NC Cooperative Extension Resources

Introduction

The commercial apple industry worldwide is in the midst of a major change in fruit production management systems. With size-controlling rootstocks tree size has been reduced and the number of trees per acre, referred to as tree density, has increased significantly. Some orchards in Europe have exceeded 5,000 trees per acre. However, in North Carolina, tree densities that are being commercially evaluated are around 450 trees per acre with a maximum of approximately 1,100 trees per acre.

All apple trees sold commercially are either budded or grafted trees. The selected variety or top of the tree, referred to as the scion, is grafted or budded on a rootstock of choice. The rootstock determines the relative size of the tree but does not affect the type of fruit or quality of the fruit that the tree bears. With currently available commercial rootstocks, there is a wide range in tree size potential as well as some degree of resistance to certain root borne insects and disease problems.

Size-controlling Rootstocks

The terms "dwarf tree" or "dwarfing rootstock" are frequently used to describe trees smaller than full size. However, this term is misleading as a dwarf tree to one person is a medium size tree to the next. It may be best to use the term size-controlling rootstock to avoid misunderstanding. Table 1 lists the relative size of mature trees on the commercially available rootstocks as well as some important characteristics of each.

Table 1
Tree Size as Percent of Seedling Resistance to Selected Pests
Rootstock NONSPUR SPUR Anchorage Root Suckers Fruit Bearing Age (yrs) Crown Rot Wooly Apple Aphid
Fire Blight Burr Knot
Seedling 100 80 Excellent Slight 6 - 10 Medium Variable High Variable
MM.111 85 70 Excellent Slight 4 - 6 Medium High Low Low
MM.106 80 70 Excellent Slight 3 - 4 Very low Medium Low Medium
M.7a 70 60 Fair Heavy 3 - 4 Medium Low High High
M.26 50 40 Poor Slight 2 - 4 Medium Low Very low Low
MARK 50 40 Good Slight 2 - 3 Medium Low Low Medium
M.9 35 20 Poor Slight 2 - 3 Medium Low Low Medium

As a general rule, the smaller the size class of the tree (scion/rootstock) the sooner the tree will bear fruit once planted. It is recommended that trees smaller than M.7a be staked or supported for optimal tree growth. Staking also helps to support the tree and fruit load in the early years as the smaller trees bear fruit earlier. Ten foot stakes are generally recommended with 2 feet driven into the ground and 8 feet above the ground. The tree is loosely tied, approximately every 2 feet, to aid in supporting the leader. Materials commonly used are one inch aluminum conduit or 3 inch wooden posts.

The majority of size-controlling rootstocks originated in England. The "M." prefix, refers to the East Malling Research Station, England, where much of the initial research was conducted in the early 1900's. The "MM." prefix, Malling-Merton, refers to hybrid trees of the Malling series crossed with "Northern Spy" in Merton, England in the 1920's. Some of the "M." and "MM." rootstocks are available with the EMLA suffix, which stands for East Malling / Long Ashton. The EMLA rootstocks were put through a program to eliminate viral pathogens in the 1960's and are classified as virus-free. Because the trees are virus-free they may be slightly more vigorous than trees on standard rootstocks.

When purchasing apple trees, the tree should be labeled with rootstock and variety in order to correctly space and plant the tree. For example, many apple trees are sold as semi-dwarf trees but without any reference to the rootstock used. Many nurseries define semi-dwarf trees as trees smaller than seedling and larger than the M.7a size class. With a potential difference of 15% in tree size, determining accurate tree spacings for this tree is impossible. Identifying the rootstock when buying your trees will also help to avoid potential failures that are not evident until several years after planting. For example, if the tree was bought as a semi-dwarf on MM.106 and planted in a poorly drained site, the potential for tree loss is very high as MM.106 is not resistant to crown rot (Table 1).

How Far Apart Should I Plant My Apple Trees?

The next question that is frequently asked is, "How far apart should I plant my apple trees?" This question is very difficult to answer because the size of the tree can be influenced by many factors. However, soil fertility is probably one of the greatest factors influencing tree size other than the rootstock. Table 2 has a suggested range of planting distances with the wider distances for trees planted in good, fertile soils and optimum growing conditions. Two multiplication factors are also included, 0.65 for spur type varieties and 1.4 for very vigorous varieties. The spur type varieties do not grow as large as standard varieties and the very vigorous varieties grow more than standard varieties and the multiplication factor compensates for the difference in growth. Varietal growth information is available from the Cooperative Extension Bulletin Apple Varieties for North Carolina (AG-311, Cost $1.00) or in commercial nursery catalogs. For trees planted in rows, it is recommended that the distance between rows be twice the distance between trees within a row to allow maximum sunlight to the tree. The desired tree height is generally equal to the distance allowed between trees in the row, and trees should be pruned and trained accordingly.

Table 2
Rootstock Distance Between Trees in the Row (ft)
Seedling 18-25
MM.111 14-18
MM.106 12-16
M.7a 10-14
M.26 8-12
Mark 6-8
M.9 4-8
  • For spur type varieties multiply by .65 (i.e. Redchief Red Delicious, Starkrimson Red Delicious, Lawspur Rome, Oregon Spur).
  • For very vigorous varieties multiply by 1.4 (i.e. Rome Beauty, Granny Smith, Jonagold).

Newest Rootstocks

Since the 1920s there has been much research conducted worldwide to select size-controlling rootstocks that are well anchored with insect and disease resistance. Table 3 lists some of the newest rootstocks with general comments about each rootstock. At present, these rootstocks are still being evaluated but have shown potential for commercial use. Several of the newest rootstocks are currently available in limited quantities from commercial nurseries for evaluation.

Table 3

Rootstock

Equal to Size Class of Table 1

Desirable Characteristics

Budagovsky (Bud.) 9

M.9

Hardy, crown rot resistant

Bud. 490

MM.106

Precocious, crown rot resistant

Bud. 491

Precocious, crown rot resistant

Footnotes

For further information refer to Rootstocks for Improving Apple Production Efficiency in North Carolina, by Drs. Eric Young and C. Richard Unrath, AG-292, 1989. The cost is $1.00 and can be purchased through Ag. Communications, Box 7603 - NCSU, Raleigh, NC 27695-7603.

Author:

Extension Horticulture Specialist and Associate Professor (Tree Fruits/Pecans)
Horticultural Science

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Publication date: Jan. 1, 1993