A. Grafting for Clonal  Selection and Propagation of Otherwise Difficult-to-Clone Plants

1. When a plant must be clonally propagated to maintain a selected genotype (cultivar , new sport ), but is difficult to propagate vegetatively by cuttings or other means, it is often grafted or budded.

a. Shade tree cultivars of several difficult-to-root species are routinely budded:

b. Other ornamental cultivars

(1) Cultivars of selected Pinaceae (Pine Family) species with unusual growth forms

(2) Bloodgood Japanese Maple (A. japonicum 'Bloodgood')
(3) Taxus bacatta 'Repandans'
(4) Upright juniper cultivars

c. Find out about these and other ornamental plants. The Nursery Web has links to many Plant Identification websites

2. Economics - sometimes grafting is less expensive than cuttage

  • This is a corollary to A.1. above, since if a selection is difficult to root, grafting is usually cheaper than cuttage.
  • a. Although labor for grafting perse is usually more costly than cuttage (more time-consuming per unit, and more skilled), the cost of materials and equipment may be lower if cuttage requires long periods in a heated greenhouse, with bottom heating, mist, etc.
    b. Cost analysis: Flowering dogwood cultivars: This is the result of an economic analysis of costs for production of flowering dogwood cultivars by either budding or cuttings ( Badenhop, 1986).

    3. Budding for delayed self rooting of slow to root species / Nurse (root) grafting  (NRG)

    a. Some species are difficult to root from cuttings, because a conventional cutting cannot stay alive long enough for rooting to occur. Such "cuttings" may be grafted to a piece of root to keep them alive long enough for them to become self rooted. This is called nurse root grafting. The graft union is planted below the soil line (unlike most grafting), and eventually the scion becomes self rooted. Afterwards the rootstock can either be deliberately removed or it will die off, especially in cases where the scion and rootstock are not closely related, resulting in a delayed graft incompatibility. Incompatibility is discussed in the section on Compatibility  

    Question In what way is the process of nurse grafting similar to layering?

    Examples:

    (1) Lilac (Syringe vulgaris), previously, was commonly nurse root grafted to California privet (Ligustrum ovalifolium). Both of these genera are in the Oleaceae family.

    (2) Avocado. Nurse seedling graft of avocado rootstocks by the Frolich method and modifications are described by Reuben Hofshi in the Subtropical Fruit News, (vol. 4, no. 2, Spring, 1997).  The method was developed as a means of cloning avocado rootstock varieties. Avocado is very difficult / slow to root from cuttings; hence, grafting. The method involves grafting a scion, from a clone that is ultimately intended to be used as a rootstock, onto a nurse seedling. This nurse seedling will serve as a temporary rootsystem for the scion of this rootstock variety. New growth from the scion is then etiolated, and then air layered, in order to induce its own adventitious root system. The rooted layer is then detached from the nurse seedling and grown on. Subsequently a scion of a fruiting variety is grafted onto the rootstock clone.  

    (3) In the past, apples were sometimes nurse root grafted before the use of size controlling clonal rootstocks became common. This illustration is from Liberty Hyde Bailey's Standard Cyclopedia of Horticulture, written in the early part of the 20th century ( Bailey, 1924). NRG is sometimes still used to "bulk up" (rapidly increase the numbers of) newly selected apple rootstock selections in rootstock breeding programs such as the one at the New York Agricultural Research Station at Geneva project Website.

    (4) Other examples: Large-flowered Clematis hybrids, Peony, and Catalpa cvs.

    B. Grafting for repair

    1. Grafting to repair a girdled stem - Bride Grafting

    a. young bridge graft
    b. older bridge graft
    c. Ron Perry describes bridge grafting in an MSU Extension bulletin.
    d. Guy Witney, WSU Area Extension Faculty, describes the use of Bridge Grafting to repair mouse damage (girdling) of apple trees in Washington State.

    How is bridge grafting like double working (described in the section on Concepts and Definitions)?
    What time of year (season) is bridge grafting performed? (see Seasonal Considerations in the section on Required for Successful Grafting and Budding)

    2. To replace a damaged (girdled) trunk base - inarching or bridge grafting

    3. To replace a damaged or diseased root system - inarching.

    4. To overcome a delayed incompatibility - bridge grafting or inarching.

    C. Grafting to create unusual growth forms - Highworking (see Grafting by Position)

    D. Grafting to change fruit varieties

    1. Replacing an old variety on an established tree with an new one for economic or other reasons is known as Reworking, which is a form of topworking (see Grafting by Position spatial diagram)
    2. Seedling fruit trees can take 7 years or so to flower, and even grafted nursery stock can take several years. An alternative to waiting this long was, and still is, to a limited extent, to cleft graft a new variety up into the crown of an established tree. This could hasten production of the new fruit variety by several years.  

    E. Grafting to put multiple scion varieties on a single tree. This is an example of Topworking (see Grafting by Position spatial diagram

    1. Rarely practiced commercially, but an enjoyable home gardening option.
    • e.g. apple with Macintosh, Granny Smith, & Red Delicious, etc. all on one tree
    • e.g. citrus tree with orange, lemon, grapefruit all on one tree.
    • e.g. hibiscus with several cultivars differing in flower color.
    b. cleft grafting would typically be used for this topworking objective.

    F. To provide a pollinizer branch for self-incompatible fruit tree species

      1. Apples, cherries and some other fruit tree species are self incompatible within a clone.

      • e.g. Macintosh apple will not self-pollinate, but it will cross-pollinate with another domestic apple or a crabapple

      2. In commercial apple production one pollinizer (often crabapple) tree is usually planted at the end of each row, but grafting a pollinizer branch of another variety may be practical (and fun) in the home fruit  garden.

    Note: a cautionary note about orchard topworking - Topworking for changing varietes, multiple varieties, or pollenizer brancts is inefficient and rarely practices in modern orchard production but may be useful on a home-owner landscape scale. Quick-fix grafting solution can lead to long-term problem, From the Good Fruit Grower Web site, Dr. Lee Parish, a virus expert who retired last year from the U.S. Department of Agriculture (USDA) in Wenatchee, Washington, urges growers to consider the risk of spreading tree fruit viruses.

    G. Grafting to Influence Growth Phase

    1. Grafting to avoid rejuvenation

    a. Generally a grafted tree will come into bearing sooner than a seedling

      (1) This is because the (adult) growth phase of the scion tends to be maintained, whereas a seedling is naturally rejuvenated by the process of embryogenesis (seed formation) compared to the seed bearing parent tree.
      (2) Furthermore, dwarfing rootstocks tend to induce scion precocity, i.e. they cause a scion to come into flowering one or more years sooner than it would on its own roots or grafted onto a non- (or less) dwarfing rootstock.  

    b. This avoidance of rejuvenation is exploited by tree fruit growers, to avoid the long delay (several years) associated with orchard establishment from seedlings.

    • e.g. Avocado would come acceptably true-to-type from seed, but several years of bearing would be lost. Hence top wedge grafting of a scion from a mature bearing tree onto a seedling understock is commonly practiced in the nursery production of this tropical crop.  
    • e.g. Many Citrus varieties would come true-to-type from apomictic seed, since apomixis is a natural form of asexual propagation.  Apomictic seedlings, however, like zygotic seedlings, are juvenile, and flowering would be delayed for several years, as explained above. Hence, one reason for T -budding citrus is to avoid seedling rejuvenation.  
    • It should not be inferred that avoidance of rejuvenation is the only or even the principle reason for grafting fruit trees. Grafting is also practiced simply because many fruit tree species (e.g. apple, avocado, etc.) are difficult to root from cuttings (described above), and in order to take advantage of specific rootstock effects (described below) 

      2. Grafting for Scion rejuvenation to facilitate subsequent rooting of cuttings ( Serial Grafting)

      a. Even though a scion from a mature tree tends to retain its adult growth phase (as pointed out in the previous section) an adult scion will be slightly rejuvenated by grafting onto a juvenile (seedling) understock. Because this rejuvenating effect is only slight a scion may have to be sequentially regrafted onto a series of juvenile rootstocks before a useful degree of rejuvenation has been achieved. Cuttings taken from this rejuvenated growth tend to root more easily than from mature growth.

      b. Ease of rooting is a general property of juvenile compared to adult growth.

      c. This rootstock-influenced gradual rejuvenation of the scion is called serial grafting.

      d. Serial grafting is an extreme (last resort) method for facilitating the rooting of cuttings from the mature wood of extremely difficult-to-root species such as 100 year-old Sequoia ( Tranvan, et al., 1991).  

    H. Grafting for virus detection (Virus indexing)

    1. Essentially all viruses are graft transmissible - This is, of course, a disadvantace of grafting unless the objective is viral detection via graft indexing (see below)

    2. In a given crop species a virus may or may not cause obvious disease symptoms. Those virus-infected plants which are asymptotic may still exhibit decreased vigor and yield.

    3. Graft Indexing. Grafting a scion from a tree of an asymptotic species suspected of harboring a virus onto a more sensitive (symptomatic) indicator species will result in viral transmission from asymptotic scion to susceptible stock, which will then develop visible symptoms (mottling, streaking).  

    • Why is it important that the indicator is used as understock, not as the scion?
    • Note: for many viruses there are other newer, more specific and/or more sensitive virus indexing techniques such as ELISA (enzyme linked immuno sorbant assay). Hence, graft indexing tends to be used less frequently.

    a. Examples:

    (1) Strawberry - petiole wedge graft - terminal leaflet of trifoliate leaf removed, and small split made, where rachis of suspect terminal leaflet is inserted.
    (2) Prunus subhirtella is used as an indicator for cherry, plum, etc. - compatible union doesn't even form, but virus is transmitted, and detection can be made.

    I. Grafting to achieve independent optimization of component genotypes - Specific Rootstock / Interstock Benefits

    1. Grafted plants are compound genetic systems

    a. The rootsystem and the shoot system of a plant exist in different environments. Each has a different role in plant development and each makes a different contribution to agricultural productivity.  Given the long generation time of trees (years), it could take a very long time, using standard plant breeding methods, to breed a tree to genetically optimize both the root and the shoot systems. Grafting, on the other hand, has allowed agriculturists to mix and match different genotypes in the root and shoot systems, resulting in a genetically compound plant that performs better overall than either genotype alone.  

    b. Of course, in modern times, genetic engineering, is another way to "construct" a plant with genes from more than one organism. However promising, genetic engineering is still in its infancy with respect to "designer" trees.

    Do you think genetic engineering will ever make traditional grafting obsolete?

    2. What is the difference between "specific" rootstock effects and non-specific rootstock effects?

    a. "Specific" rootstock (or scion, or interstock) benefits, in the context of this discussion, are advantages gained by grafting that are due to the specific genotype of stock, or interstock.  

    • e.g. grafting Macintosh apple (scion) onto an M9 (dwarfing) rootstock, results in size control (dwarfing) of the scion because hormonal or other aspects of M9, under genetic control, are translated to the scion, affecting its vigor. Size control and other specific rootstock benefits in apples will be discussed in the section on Clonal Apple Rootstocks.
    • e.g. Grafting Arabica Coffee (Coffea arabica, higher quality but nematode susceptible) onto another species of coffee, C. robusta, which is nematode resistant

    b. Non specific rootstock effects would be grafting to achieve an objective that could be achieved by any compatible rootstock, regardless of its genotype. This includes many of the reasons for grafting & budding stated above such as the following examples in which the scion but the rootstock genotype is important:  

    • e.g. grafting onto a seedling rootstock merely to propagate difficult-to-root clone.
    • e.g. grafting onto a seedling rootstock to produce a plant with an unusual growth form, such as a weeping cherry.

    What are some other non-specific rootstock effects?

    3. A List of Specific Rootstock benefits

    a. Size control of the Scion.

    "Size control" most often refers to some degree of dwarfing, especially in the case of fruit trees caused by a mutation, or sport, of the normal scion genotype. Note: rootstock genotype effects on scion growth and vigor.

      e.g. Apple - the use of clonal rootstocks for size control (and other reasons) is a major part of modern apple production. The "Malling" and Malling-Merton apple rootstocks, introduced in the early 20th century, revolutionized apple production. Progress has been made since then by a number of other apple rootstock breeding programs, around the world. Since apple is perhaps the best example of crop improvement through selection of clonal rootstocks, this topic is discussed at length in the section on Clonal Rootstocks.
      e.g. Pear - pears are sometimes dwarfed by grafting them onto quince rootstocks.

      b. Effects of rootstock on precocity (early flowering) of scion

      Flowering and fruiting of an adult phase scion occurs more rapidly (precociously) when grafted on some rootstock genotypes than on others. In particular, the more dwarfing the rootstock causes dwarfing the scion will flower and "come into bearing" from the standpoint of fruit production.

      c. Pathogen resistance

      Many rootstocks have been selected for disease or pest resistance but in most cases the resistance is not transmitted to the scion (in contrast to dwarfing).

      (1) e.g. Fungal pathogens

      (2) e.g. Bacterial pathogens  

      (3) e.g. Viral pathogens 

      • Tristeza virus  
        • Tristeza causes greening disease in citrus which is a serious problem in Africa and other parts of the world
        • Resistance to tristeza is conferred by rough lemon rootstock.  A table of citrus rootstock resistance to Tristeza and other diseases is presented by the Florida Agricultural Information Retrieval System.

      d. Pest resistance

      (1) e.g. Insect pests

      • Wooly aphid (WA) (Eriosoma lanigerum) is an insect pest of apple. The Malling-Merton series rootstocks were developed by crossing wooly aphid-susceptible East Malling selections with WA-resistant Norther Spy apple (Information sheet from UC Davis IPM Pest Management project).
      • Phylloxera, described in an Information sheet from Univ. of California Integrated Pest Management project, is an aphid-like  sucking insect pest of grape which parasitizes the rootsystem.  Phylloxera infestation in the wine regions of France in the 19th century virtually destroyed production of European (wines) grapes (Vitus vinifera) until they began grafting them onto resistant American grape rootstocks (Vitus lambrusca).

      (2) e.g. Nematodes  

      • Nematodes are microscopic "eel worms" which parasitize the rootsystems of many agricultural crops
      • Almond (Prunus amygdala) scions are grafted on Mariana plum 2624 rootstocks which are nematode-resistant. ( Information sheet from Univesity of California IPM Pest Management Project)
      • Arabian coffee (Coffea arabica) produces a higher grade of coffee than Canefera coffee (Coffea robusta, the kind used to make instant coffee), but the former is nematode-susceptible while the latter is resistant. Scions from seedlings of C. arabica are grafted onto seedling understocks of C. robusta for coffee plantations in Guatemala and other parts of Latin America.

      (3) Rodents - the cultivar 'Novole,' crabapple rootstock, selected at the NY Agriculture Experiment Station at Geneva inhibits meadow voles from feeding on the bark during the winter.

      e. Cold hardiness

      (1) Trifoliate orange  (Poncris trifoliata) is deciduous (unlike the citrus species cultivated as fruit crops) citrus species. It is sometimes used as a rootstock for citrus. Orange or other citrus worked on trifoliate rootstock are better suited for more northerly Florida growing conditions because of the cold hardiness of the trifoliate orange. This is one of the few examples of evergreen/deciduous grafting.
      (2) Apple - for a summary of apple rootstock (cold) hardienss consult Paul Domoto's Characteristics of Apple Rootstocks table

      Question For what (other) reason is it surprising that Poncris trifoliata is used as a rootstock for Citrus sp. (hint, check the section on Requirements for Successful Grafting and Budding)?

      f. Tolerance of specific soil types  

      (1) The apple rootstock M7 is tolerant of wet soil conditions; conversely MM104 is tolerant of dry soil conditions. For a summary of apple rootstock soil adaptability consult Paul Domoto's Characteristics of Apple Rootstocks table

    4. Specific Interstock benefits

    Just as single working (scion/understock grafting) allows the grafter to combine the best possible scion genotype with the best possible rootstock genotype, double working (scion/interstock/understock grafting) allows for further optimization of each of the three components of a tree - rootsystem (nutrition, anchorage, dwarfing, etc.), trunk (support), canopy (fruit). See Grafting by Position

    a. Size control

    (1) A genotype which causes dwarfing when used as a rootstock has a similar dwarfing effect (but to a lesser extent) when used as an interstock
    (2)
    In addition, the degree of dwarfing by a given interstock genotype is proportional to the length of the interstock, i.e. a relatively long section of M9 used as an interstock has a greater dwarfing effect than a shorter section of the same genotype.

    Why use an intestock for size control of a double worked tree rather than using the same genotype as a rootstock in a single worked tree? (Hint: see Apple Grafting autotutorial slide set)

    b. To achieve an arborescent growth form of an otherwise shrubby scion variety (see Grafting to achieve special growth forms in this section, above)

    c. Avoid incompatibility of an otherwise incompatible stock / scion combination by inserting a mutually compatible interstock. (see discussion of Bradford pear/Quince incompatibility in the section on Compatibility)

    5. Summaries of specific rootstock characteristics for specific crops
      a. apple
      b. stone fruits

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