Commercially available apple rootstocks

The fundamental success of apple production systems depends substantially on the proper selection of rootstock since it is a major factor influencing the viability and sustainability of productive orchards. However, the choice of the appropriate apple rootstock to establish a new orchard itself require specific knowledge of the capabilities and limitations of each rootstock. From better growth and yield performance, dwarf tress size, resistance to biotic and abiotic stress, increased precocity, efficient mineral nutrients absorbent, and better portioning and translocation on nutrients from root to the grafted variety, and adaptability to higher planting densities (Autio et al., 2008).

There are many series of apple rootstocks from a different breeding program that commercially available and used in the apple growing areas worldwide. The list includes; Budagovsky (Bud or B), Cornell/Geneva (CG or G), Malling (M) & Malling Merton (MM), Michigan Apple Rootstock Clones (MARK), East Malling/Ashton Long

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(EMLA). Ottawa (O), Pillnitzer Supporter (Pi), Poland (P) and Vineland (V).

Following is a description of the most commonly used and those under investigation in this study;

B.9: Dwarf rootstock from the Budagovsky breeding program in Russia, a result of crossing M.8 × ‘Red Standard’ (Krasnij Standard). This rootstock has been evaluated and widely used in many apple-growing areas. B.9 is a little more dwarfing and marginally more productive than M.9. Like other rootstocks in this B series, their leaves are red (Auvil et al., 2011; Crassweller and Schupp, 2005). B.9 is very precocious and winter hardy. It produces few suckers and requires support. It thrives in well-drained soil and is very resistant to Phytophthora crown rot and has shown greater fireblight resistant than M.9 (Russo et al., 2007).

G.11®_{: Dwarf rootstock a product from a cross between M.26 × Robusta 5 that}

was released by the Cornell Geneva apple rootstock breeding program in 1997. Depending on soil fertility and irrigation, trees on G.11 are similar in size to M.9 T337 and M.26 and equally precocious but more productive with very high yield efficiency. G.11 is reported with medium resistant to fire blight (E. amylovora) and Phytophthora crown rot. It is moderately susceptible to woolly apple aphids. This rootstock requires support in the early years and develops few burr knots and root suckers. G.11 grows well in most soil conditions. (Robinson et al., 2003)

G.41®_{: Dwarf rootstock similar in size to M.9 NAKBT337. Released by the}

Cornell Geneva apple rootstock breeding program in 2003 from a cross of M.27 × Robusta 5 made in 1975. G.41 is a highly productive rootstock with very high yield efficiency. It very precocious and is winter hardy but also does well in a warmer climate.

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It produces no suckers or burrknots and requires tree support. This rootstock is highly resistant to fire blight, Phytophthora Crown rot, Wooly Apple Aphid (E. lanigerum) and it appears to be tolerant to replant disease (ARD). A five-year study shows that G.41 produces trees parallel in size to M.9, but with higher yield efficiency than M.9 and produces few root suckers. It also has excellent fruit size and induces wide crotch angles (Crassweller and Schupp, 2005).

G.202®_{: A semi-dwarfing rootstock the result from a 1975 cross of M.27 ×}

Robusta 5 that produces a tree similar to M.26. It was released in 2002 by the Cornell Geneva apple rootstock breeding program. It was characterized as precocious and productive rootstock that carries resistance to fire blight (E. amylovora), Phytophthora Crown rot, Woolly Apple Aphids (E. lanigerum) and tolerant to replant disease (ARD). This rootstock is considered to be an appropriate selection for weak growing cultivars like Honeycrisp™. A study evaluated ‘Liberty’ scion grafted on G.202 reported that trees were about 50 percent smaller than M.7 but with much greater production efficiency (Crasswellerr et al., 2005; Robinson et al., 2003).

G.214®_{: A dwarfing rootstock resulting from a cross of Robusta 5 × Ottawa 3.}

It is about 30-35% of the size of the seedling tree. The vigor of G.214 is similar to M.9,Pajam2, and M.9Nic29. However, it is more productive than those rootstocks with high yield efficiency and good cold hardiness. Trees on G.214 need support to withstand extra fruit weight. It is resistant to fire blight (E. amylovora), wooly apple aphid (E. lanigerum) and Phytophthora root rot. It is tolerant to replant disease (ARD). (Tworkoski et al., 2016).

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3 and Robusta 5 and introduced by the Cornell Geneva apple rootstock breeding program in 2003. G.935 produces a tree similar to M.26 (between M.9Pajam2 and M.26) in virgin soil and has very high yield efficiency similar or better than M.9. G.935 is precocious with excellent fruits size. It is a mid-winter hardy rootstock but produces some suckering. Since it produces a very productive tree, it requires support and it is well adapted to most soils. It produces wider branch angles in the scion (Crassweller and Schupp, 2005). G.935 is highly resistant to fireblight (E. amylovora), Phytophthora Crown rot has good tolerance to replant disease (ARD) but is susceptible to wooly apple aphid (E. lanigerum). (Robinson et al., 2003).

M.9: The pedigree is unknown, but it was selected in England from a group of French genotypes called “Jaune de Metz” in the late 1800s. M.9 is the most common and widely used dwarfing rootstock in the world. It prefers a well-drained site and requires leader support. It is very susceptible to fire blight (E. amylovora), wooly apple aphids, tolerant to crown rot and can develop burr knots. Many clones of M.9 have been developed and sold by nurseries, including;

M.9EMLA which is a virus-free clone produced by the East Malling/Long Ashton research stations. This rootstock produces a tree approximately 25-30 % more vigorous than M.9.

M.9 NAKB 337, is a virus-free clone produced by in the Netherlands and has become the mostly widely used a clone of M.9. It produces a tree with 5-10 % less vigor than M.9EMLA.

Pajam 1 and Pajam 2, are virus-free clones of M.9 produced in France. Pajam2 produces a tree with 35 to 40 % more vigor than M.9 NAKB 337.

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M.9 RN 29 or Nic29 is another virus-free clone clone from Belgium which produces a tree about 30 % larger than M.9 NAKB 337 (Crassweller and Schupp, 2005).

M.26-EMLA.26: A semi-dwarfing rootstock from a cross between M.9 × M.16 (Metziner Ideal) at East Malling Research Station in England. It is in the intermediate vigor rootstock between M.9 and M.7 and produces a tree about 40 to 45 % of a standard tree. It is a highly productive rootstock thus needs some support in the early years but reported to be self-supporting after establishment. This rootstock is very precocious with heavy fruit bearing and good adaptability for close plantings and double rows. It is winter hardy but produces a few suckers and sometimes the bud union can be brittle. It tolerates most well drained and sandy soils but is susceptible to Phytophthora. and highly susceptible to fire blight (Crassweller and Schupp, 2005).

MM.106: A cross of M.2 × Northern Spy by the John Innes Horticultural Institute and the East Malling Research Station in England. It is a semi-dwarf rootstock, slightly larger than M.7 which produces a freestanding tree. It is a precocious and productive rootstock. Trees on MM.106 are resistant to wooly apple aphid (E. lanigerum) but is susceptible to fire blight (E. amylovora), collar rot (P. cactorum) and is not recommended for poorly drained fields. A study shows that Delicious cultivars on MM.106 are susceptible to apple union necrosis caused by Tomato ringspot virus (ToRSV) (Crassweller and Schupp, 2005).

O.3: Is a semi-dwarfing rootstock that was bred by the Agriculture Canada Research Station in 1974 as a cold hardy rootstock. It is a cross of 'Robin' crab × M.9. Trees similar to the size of M.9EMLA but smaller than M.26. It is resistant to collar rot (P. cactorum) but susceptible to fire blight (E. amylovora), woolly apple aphids and very

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susceptible to apple mosaic virus. O.3 is an older rootstock and is not currently propagated by apple rootstocks nurseries. (Crassweller and Schupp, 2005).

P.18: Is a semi-vigorous rootstock which resulted from a cross of M.4 × Common Antonovka and was released by the Research Institute of Poland. It is not a dwarf rootstock and produces larger tree about the size of MM.111. However, it is tolerance to fire blight (E. amylovora) and resistance to collar rot (P. cactorum) and perform well in wet or heavier soils.

Supporter 4 (Pi.80), a semi-dwarf rootstock from Dresden -Pillnitz, Germany. It is a cross between M.9 × M.4 and is similar in size to M.26 with the same anchorage features. It has a better yielding capacity than M.26 and MM.106 but lower than M.9. It has good winter frost resistant and resistance to woolly apple aphid (E. lanigerum), crown gall (Agrobacterium tumefaciens) but is susceptible to fire blight (Fischer, 1997). V.1 Released by the Vineland station breeding program in Ontario, Canada. It is a product of open-pollinated hybrids of ‘Kerr’ crabapples and M.9. Tree size is similar or slightly bigger than M.26. However, its yield efficiency and fruit size are equal or larger than M.26. V.1 was reported to be highly resistant to fire blight (E. amylovora) with little production of suckers (Crassweller and Schupp, 2005).