HORT 413 Tree Fruit Production

Rootstocks

History of Apple Rootstocks Around the World

Budding and grafting techniques were developed by the 15th century, making the use of rootstocks possible. Dwarfing rootstocks were in use in Europe during the 1800's and reference was made to 'Paradise' (very dwarfing) and 'Doucin' (less dwarfing) stocks. Beginning in 1912, Wellington and then Hatton gathered rootstocks from around the world, to determine their identity, trueness to type and classification. Prior to this there was considerable confusion over the names and identity of apple rootstocks. Hatton collected 71 apple stocks from 35 sources. He confirmed the mixed nature of stocks and found that the majority of them were improperly named. Because of the confusion over the names of these rootstocks, Hatton abandoned the common names and assigned Roman Numerals to them I-XXIV. These numbers were not ascribed according to any particular order with respect to tree size.

In 1917, the first apple rootstock breeding program was started, in England at the East Malling Research Station (EMRS). At that time many orchards in part of the British Empire were infested with Woolly Apple Aphid (WAA), especially New Zealand and Australia. In order to develop a series of rootstocks that were resistant to this pest, a breeding program was set up between EMRS and the John Innes Institute. The Merton Immune series (MI 778-793) was introduced in the 1930's and the Malling Merton series (MM 101-115) in 1952. Both these series of rootstocks derived their resistance to WAA from Northern Spy.

Apple production in the Unites States was initially based on seeds imported with the early European settlers. Dwarfing rootstocks were imported into the US in the early 1800's but little interest was shown by horticulturists except for home garden and ornamental uses. In the 1920's a range of the Malling rootstocks was imported for a study at Pennsylvania State College and New York. In Europe, there was a move towards more dwarfing rootstocks, seedling to M.2 and MM.106, and in France (espalier) and Holland (spindle bush) M.9 was the popular stock being used.

Seedlings were used in many places, and in some developing countries even today they remain the predominant rootstocks. Seeds are usually procured from juice or processing factories. Seedling rootstocks usually result in a reasonably uniform stand of vigorous trees although precocity and productivity is low. In some countries in Eastern Europe, Antonovka seedlings are used because of their cold-hardiness. In other places however, there was a move towards clonal rootstocks to ensure more uniformity in the orchard. In North America, the most common rootstocks are M.26, M.7, MM.106 and MM.111, although some M.9 and M.9 interstems are also grown. In Europe, most orchards use intensive planting systems on dwarfing rootstocks such as M.9, although in Northern Europe, M.2 and Alnarp 2 are widely planted. In Eastern Europe, some trees are still grown on Antonovka, but the Budagovski and Polish rootstocks are becoming more common. In much as Asia, M.26 and MM.106 are common while MM.106 and M.793 are more common in New Zealand and Australia.

Why Do We Use Rootstocks?

Rootstocks influence many aspects of the growth and development of an apple tree, and choosing the right rootstock is a critical element in planning an orchard. To make a wise decision in choosing a rootstock, it is important to understand what rootstocks do. The following are some of the influences that rootstocks can have, and therefore need to be taken into account during the planning stages of an orchard.

1. Vigor control

The most obvious effect of rootstocks is their influence on tree size. This is shown diagrammatically in Figure 1. As an example, in a rootstock evaluation planting at Purdue, we assessed 17 apple rootstocks over 10 years. All these trees were planted at the same time, and all were spur-type Red Delicious trees, so the only thing that differed was the rootstock. After 10 years, trees on P.22 rootstock were about 3 feet tall while those on vigorous rootstocks like M.4 or seedling were over 20 feet tall.

Figure 1. Relative size of some common apple rootstocks (from Westwood, 1993)

2. Precocity

In fruitgrowing, precocity refers to how quickly after planting the tree starts to produce fruit. Generally, the more dwarfing a rootstock, the more precocious it will be. Bear in mind this is a general rule and there are exceptions. A tree on a dwarfing rootstock may produce some fruit in the second year and considerable production by the forth year. In contrast, trees on vigorous rootstocks may not even begin producing fruit until the fifth or the sixth year. Developing an orchard is an expensive venture, therefore the sooner there is some return on the investment, the better. Therefore precocity has a big effect on the bottom line, and the best way to increase precocity is to choose a precocious rootstock.

3. Pest and disease resistance

Apples are attacked by a wide range of pests and diseases, which is obvious if you look through the commercial spray guide. The most important diseases affecting rootstocks are fireblight and crown or collar rot. Fireblight can be severe with susceptible rootstocks such as M.9 and M.26. In fact one of the problems we have is that there currently no commercial dwarfing apple rootstocks that are resistant to fireblight. Some of the new rootstocks being developed at Cornell University look promising in this regard, but as yet they have not been adequately tested and should only be planted on a trial basis. Rootstocks also affect the sensitivity of the scion to fireblight. For example, Golden Delicious is more susceptible to fireblight when on M.26 rootstock than on MM.111 rootstock. This effect may be due to early flowering or extended bloom which translates into more chance for infection. For collar rot, M.9, M.7, M.26 and MM.109 have higher resistance compared with M.25, MM.104, and MM.106. Woolly apple aphid can be a severe problem in warmer climates. The MM rootstocks gained their resistance to this pest from Northern Spy, but as yet there are no resistant dwarfing rootstocks. Dwarf rootstocks generally have thicker bark and are more attractive to voles. A recently introduced rootstock called Novole is apparently resistant to vole feeding.

4. Anchorage

Vigorous rootstocks generally have larger root systems, but dwarfing rootstocks such as M.9 do not necessarily have shallow root systems. Rooting depth depends to a large extent on soil characteristics and genotype. The roots of M.9 though, tend to be brittle and can break when put under stress. All plantings on dwarf rootstocks need to be supported. This can be with wooden posts, metal stakes, conduit, a trellis, or whatever, but a dwarf orchard system without support will fail. Tree leaning and inadequate anchorage have been associated with M.7, M.4, M.2 and MM.109 rootstocks.

5. Suckering

Some rootstocks tend to sucker very badly from the base of the tree or even further out from the base, under the tree. Suckers cause problems by interfering with weed control practices and providing a habitat for pests. Some chemicals have been somewhat successful in burning these suckers off, but removal by pruning them off by hand is still widely practiced. M.7 is particularly well known for producing many root suckers.

6. Adaptation to climate (hardiness, drought, flooding, temperature)

Seldom are orchards provided with perfect growing conditions, therefore trees must tolerate unfavorable conditions. Some rootstocks perform better under adverse conditions than others.

Soil moisture - M.26 and MM.106 are fairly sensitive to wet feet, while M.7 is more tolerant of flooding. MM.111 is not very tolerant of flooding, but quite tolerant of drought conditions (many fine roots which are able to extract moisture from the soil). Water use efficiency also differs among rootstocks. For example, MM.104 and MM.111 consumed less water than trees on seedling rootstocks while making the same growth.

Temperature - the level of tolerance to cold conditions is referred to as 'hardiness'. The most important rootstock influence in this regard is in speeding maturity in the fall and delaying budbreak in the spring. Hardiness of the rootstock material itself is also important. M.9 is not very hardy whereas B.9 is quite hardy. If planting trees on M.9 rootstock, make sure the graft union is close to the ground so that little of the sensitive rootstock shank is exposed.

Interstems

An interstem is a piece of wood grafted between the rootstock and the scion. These trees therefore have 2 graft unions. Often, the interstem is a stem from a dwarfing rootstock grafted onto a vigorous rootstock which helps to reduce the tree size. Why use an interstem rather than just use the clone as a rootstock? Better anchorage, tolerance of soil conditions, pest and disease resistance, etc.

Specific Apple Rootstocks

There are more rootstocks, covering a wide range of vigor, for apple than any other crop. Table 1 lists some of the more common rootstocks by their vigor class. Bear in mind, that they classifications are just that, classifications. The difference in tree size among these rootstocks can be subtle, and in reality they do not separate into distinct classes as shown below; this is merely an attempt to give an idea of the approximate degree of size control by each of these rootstocks.

Apple rootstocks currently in use in the United States by vigor class.

Very dwarf	Dwarf	Semi-dwarf	Semi-vigorous	Standard
M.27 EMLA	M.9	M.26 EMLA	MM.106 EMLA	M.4
P.16	B.9	M.7 EMLA	MM.111	MAC 24
P.22	Mark	MAC 1	MM.104	M.16
B.146	P.2
	O.3
	G.65
	G.16

Recommended rootstocks for Indiana

As can be seen from the above listing, there are a number of rootstocks of each size class that are possibilities for any particular orchard. The following are the rootstocks I would recommend for plantings in Indiana:

Dwarfing

Bud. 9 (B.9); cold tolerant Russian rootstock. Same vigor class as M.9, and should produce a tree 6-10' in height. Very precocious and productive in the orchard. Trees require support and well drained soils. It tolerates dry conditions fairly well. This rootstock is very resistant to collar rot but is susceptible to fireblight. Not as susceptible to fireblight as M.9, but B.9 is still susceptible so fireblight may pose a significant risk with this rootstock.

Semi-dwarfing

M.7; the most widely grown rootstock in the United States. Moderate in terms of precocity and productivity. Tree height should be 12-15' depending on variety, site and management. Support is not essential, but trees tend to lean somewhat. Suckers badly. Well adapted to a wide range of soil conditions.

Semi-vigorous

MM.111; been around for a number of years and is tolerant of many stresses. Fairly slow at coming into bearing and is of low-medium productivity. Very well anchored and is pretty hardy to the cold. Is drought tolerant but does not like wet feet. Tolerant of both collar rot and fireblight.

Summary

The choice of rootstock is one of the critically important decisions of planning an orchard. As a fruitgrower, there is nothing you can do that will affect tree size as much as your choice of rootstock. Choose a rootstock that will produce a tree of an appropriate size for the planting. Within the desired vigor range, try to select a rootstock that will perform well under the conditions - soil type, temperature (especially during the winter), pest and disease pressure and management style.