits). Many of these are also the object of current, ongoing apple rootstock improvement programs discussed below.
b. Problems with clonal apple rootstocks
The problem with breeding apple trees or any crop is that seedlings selected for one desirable characteristic often have many characteristics that are not desirable. Hence it takes screening for multiple characteristics over multiple generations to obtain anything approaching the "perfect" genotype. This problem is particularly acute for trees since they typically have long generation times (years).
(1). Burr knots
Burr knots are tight clusters of preformed adventitious root primordia that occur on the stems of susceptible apple genotypes. Since they are associated with ease of clonal propagation (layering and cuttage primarily), they were actively selected for in early rootstock breeding programs. Today, however, they are actively selected against since they are considered more of a problem than a blessing. Since normal stem / root xylem and phloem conduction is interrupted by the tightly massed cluster of adventitious root primordia that make up a burr knots, complete or partial stem girdling, with ultimately fatal consequences, may occur.
(2). Wooly apple aphid (Eriosoma lanigerum)
Wooly apple aphid is an insect pest that can cause considerable damage to apple nursery stock of susceptible genotypes. The early East Malling rootstock selections (M9, etc., described below) were especially susceptible to wooly apple aphid, but since this pest was not a problem in Europe, at the time, wooly apple aphid susceptibility was not considered in seedling selection. However, in subsequent years, apple producers in Australia and New Zealand, where wooly apple aphid was a serious problem, reported that many of the early East Malling (EM) rootstocks were highly susceptible to the pest. Hence, resistance to wooly apple aphid became one of the primary goals of the subsequent Malling Merton (MM) breeding program that began in 1928. Since Northern Spy apple was known to be resistant to wooly apple aphid, it was used as a parent in crosses with the EM rootstocks. Only seedlings that were wooly apple aphid resistant and that had other desirable characteristics (especially various levels of size control) were selected for and ultimately released as MM rootstocks. Wooly apple aphid resistance is still a selection criterion of modern rootstock breeding programs described below.
Suckering refers to the formation and growth of adventitious shoots from the root system of an established tree. In the case of apple varieties grafted onto clonal rootstocks, the suckers are genetically identical to the rootstock genotype, and hence genetically different from the scion variety, and not at all suited for fruit production (many produce small crabapple-like fruit). Hence, suckers must be removed to prevent them from growing to the point when they would become part of the mature fruit bearing canopy of the tree and give rise to off-type apples. Furthermore, suckers of fireblight susceptible rootstocks may become infected with this bacterial pathogen to the detriment of the entire tree. Manual removal of suckers is labor intensive and hence it is desirable to select against suckering during the process of apple rootstock seedling selection.
Some dwarfing rootstocks like M9 tend to exhibit brittle wood due to relatively short fusiform initial cells (cells in the vascular cambium that give rise to wood fibers) resulting in shorter, minimally interlocking wood fibers between stock and scion. Consequently, breakage may occur under a heavy crop load.
The following Web sites have additional information about common apple and other tree fruit rootstock resistance to various "problems":
c. Rootstock breeding programs around the world
(1). East Malling Rootstocks (England)
The East Malling apple rootstock program involved evaluation and selection of existing rootstocks (not breeding per se).
- This was the first of several apple rootstock improvement programs which have continued up to the present day. The objective was to systematically evaluate and select the best of the many clonal rootstocks being used throughout Europe.
- In 1912, R. Wellington, director of the East Malling Research Station in Kent, England, brought together 71 collections of Paradise apple rootstocks acquired from Britain, France, Holland, and Germany. Wellington was succeeded by RG Hatton in 1914. Initially, these rootstock collections were clonally multiplied to generate sufficient numbers of each to begin orchard evaluation. The first report of this work was published in1917, with the description and release of the first nine "Malling" rootstocks.
- These original 9 clonal rootstocks were introduced with a new systematic naming system, Malling I, Malling II ... through Malling IX. For example, Malling IX was the new name given to the original Jaune de Metz clone selected in France in 1879. Subsequently, in 1938, the "Malling" was shortened to "EM", and the roman numerals were changed to Arabic numerals, but more recently "EM" was changed back to "M" for "Malling" (not to be confused with "MM" for the more recent breeding program, "Malling Merton" that is described below).
- Size control (rootstock effect on scion) was the principal selection criterion used in the selection of these original East Malling rootstocks. They were categorized as very dwarf, semidwarf, semistandard, and standard.
- In addition to size control, other desirable characteristics were considered in the selection of these rootstocks including precocity (earliness of bearing), prolific fruiting (effect on fruiting of the scion variety), and ease of propagation. To the extent that these early selections were descended from the original Paradise apples, they were inherently more or less easy to propagate vegetatively, mostly by layering. In fact, a tendency towards excessive spontaneous rooting, know as burr knots, eventually came to be recognized as a problem since it sometimes resulted in partial or complete stem girdling. This tendency to form burr knots has been actively selected against in more recent apple rootstock breeding programs such as the one at Geneva, NY (described below), with the trade off being that such selections are more difficult to clonally propagate.
Later releases from the East Malling program
- Between 1918 and 1970, eighteen more rootstocks were released from the EM program, bringing the total to 27 clones. EM 26 was released in 1959 and EM 27 in 1970. Some of these later releases were not simply selections of preexisting European rootstocks but rather the result of deliberate breeding. Most of the breeding work, however, was conducted in collaboration with the John Innes Horticultural Research Station at Merton, England, which was considered a separate rootstock improvement program known as Malling-Merton (see below).
- Widespread adoption by growers of these EM rootstocks did not occur until the 1930s and 1040s, and have remained an important component of modern apple production.
(2). Malling-Merton Rootstocks (England)
- The Malling-Merton breeding program that got underway in 1928 was motivated in part by a serious pest, known as the wooly apple aphid (Eriosoma lanigerum), which was threatening apple production in Australia and New Zealand.
- The East Malling research station collaborated with the Long Ashton station at Merton, England to cross the highly wooly-apple-aphid-susceptible East Malling rootstocks with the wooly-aphid-resistant Northern Spy apple. The goal was to select seedling genotypes that combined the size control and other favorable characteristics of the East Malling series with the Northern Spy resistance to wooly apple aphid. From a total of 3758 seedlings only 15 were eventually selected for release in 1952. These 15 rootstocks were named MM 101 through MM115.
(3). East Malling Long Ashton (EMLA) Rootstocks (England)
- The EM and MM series rootstocks had accumulated a heavy load of asymptotic viruses, which reduced tree vigor and fruit yield.
- This program was not involved in breeding per se, but rather in virus elimination, accomplished through "meristem culture", a form of micropropagation (tissue culture).
- An interesting outcome was that the rootstock clones free of specific viruses were 10 to 15% more vigorous (less dwarfing).
(4). Budagovsky Rootstocks (former Soviet Union)
- Extreme cold is a major limiting factor for apple production in the region formerly known as the Soviet Union.
- The Budagovsky series was developed by crossing of M8 and a number of cold hardy domestic apples from that region.
- The Budagovsky or B series of apple rootstocks selected for cold hardiness and the full range of size control.
(5). Polish Rootstocks
- Cold hardiness and size control were the objectives in breeding the Polish (P) series rootstocks
(6). Geneva Rootstocks (Eastern United States): USDA-ARS / Cornell University apple rootstock breeding project
- Initiated in 1968 by Dr. James Cummins at the NY Agricultural Research station at Geneva, NY.
- Original objective was to develop rootstocks better adapted than the EM, MM and others previously available, to the biotic stresses (soils, climate) of New York. In the 1970's emphasis shifted to include resistance to biotic stresses including root rot (Phytophthora cactorium), fireblight (Erywinia amylovora), particularly in the context of modern high-density orchard planting systems.
- Unique features of the program are described by the project Web site (link above):
"All seedlings are subjected to rigorous screening programs to eliminate disease susceptible individuals prior to planting in the field. Rootstock genotypes are selected on the basis of abiotic stress tolerance, disease resistance, productivity, and precocity in orchard tests. This breeding protocol contrasts sharply with those of other breeding programs, which have concentrated effort on improving the stoolbed and nursery performance of rootstock cultivars and have employed the much narrower range of traditional rootstock genotypes as parents of breeding populations. As a result, the USDA-ARS/Cornell University rootstock breeding program is recognized worldwide as the most promising source of new, disease resistant and productive rootstock genotypes for the future."
- Four rootstocks have been commercially released so far, but availability is limited, i.e. there is a need for rapid clonal propagation. Characteristics of these rootstocks are listed in the Apple Rootstock Table. Many of these General rootstocks are available from Cummins Nursery.
G65 (M27 x 'Beauty' crabapple)
G16 (Ottowa3 x Malus floribunda)
G11 (M.26 x Robusta 5 Crabapple )
G30 (Robusta 5 x M9)
Three additional Geneva rootstocks are still under evaluation including CG.41, CG.935, and CG.210
(7). Ongoing long term evaluation in North America of clonal rootstocks from many sources - NC-140 project
Most rootstock breeding programs have been primarily concerned with selection and breeding of rootstocks for a particular region with its associated set of environmental and biological (diseases and pests) stresses and constraints. Nonetheless, many of them, particularly the EM and MM series rootstocks, are used outside the region they were developed for. In an attempt to better understand rootstock performance under a wide range of conditions and to allow an objective comparison among rootstocks from different programs under a given set of conditions, the US Department of Agriculture has funded the NC-140 Regional Hatch project. NC-140 involves evaluation of many different rootstocks at many different locations. The NC-140 Web site contains a great deal of useful information about modern research in apple rootstock improvement.
d. Propagation of clonal apple rootstocks - this topic is reviewed in the autotutorial on Commercial Apple Production via Grafting and Budding
e. Additional information about clonal apple and other fruit tree rootstocks and modern fruit tree production can be found at:
H401 CI Homepage
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