Dwarf and high-density apple cultivation utilizes dwarf rootstocks, dwarf (short-branch) varieties, and dwarfing techniques to create compact trees that are reasonably densely planted, achieving the goals of early fruiting, high yield, quality, low consumption, and high efficiency. With the development of apple production, the cultivation system has shifted from standard rootstock with sparse planting to dwarf and high-density cultivation, which has become a current trend both domestically and internationally.

1. The Significance of Dwarf and High-Density Apple Cultivation

1. Early fruiting and high yield: Standard apple trees typically start bearing fruit in about 6-7 years and reach full production in 10 years, while dwarf and high-density trees can start producing in 2-3 years and reach full production in 6-7 years, demonstrating significant early fruiting and high yield characteristics. They show early fruiting, early high yield, and high yield per unit area.

2. Early maturity and good quality: Dwarf varieties mature 5-10 days earlier than standard varieties, with larger fruit, vibrant color, rich flavor, and high marketability.

3. Maximizing land and light utilization, leading to high yield and good economic benefits: The highest yield occurs when the tree crown coverage is at 80%, while sparse planting is only at 70%. The economic benefits are about twice as high as those of standard planting.

4. Easier management, saving time and labor, and high work efficiency: The compact tree structure allows for easy management and high production efficiency.

5. Easy variety renewal and quick recovery of yield: Due to early fruiting, trees can bear fruit within 3-4 years after planting, allowing for quick variety updates. Densely planted fruit trees exhibit a short lifespan, facilitating variety replacement.

6. Higher requirements for cultivation management techniques: Improper management can lead to poor ventilation and light conditions in the orchard, resulting in decreased yield and quality, and complicating management. Densely planted orchards have a shorter economic lifespan.

2. Principles and Methods of Dwarf Apple Cultivation

(1) Methods of dwarf and high-density cultivation

1. Selecting short-branch varieties: Short-branch varieties are characterized by small crowns, dwarf trees, densely packed short branches, and primarily fruiting on short fruiting branches. They mainly include two aspects: dwarfing in growth habits and short fruiting branches. Existing short-branch varieties have all evolved from standard varieties, featuring short internodes, easy formation of short fruiting branches, and compact tree structures that are only 1/2 to 3/4 the size of standard trees. Additionally, they have early fruiting, good fruit coloration, and other advantages. By selecting appropriate rootstock combinations and grafting onto dwarf rootstocks or dwarf interstocks, the trees can be made even shorter and more suitable for high-density planting. Due to the inherent dwarfing characteristics of short-branch varieties, adaptive rootstocks can be chosen, resulting in broad application prospects, which are highly valued both domestically and internationally. Notable examples include the Marshal series of short-branch varieties (New Red Star, First Red, Good Dwarf, etc.), Green Banana short-branch (Yankong), Fuji short-branch (Huimin short Fuji, Guanzaki short Fuji, Fudian short Fuji, etc.), and Jin Guan series short-branch (Jin Dwarf, Huang Dwarf, etc.). These dwarf varieties are characterized by small tree size, short internodes, few branches, large and thick leaves, compact crowns, bright colors, and stable high yields.

2. Selecting dwarf rootstocks: Using dwarf rootstocks or dwarf interstocks can make the standard varieties grafted onto them become compact and dwarf. This dwarfing method is currently the most widely adopted and effective in the world of fruit tree cultivation. Dwarf rootstocks can not only limit the growth of shoots and control tree size but also promote early fruiting, increase fruit set, enhance yield, improve quality, and provide a long-lasting and stable dwarfing effect. Different dwarfing rootstocks can be selected based on various site conditions and cultivation requirements, such as M26, M9, M24, and MM106.

3. Employing dwarfing techniques: Dwarfing can be achieved through cultivation techniques, primarily including three aspects. First, creating certain environmental conditions to control tree growth and achieve dwarfing; second, employing shaping and pruning techniques that promote dwarfing; and third, using chemical dwarfing techniques.

(1) Environmental dwarfing: Selecting or creating unfavorable environmental conditions for nutrient growth, such as sandy soil that is easy to control for fertilization and water; appropriately reducing nitrogen fertilizer while increasing phosphorus and potassium fertilizer, and controlling irrigation to achieve dwarfing of the trees.

(2) Pruning for dwarfing: There are many pruning techniques to promote dwarfing, such as ring barking, girdling, inverted peeling, strangling, branch pulling, branch holding, and root pruning.

(3) Chemical dwarfing: Using plant growth retardants, such as CCC, MH, and PP333, can inhibit the division and elongation of the terminal meristem, thereby hindering branch elongation and achieving tree dwarfing.

By controlling tree growth through cultivation techniques, the trees can be kept compact and small. Techniques include using growth inhibitors, dwarfing agents, short-branch pruning, forced flowering techniques, and controlling root growth.

(2) Physiological Mechanisms of Dwarf Rootstocks

1. Differences in the tissue structure of rootstocks and scions: Studies show that the cross-section of the roots of dwarf rootstocks has a larger cortex area and a smaller xylem area, with fewer and finer vessels in the xylem. Additionally, dwarf rootstocks have more stored substances and living cells in the xylem, with living tissue being 2-3 times more than dead tissue, while standard rootstocks are roughly equal in both. The phloem of dwarf rootstocks contains fewer and smaller sieve tubes. This affects nutrient transport, limiting tree growth and resulting in dwarfing.

Dwarf rootstocks have shorter root hairs and a smaller contact area with the soil, leading to weaker absorption capacity. Coupled with a high metabolic demand from living tissues, the dwarfing effect is significant.

2. Differences in physiological functions of rootstocks and scions: Due to the differences in tissue structure, nutrient supply becomes limited. The respiration and transpiration rates of dwarf rootstocks are lower than those of standard rootstocks. This results in fewer main branches, better light exposure, higher photosynthetic efficiency, and greater nutrient accumulation in dwarf trees compared to consumption. Studies show that standard rootstock apple trees require an average of 35-50 leaves to meet the nutrient needs of one fruit, while dwarf rootstocks require only 20-25 leaves.

3. Differences in plant hormone content: The reduction in nutrient growth leads to an increase in growth inhibitors in dwarf rootstocks, while the content of growth promoters decreases. This has a significant inhibitory effect on the nutrient growth of apple trees.

(3) Main Dwarf Rootstocks for Apples

1. The UK’s M and MM series: The dwarf rootstocks developed by the East Malling Research Station in the UK are called the M series; the rootstocks developed in collaboration between the John Innes Horticultural Research Institute and the East Malling Research Station in Meiton are called the MM series.

M9: Dwarf rootstock, brittle roots, shallow roots, poor anchorage, not drought-resistant, not flood-resistant, early fruiting. Tree height is 2-3 meters. Moderate density planting. In recent years, Italy has bred better dwarfing rootstocks from M9, showing strong drought resistance, easy flowering, uniform fruit size, and good yield characteristics, especially suitable for developing high-density spindle-shaped trees.

M26: Dwarf rootstock, relatively brittle roots, good anchorage, can tolerate short-term soil temperatures of -17.8°C, tree size is between M9 and M7, suitable for moderate density planting.

MM106: Semi-dwarf rootstock, well-developed roots, good anchorage, deep distribution, tolerant of poor soil, drought-resistant, cold-resistant, can tolerate short-term soil temperatures of -17.8°C, resistant to apple woolly aphids, suitable for low to moderate density planting.

2. The Budagovsky series from the former Soviet Union, including Bud-57-490, Bud-54-146, and Bud-57-491, among others, with Bud-9 being widely adopted as a major cold-resistant interstock in the former Soviet Union and Poland, is also recognized globally for its excellent cold resistance.

3. The CG and MAC series from the United States.

CG series rootstocks: Rootstocks with a dwarfing effect equivalent to M9 and M26 include CG10, CG23, CG24, CG47, CG57, and CG80. These rootstocks have not been widely adopted in the US due to threats from bacteria and fire blight.

MAC series M9 rootstocks: Developed from M9 through seed selection in the US. Registered rootstocks include MAC9, MAC1, MAC10, MAC25, MAC39, and MAC46. MAC9 (also known as Mark) has the same dwarfing degree and early fruiting habits as M26, and due to its hard wood structure, it has received high praise globally as a non-supporting rootstock, and can adapt to sandy and clay conditions, showing cold resistance and good resistance to cotton aphids.

4. The P series from Poland.

P1, P2, P6, P16, P22: This series includes semi-dwarf rootstocks that are more resistant to cold than the M series but less so than M4. Grafted varieties yield early and are high-yielding.

5. Dwarf rootstocks bred in China.

SH series: After years of systematic observation and identification, three rootstocks SH40, SH6, and SH1 have shown comprehensive economic traits that surpass foreign M, MM, and P series dwarf rootstocks, showing outstanding and stable performance in 13 provinces (municipalities) across the country.

GM256: Compared to foreign dwarf rootstocks, it not only has strong compatibility with multiple grafted varieties, early fruiting, and dwarf tree characteristics, but also outstandingly tolerates temperatures as low as -40°C, showing wide adaptability. GM256 is resistant to early leaf drop disease and black spot disease, resistant to aphids, and not easily infected with rot disease. Grafting with various varieties can significantly increase yield. When grafted with cold-resistant varieties, the results are excellent.

KM: Grafted trees have small crowns, strong compatibility, early fruiting, and high yield.

(4) Short-branch varieties (taking the Marshal series as an example)

Marshal (also known as Red Banana, Red Marshal) was discovered in 1881 in Peru, Iowa, USA, and began to be promoted in 1895. It was gradually replaced by the later superior varieties Red Star and Red Crown (Richared Delicious) after the emergence of their bud mutations. The fruit is conical with noticeable five ridges at the top, with an average weight of 250g, and the skin’s base color is yellow-green with bright red blush and deep red stripes. New Red Star is entirely deep red, while Red Crown is also entirely deep red. The flesh is light yellow-white, crisp, sweet, and aromatic, of high quality. The bud mutation series of Marshal mainly includes: First Red: a bud mutation of New Red Star discovered in Washington State, the 4th generation of the Marshal series of short-branch varieties, published in 1976. Giant Red: discovered in 1964 in Buneuster, Washington, after radiation treatment of the top dormant branches, selected in 1971 and named in 1975. Brilliant Red: discovered in Maryland in 1967 as a bud mutation of Red Star, named in 1974 as the 3rd generation of Marshal’s short-branch varieties. Super Red: discovered in Yakima, Washington, in 1967 as a bud mutation of Red Star, named in 1972 as the 3rd generation of Marshal’s short-branch varieties. Silver Red: discovered in 1976 in Shelvies Orchard, Zeila County, Washington, as a bud mutation of Early Red, named as the 3rd generation of Marshal’s short-branch varieties. Red Ruby: a bud mutation of New Red Star, the 4th generation of Marshal’s short-branch varieties, introduced to the Chinese Academy of Agricultural Sciences in 1984. New Red Dwarf: discovered in Washington State in 1973 as a bud mutation of New Red Star, the 4th generation of Marshal’s short-branch varieties, registered in 1983. Wali Short Branch: a bud mutation of First Red, the 5th generation of Marshal’s short-branch varieties, introduced to the Chinese Academy of Agricultural Sciences in 1984. Red Rose: discovered in 1974 in Dongnan Ditch Orchard, Pingyin County, Shandong, as a bud mutation of Red Star, short-branch type, named in 1979. Liulin Red: discovered in 1978 in Dongguo Orchard, Liulin County, Shanxi, as a Marshal mutation, short-branch type, named in 1988. Dai Red: discovered in 1989 in the orchard of Houjiadian, Tai’an City, Shandong, as a bud mutation of Red Star, short-branch type, named in 1989.

(5) Physiological Mechanisms of Dwarfing in Fruit Trees

1. Characteristics of growth and development in dwarf and high-density apple trees

(1) Growth characteristics of dwarf and high-density apple trees

1. Roots: The total root mass of mature dwarf rootstock apple trees is less than that of standard rootstock trees, with shallower root distribution; dwarf root systems have fewer main roots and more fibrous roots, making them sensitive to soil conditions; the roots have fewer dead cells formed by thick-walled or thick-angled cells, while living cells (thin-walled cells) are more abundant, affecting the anchorage and cold resistance of dwarf rootstocks.

2. Above-ground: Dwarf rootstock trees grow vigorously at a young age, similar to those on standard rootstocks, but have more short branches. After entering the fruiting period, growth gradually slows, and the crown volume is significantly smaller than that of standard rootstock trees. As fruiting increases, the volume difference of the crown becomes more pronounced. Apple trees on dwarf rootstocks have significantly more branches at a young age compared to standard rootstock trees, especially with a strong ability to form short branches, which is beneficial for flower bud formation and early fruiting. As they age, their ability to sprout long branches decreases, so care must be taken during pruning to encourage the sprouting of a certain amount of medium and long branches to ensure continuous high yield.

(2) Fruiting characteristics of dwarf and high-density apple trees

Due to the small tree size and abundant short branches of dwarf rootstock trees, their fruiting characteristics are similar but different from those of standard rootstock trees. Compared to standard rootstock trees, dwarf rootstock trees or short-branch trees show early fruiting, strong yield capacity, and primarily fruit on short fruiting branches from the start of fruiting.

Whether using dwarf rootstocks or short-branch varieties, they generally start fruiting 2-3 years after planting. Dwarf rootstock and short-branch trees also have high fruit set rates, mature early, produce uniform fruit sizes, have smooth surfaces, and good coloration.

(3) Environmental requirements for dwarf and high-density apple trees

Due to the higher number of trees planted per unit area, early fruiting, and high yield, the environmental requirements are also higher. Dwarf and high-density fruit trees require well-drained sandy loam, loam, or clay loam to ensure normal growth and fruiting. Poor soil conditions should ideally be improved before planting. Under good soil conditions, the root systems of dwarf rootstock trees or short-branch varieties grow deeper and wider, with more roots, resulting in healthy tree growth that extends the productive lifespan. Generally, dwarf and high-density orchards require more water and nutrients than standard orchards.

2. Physiological differences between dwarf and standard apple trees

(1) Differences in tissue structure

Dwarf rootstocks or short-branch varieties have well-developed root cortexes, while the xylem is small, with fewer and finer vessels. The ratio of cortex to xylem (root cortex ratio) is significantly increased. At the same time, the content of living tissues such as ray and thin-walled cells in the xylem is higher, while the phloem contains fewer and smaller sieve tubes. The root hairs of dwarf rootstocks are coarse and short, resulting in lower absorption capacity. These structural differences affect the supply of water and inorganic salts to the above-ground parts and restrict their growth, while also impacting the transport of photosynthetic products from the above-ground parts to the roots, limiting root growth and absorption capacity, which in turn affects the growth of the above-ground parts.

(2) Differences in light energy utilization

Dwarf fruit trees, especially those using dwarf rootstocks, have thicker leaves and well-developed palisade tissues, with higher chlorophyll content per unit leaf area, leading to higher net carbon dioxide absorption rates and stronger photosynthesis. The same weight of leaves in dwarf trees accumulates significantly more photosynthetic products compared to standard rootstock trees, while their respiration and transpiration rates are lower than those of standard rootstock trees, thus favoring the accumulation of nutrients and promoting flower bud differentiation, resulting in early and high yields.

(3) Differences in nutrient allocation in the tree

During the growing season, the nutrient levels of dwarf apple trees are significantly higher than those of standard sparse-planted trees in late autumn. Dwarf apple trees have higher levels of ammonium nitrogen, sugars, starch, and potassium than standard sparse-planted apple trees, which is beneficial for flower bud formation.

Dwarf apple trees consume less of their photosynthetic products for vegetative growth. In terms of dry matter allocation within the tree, the dry matter allocated to fruits and branches in standard sparse-planted apple trees is roughly equal, while in dwarf dense-planted apple trees, the dry matter in fruits is more than five times that in branches. This indicates that the assimilated substances formed by photosynthesis in dwarf dense-planted trees are largely directed towards fruit growth, with minimal consumption for vegetative growth, leading to higher production efficiency compared to standard sparse-planted trees.

(4) Differences in growth inhibitor content

Varieties grafted onto dwarf rootstocks have higher levels of abscisic acid and lower levels of gibberellins, and the content of abscisic acid in dwarf rootstocks is positively correlated with the degree of dwarfing. Extremely dwarfing, dwarfing, and semi-dwarfing apple trees have abscisic acid levels that are 5 times, 3 times, and 2 times higher than those of standard rootstocks, respectively. Abscisic acid is a growth inhibitor that can suppress cell division and elongation, thus inhibiting the growth of shoots and roots, resulting in tree dwarfing.

(5) Differences in enzyme content and activity

3. Differences between short-branch apples and standard trees:

Standard apple trees can undergo mutations into short-branch trees due to strong light exposure, high temperatures, or cold waves, leading to the emergence of short-branch varieties. Compared to standard varieties, short-branch varieties have the following characteristics:

(1) Small crowns and compact tree structures: Mature trees are generally only about 3 meters tall, with a crown diameter of 3-3.5 meters, which is generally 4/5 and 2/3 the size of standard varieties (semi-dwarf), equivalent to the size of apple trees using nutrient rootstocks M7 and MM106. New shoots are robust, with short internodes, and the compact crown is suitable for high-density planting, with 825-1335 trees per hectare. This is 2-3 times the number of standard apple trees, but the lifespan is shorter.

(2) High sprouting rate, low branching ability, many short branches, few long branches: The sprouting rate of short-branch apple trees is generally above 70%, such as New Red Star at 78.5%, Good Dwarf at 79.4%, Yankong at 74%, Yan Hong at 71.8%, and Green Light at 80%. The branching ability is low, with only 1-2 long branches sprouting from the tips of one-year-old branches after pruning, while the rest are all short branches. The entire tree has few long branches and many short branches, making it prone to weak growth. Therefore, young short-branch apple trees should be pruned heavily to promote branching and increase the overall branch count, which is beneficial for cultivating fruiting branches and achieving high and stable yields.

(3) Easy flowering and early fruiting: Short-branch varieties can form flower buds on low-age branches. Investigations show that the flowering rate on one- to two-year-old branches of New Red Star ranges from 39.1% to 63.7%. When using bud grafted seedlings to establish orchards, they can start fruiting in 2-3 years and yield abundantly in 5 years. Short-branch apple trees primarily fruit on short fruiting branches, with a certain number of axillary flower buds; medium and long fruiting branches are fewer, with full flower buds and a high fruit set rate. The flowering rate of Yankong is as high as 92.7%, with the flower set rate also exceeding 60%. After starting to bear fruit, the yield increases rapidly, and they can enter the peak fruiting period by 6-7 years, which is about 5-6 years earlier than standard varieties. However, after abundant fruiting, short-branch trees tend to exhibit significantly weaker growth and are prone to alternate bearing.

(4) Young trees exhibit upright growth and vigorous growth with small angles of main branch divergence: Varieties like Yankong, Yan Hong, and New Red Star generally have main branch base angles between 35-45 degrees, with fewer open types. Therefore, when shaping and pruning, attention should be given to methods such as propping, pulling, and weighing to open up the angles of the main branches.

(5) Convenient management: Due to the small size of the trees, management such as spraying, pruning, and harvesting is very convenient. Cultivating short-branch varieties can reduce labor by 60-70%.

Thus, young short-branch apple trees have strong sprouting rates and branching ability, but their branching ability significantly weakens after entering the fruiting period. Generally, they only produce 1-2 robust branches at the extension points, and even after heavy pruning, it is difficult to sprout vigorous branches. Therefore, short-branch apple trees should cultivate the main branches well before abundant fruiting and retain an appropriate number of auxiliary branches to avoid having too few branches after peak fruiting, which would affect yield.

In addition, during the juvenile period of short-branch apple trees, care should be taken to increase the number of branches and accelerate the expansion of the crown; after abundant fruiting, strict control of flower bud retention and thinning of flowers and fruits should be implemented to maintain an appropriate branch-to-fruit ratio. On one hand, attention should be paid to increasing the number of branches, rapidly expanding the crown, and increasing the photosynthetic area to create conditions for early fruiting and high yield. On the other hand, short-branch varieties are easy to flower, entering the fruiting period and peak fruiting period earlier with high yields. After abundant fruiting, it is important to strictly control the retention of flower buds and not leave too many fruits to avoid the phenomenon of alternate bearing.

(6) Relationship between the growth and fruiting characteristics of dwarf rootstocks and short-branch high-density apple trees and shaping and pruning

1. In dwarf rootstock high-density orchards, young trees grow vigorously, and their growth is not significantly different from that of trees on standard rootstocks, but they have more short branches. After entering the fruiting period, the growth gradually slows down, and the size of the trees is significantly smaller than that of standard rootstock orchards. Different types of dwarf rootstocks also greatly affect tree growth. Dwarf rootstock high-density orchards start bearing fruit early, generally beginning to fruit 2-3 years after planting. The individual yield of dwarf rootstock trees is lower, but the yield per unit area is higher. Dwarf rootstock trees have many axillary flower buds, high flower set rates, and generally mature fruit earlier, with larger fruit size, greater firmness, and better quality compared to standard rootstock trees.

2. Dwarf interstocks can help dwarf trees, with the degree of dwarfing increasing with the length of the interstock. Dwarf interstocks have characteristics of early fruiting, early high yield, high yield per unit area, and good quality.

3. Short-branch apple trees have small tree sizes and compact crowns. Under the same soil conditions, their size can be 1/3 smaller than that of the original varieties. They have high sprouting rates, low branching ability, fewer main branches, and many short and medium branches. Their new shoots are short and robust, with short internodes, and they grow upright (some are open types), with dense and large leaves, resulting in a higher net photosynthetic rate and good light conditions for flower bud formation. Grafted short-branch varieties on standard rootstocks can flower in 2 years and yield abundantly in 3-4 years, with high fruit set rates and high yields per unit area.

4. The shaping and pruning principles for dwarf rootstock and short-branch high-density apple trees are similar to those for standard rootstock orchards, but the methods have the following characteristics: it is advisable to choose small crown shaping; when pruning dwarf rootstock trees, consider what rootstock is used and its dwarfing effect. The height of dwarf dense-planted trees should be appropriately reduced, and the central leader and extension branches should be controlled to flower and fruit minimally to prevent weakening of tree vigor; both the orchard and individual trees should reasonably control flower quantity to avoid excessive load that could affect growth; timely renewal of branch groups and appropriate heavy pruning should be performed.

5. In dwarf dense-planted orchards, trees are planted closely together, and their crowns should not exceed the planting distance, and their height should not exceed the row spacing. If the crowns are too wide, branches may cross, and there will be no passage between rows, affecting ventilation and light, making field management inconvenient. If the crowns are too tall, for example, if the row spacing is 2-3 meters and the trees exceed 3 meters in height, they will shade each other, affecting ventilation, leading to premature aging of lower branches, shortening the fruiting lifespan, and reducing fruit quality. To keep the crowns small, it is essential to consider adopting smaller tree shapes, such as cylindrical, spindle-shaped, natural fan-shaped, or palm-leaf-shaped.

6. Pruning of 1-3 year old young trees in dwarf dense-planted orchards before fruiting should involve minimal pruning except for retaining the branches that will become the main branches. During the juvenile period, fewer main branches should be left, and the proportion of fruiting branches should be maximized, cultivating more branch groups. Unlike medium crown trees, the branch groups of dwarf dense-planted trees mostly grow on the central leader and main branches.

7. Dwarf dense-planted trees generally enter the early high-yielding period by the age of four. After this, pruning should pay attention to the relationship between tree growth and fruiting, preventing excessive fruiting while maintaining a certain growth amount each year. The principle of renewing and strengthening before aging should be observed, utilizing robust developing branches to renew the crown, stabilize the framework, and replace branch groups. Control the branch groups to prevent excessive fruiting and premature decline.

8. In 5-6 year old dwarf dense-planted trees, the plants connect to form a tree wall, interlinking with each other. The subsequent pruning aims to balance growth and fruiting, while timely controlling the size of the plants and the density of the canopy. Control the height of tree crowns, bending upwards the vigorous growing tips; adjust the direction of main branch elongation, controlling the expansion and crossing of the tree crown; thin dense branches to improve light conditions; control flower and fruit quantities, renew and strengthen branch groups; and thin the layers, keeping weak branches at the top, sparse below, and allowing light to penetrate from outside to inside, ensuring that branches temporarily left on the trunk during the first 1-3 years are cleared to create a 30-50 cm space below the tree crown to utilize reflected ground light, improving light conditions for the lower layers of the crown.

3. Current Status of Dwarf and High-Density Apple Cultivation

(1) Current Status and Trends of Dwarf Apple Cultivation Abroad

Regarding the cultivation of dwarf apples, in 1472, Frenchman Champier mentioned the Paradise apple in Normandy, France. As for the Dousheng apple, it first appeared in literature in 1519. At that time, the names for Paradise and Dousheng apples were quite confused across various European countries until the British Horticultural Society first proposed classification research in 1872. In 1912, Hatton from the East Malling Research Station in the UK named it the nutrient system rootstock, and later selected the EM series rootstocks. In 1972, Perlira, the director of the East Malling Research Station, proposed changing EM to M and promoting it worldwide. The following outlines the research and utilization of dwarf apple rootstocks abroad.

1. UK

East Malling Research Station began utilizing M series and various rootstock hybrids since 1929, selecting some excellent dwarfing rootstocks. For instance, the M26 rootstock, published in 1959, is a cross between M16 and M9, classified as semi-dwarf; the M27, published in 1974, is a cross between M13 and M9, classified as a dwarf rootstock. In the 1980s, the 3426 rootstock, with parents M7 and M9, was released, classified as extremely dwarfing, with ten-year-old grafted trees measuring less than a meter in height. Since 1940, the East Malling Research Station and Long Ashton Research Station have jointly conducted virus elimination research, providing M9EMLA, M26EMLA, M27EMLA, and MM series virus-free rootstock production. Currently, new orchards in the UK primarily promote the superior M9 variety, T337.

2. France

In France, the application of nutrient system rootstocks is widespread, with over 85% of apple trees using M series dwarf and semi-dwarf rootstocks. In the 1980s, M9 accounted for about 30%, M2 about 40%, and M5 about 15%. Other rootstocks include M4, M26, and MM106. Currently, French apple seedlings primarily use M9 dwarf rootstocks, with high planting densities (2500-3500 trees/hectare). The shaping methods combine techniques such as central axis, sparse layer, and slender spindle shapes, referred to as “high spindle shape.” Advanced orchard management techniques have achieved high yield and quality for apples, with average yields of 10 tons/hectare in the second year after planting, reaching 25-30 tons/hectare in the third year, and increasing to 40-50 tons/hectare after four years.

3. Italy

In the 1980s, over 20% of apple production areas utilized M4, M2, and MM106 semi-dwarf rootstocks, with about 5% using M9 dwarf rootstocks. M26 semi-dwarf rootstocks were also in trial use. By the 1990s, apple trees were planted at high densities of 3300-4000 trees/hectare, primarily using M9 dwarf rootstock seedlings. In recent years, the South Tyrol region of Italy has bred the T377 rootstock, which is more dwarfing and productive, primarily using self-rooted propagation suitable for large-scale development of high spindle-shaped trees. Typically, high-quality, robust, and strongly branching three-year-old seedlings are directly planted. Orchards using advanced techniques can achieve quality fruit yields of up to 50 tons/hectare. Farmers with favorable geographical conditions and rich practical experience generally exceed 60 tons/hectare in yield.

4. Netherlands

As one of the countries in Europe where dwarf rootstocks are widely applied, the Netherlands has become one of the main exporters of apples due to the use of dwarf rootstocks. In the 1980s, about 70% of apple trees were grafted onto M series rootstocks, primarily promoting M9, M1, M2, M4, and M7 rootstocks. Today, nearly all new orchards use dwarf rootstocks.

5. Germany

Maurer and others selected some nutrient system rootstocks from 200,000 naturally growing seedlings, naming them the Da nutrient system rootstocks, which have similar winter hardiness to the M series. Schindler selected a series of Pi series rootstocks from Dousheng and Paradise apples. Regional testing results for M series rootstocks indicate that M1, M2, M4, M9, and M11 perform well, with fruits on M4, M9, M5, and M7 rootstocks showing good coloration. Currently, orchards adopt single-row high-density planting (2800-3500 trees/hectare), typically selecting high-quality, strongly branching seedlings grafted onto M9 rootstocks. Vertical multi-wire trellises (wooden stakes) support the fruit trees. The average commercial yield is 35-40 tons/hectare.

6. Poland

Utilizing M9 and ordinary Antonovka hybrids, the P series cold-resistant rootstocks were developed. Most orchards in Poland use dwarf or semi-dwarf rootstocks (such as M26) that are not as dwarfing as M9, with a density of about 1000-1250 trees/hectare. Several modern orchards have recently begun importing grafted seedlings of branching M9 dwarf rootstocks, increasing planting density to 1800-2500 trees/hectare. Today’s high-density orchards can produce 30-40 tons/hectare. Currently, the main promoted rootstocks include M9, T337, and B9, with M9 accounting for over 80% of the total.

Table 1: Apple Planting Densities in Poland (Andrej.A Przybyl, 2006) (tons/hectare)

Year

Row spacing (m)

Trees/hectare

Rootstock

Initial fruiting age

Yield

1919-1950

10×10, 10×8

100-125

Standard rootstock

8-10

10-15

1951-1970

8×6, 7×5

208-285

Standard rootstock

6-8

20-25

1971-1980

6×4, 5×3.5

416-571

Standard rootstock, A2, M7, MM106

5-6

25-30

1981-1990

4×2.5, 3.5×1.5, 3×1

1000, 1905, 3333

Intermediate rootstock, B9, P2, M26, M9

4-5

30-40

1991-1998

3.5×1.2, 3.5×1, 3.5×0.8, 2.5×0.4

2380, 3333, 4166, 10000

M9, M26, P60, M9, P16, P22, P59

3-4

30-50

1999-

3.5×1.2, 3.5×1, 3.5×0.8

2380, 2857, 3571

M9, P16, P22, P60, P16, P50, P22

2-3

40-70

7. United States

The United States places great importance on the breeding of dwarf rootstocks. Cornell University’s Geneva Research Station has bred CG series rootstocks targeting disease and pest resistance, such as CG10, CG26, CG47, CG80, CG23, and CG57. Michigan State University has bred the MAC series rootstocks, including MAC1, MAC9, MAC10, MAC25, MAC39, and MAC46. The USDA Research Center has bred the USD series rootstocks, including USD1225, USD312, USD316, USD323, USD329, USD1256, and USD1263. Before 1960, apple planting density in the U.S. was 100 trees/hectare, increasing to 600 trees/hectare from 1960 to 1980, and over 1500 trees/hectare after 1980. In the early 1980s, approximately 70% of apple production areas in the western U.S. utilized dwarf rootstocks, while around 30% did so in the eastern regions. Today, almost no standard rootstock orchards are seen; nearly all have been replaced by dwarf apple trees. Among them, M9 accounts for 30%, B9 for 20%, M26 for 20%, M7 for 10%, MM106 for 9%, MM111 for 5%, G16 for 2%, and G30 for 1%. Planting densities are shown in Table 2.

Table 2: Apple Planting Systems in the United States (Robinson)

System

Density (trees/hectare)

Row spacing (m)

Rootstock

Thin Tower Shape

840

2.4×4.9

M26, G30, G202

Upright Shape

1538

1.5×4.2

M9, Nic29, G16

Long Upright Shape

2244

1.2×3.6

M9, Nic29, G16

Long Spindle Shape

3262

0.9×3.3

M9, T337, B9

High Spindle Shape

5382

0.6×3.0

M9, T337, B9

8. Japan

Dwarf apple cultivation has developed rapidly in Japan. In the early 1980s, the rootstock types used in production included M26 (approximately 68.3%), MM106 (approximately 19.5%), M9 (approximately 6.7%), M7 (approximately 2.5%), and MM111 (approximately 1.3%). The rest accounted for about 1.7%. In Japan, high-density cultivation primarily uses dwarf rootstocks, with different provinces showing varying proportions. For example, in Iwate Prefecture, dwarf rootstocks account for 74.4%, in Hokkaido for 42.2%, in Fukui for 21.8%, and in Aomori for 20%.

(2) Current Application of Dwarf Rootstocks in China

In May 1973, the Zhengzhou Fruit Research Institute organized a nationwide collaboration network for dwarf apple cultivation, involving 38 relevant units across 19 provinces, focusing on the breeding and utilization of the M series and MM series dwarf rootstocks widely used abroad in apple-producing areas such as Bohai Bay, the Yellow River ancient path, the northern foot of the Qinling Mountains, and the central and southern parts of the Loess Plateau and northwest Hubei. A total of 11 rootstock systems and 42 clonal rootstock types were introduced, establishing 10 germplasm resource nurseries. By 1987, the area of dwarf apple orchards in the country had reached 10,200 hectares, accounting for 0.7% of the total apple area of 1.44 million hectares. In 2006, Professor Han Mingyu from Northwest A&F University led a project involving over 20 units nationwide, launching the Ministry of Agriculture’s industry plan for “Selection of Apple Rootstock and Scion Combinations, Orchard Tree Shape Renovation Techniques, and Cultivation Models.” On September 26, 2008, the Ministry of Agriculture hosted an on-site observation meeting on “International High-Efficiency Cultivation Models for Dwarf Apples” in Fengxiang County, Shaanxi Province, promoting new developments in dwarf apples across the country. On January 19, 2009, the Ministry of Agriculture issued a notification recommending 100 leading varieties and 60 leading technologies, among which the “High-Efficiency Cultivation Technology Model for Dwarf Apples” provided by Northwest A&F University was promoted as a leading technology in fruit trees, marking the peak of the development of dwarf apples in China and the beginning of a transformation in apple cultivation systems.

1. Current Status of Dwarf Apple Production in China

As of 2006, the area of apple cultivation in China reached 1.8988 million hectares, with dwarf rootstock apple area at 87,400 hectares, accounting for 4.60% of the total apple area that year (see Table 3). Among the major producing provinces, Shaanxi has the largest area of dwarf rootstock apples at 40,000 hectares, followed by Henan and Shandong. In terms of the proportion of dwarf rootstocks to total apple area, Henan has the highest proportion at 21.88%, followed by Shaanxi at 8.65%, and Liaoning at 1.19%. The proportion of dwarf rootstocks in China is far behind the over 80% in advanced apple-producing countries such as Italy, France, Germany, the United States, and South Korea.

Table 3: Current Status of Dwarf Apple Cultivation Area in China

Province (City)

Total Apple Area (ha)

Dwarf Rootstock Area (ha)

Proportion of Dwarf Rootstock to Total Apple Area (%)

Shaanxi 462,200 40,000 8.65

Shandong 311,100 190 0.61

Hebei 253,100 200 0.79

Gansu 207,400 130 0.62

Henan 167,700 36,700 21.88

Shanxi 166,000 170 1.16

Liaoning 109,100 130 1.19

Other Provinces (Cities) 242,200 250 1.03

Total 1,898,800 87,400 4.60

2. Current Status of Main Dwarf Rootstocks Used in China

China introduced dwarf rootstocks in the 1940s, but the materials were not preserved. In 1951, the former North China Agricultural Research Institute introduced M series dwarf rootstocks from Denmark, followed by the Beijing Botanical Garden introducing dwarf rootstocks from Poland. Subsequently, dwarf rootstocks from the UK (MM series), Poland (P series), the former Soviet Union (B series), the US (CG series), Canada (O series), and Sweden (A series) were also introduced. Utilizing the introduced materials and local apple rootstocks for hybridization, many dwarf rootstocks have been selected across the country, including Liao No. 2, SH series, GM256, 77-34, 63-2-19, etc.

Table 4: Current Status of Application Areas of Different Dwarf Rootstocks

Province (City)

M26 (%)

SH Series (%)

GM256 (%)

Other Rootstocks Area (ha)

Proportion of Dwarf Rootstock Area in Province (ha)

Proportion of Dwarf Rootstock Area in Province (ha)

Proportion of Dwarf Rootstock Area in Province (ha)

Shaanxi 32,000 80.00 3,005 7.52 – – 4,995 12.48

Henan 31,195 84.99 667 1.82 – – 4,839 13.19

Hebei 655 32.75 998 49.90 347 17.35

Shandong 1,558 82.00 133 6.99 – – 209 11.01

Shanxi 410 24.11 1,165 68.53 – – 125 7.36

Gansu 910 70.01 101 7.76 – – 289 22.23

Liaoning – – – – 1,040 80.01 260 19.99

Others 1,750 69.99 250 10.01 125 4.99 375 15.01

Total 68,478 78.36 6,319 7.22 1,165 1.33 11,439 13.09

3. Main Problems in the Application of Dwarf Rootstocks in China

(1) Lack of unique dwarf rootstocks adapted to local conditions

Although many dwarf rootstocks have been bred in China, such as the SH series in Shanxi and 63-2-19 in Jilin, they have not been widely promoted in production, and no unique dwarf rootstocks have been formed.

(2) Unsuitable tree shapes and mismatched good rootstocks with good methods

In the past, China promoted tall tree shapes with main branches for dwarf apple cultivation, such as improved spindle shapes, free spindle shapes, and irregular slender spindle shapes. In contrast, foreign countries promoted high spindle shapes, ultra-slim spindle shapes, cylindrical shapes, etc. Due to insufficient research on tree shapes, the promotion of standard tree shapes has resulted in dwarf trees not being dwarf, leading to closed orchards with poor light conditions, causing doubts about the effectiveness of dwarfing. Many dwarf orchards still adopt shaping and pruning techniques from standard rootstock old orchards, without developing a set of techniques suited to the characteristics of dwarf rootstock trees.

(3) Unsuitable planting density

When establishing orchards, suitable planting densities should be selected based on the degree of dwarfing of the rootstock. Overemphasizing high density and early high yield without appropriate techniques has led to closed orchards. For example, in the favorable water and fertilizer conditions of the Henan Yudong Plain, using M26 as an interstock requires medium density planting, with planting distances varying between 2-3 meters depending on the variety. For varieties like Red Fuji and Huaguan, the planting distance should not be less than 2.5 meters; otherwise, there is a risk of excessive planting density.

(4) Weak central trunk growth and crooked trunks

In the cultivation of dwarf apples in China, standard techniques are often used, and generally, no support is provided. Additionally, after planting, trees are topped at full buds without removing competing branches, and some even use competing branches to promote early fruiting, often resulting in the main branch diameter approaching that of the central leader, causing the central leader to grow poorly and become crooked. This phenomenon occurs mostly in orchards with poor site conditions and rough management.

(5) Poor underground management in orchards

After planting dwarf apples, many orchards engage in intercropping for years, severely affecting the development of young trees and their shape. Some orchards either do not fertilize for many years, leading to nutrient deficiencies and weak trees, or excessively apply nitrogen fertilizers, delaying fruiting, resulting in vigorous tree growth and making canopy control more difficult, leading to closed orchard issues. In some orchards, during severe droughts, the lack of irrigation conditions affects tree growth and fruiting.

(6) Over-deep planting of interstocks, leading to dwarfing becoming standard

In production, it has been found that the planting depth of dwarf interstocks is closely related to the size of the crown. If the dwarf interstock is completely buried underground, the apple tree will grow vigorously, and the variety will root, accelerating tree growth, causing dwarfing to become standard. If the interstock is completely exposed, the dwarfing effect is strong, and the young tree grows slowly, leading to poor growth of the central trunk and making it crooked. Additionally, some interstocks will root, effectively shortening the dwarfing effect. Due to farmers’ lax adherence to planting techniques and inconsistent sizes of planting pits, there is a sinking process after planting, resulting in varying depths of interstock burial in dwarf orchards. Many orchards have either too deep interstock burial, leading to large crowns and non-dwarfing trees, or too shallow burial, leading to small crowns and excessively dwarfing trees, resulting in uneven crown sizes and poor uniformity in the orchard.

(7) Early fruiting can lead to early senescence

Due to early flowering and fruiting within 2-3 years after planting dwarf apples, many farmers see flowers and retain fruits, leading to early high yields. Additionally, since dwarf apple planting mostly involves 2-year-old seedlings, and intercropping occurs for many years after planting, young tree growth slows. Under the influence of early fruiting and high yields, early senescence of dwarf trees has been observed.

(8) Poor seedling quality and high difficulty in shaping

For many years, farmers have preferred low-priced apple seedlings, while seedling producers have aimed for rapid seedling production. These factors have led to dwarf apple seedlings being primarily produced in two years, resulting in poor seedling quality, delayed fruiting, poor adaptability, and high difficulty in shaping.

Scan the QR code below to learn more about apples.