The use of rootstocks and scions has changed, along with their systems of cultivation. Associated with climate change, fruit trees face new ecological and phytopathological challenges. Rootstocks affect the generative and vegetative performance of a scion, such as productivity, span of nonbearing period, growth vigor, shelf-life and quality of fruits. Several traits of rootstocks facilitate the growth of a grafted tree under different climatic and soil conditions. Due to the high risks of cultivation, it is extremely important to determine which rootstocks are suitable for successful apricot production.
1. Origin of Apricot
Most apricot cultivars belong to the
Prunus armeniaca L. species (
Rosaceae family,
Prunoidae subfamily,
Prunus L. genus), but other related species are also in cultivation in Asia. According to Vavilov
[1][2] apricots originated in the highlands of northern China. The mountains of Tien Shan and Dzungaria are considered as secondary gene centers, as they are rich in wild relatives of apricot
[3]. The Silk Road played an essential role in spreading it to other continents, which commenced from Xi’an (formerly Chang’an), and reached Byzantine, Venice, then Lyon
[4]. In addition to fresh consumption, dried apricots are often sold, but several cultivars are also grown for their kernels in China
[5].
2. Apricot Cultivation Worldwide and in Hungary
Apricot, after cherries and peach, are the third most economically significant stone fruit species in global production. Apricot is cultivated on 560 thousand hectares worldwide yielding up to four million metric tons of fruit every year. Mostly grown in regions with a Mediterranean climate, the fruits contain several substances that are important to human health
[6]. Turkey, Iran, Uzbekistan, Italy, and Pakistan account for 54% of global apricot production
[7]. More than 50% of production comes from Asia, followed by Europe (27%) and Africa (14%)
[8]. Turkey produces the most apricots worldwide (695.000 tons in 2009). Apricots are generally sold as dried fruits, with only 2% sold on the fresh market. The most significant apricot producers after Turkey are Italy (233.000 t), France (190.000 t), Spain (97.000 t) and Greece (77.000 t)
[7].
In Hungary, on average, 22.000 tons of apricots are grown on 5.000 hectares. The cultivation area is increasing by 100 to 200 hectares every year, resulting in an increase in production. Consistent quantity and good quality of the product are indispensable for the long-term retention of fresh markets. It has been estimated that Hungarian producers could reliably sell 40.000 tons of fruit each year, but this has been revised to 50.000 tons. About 60% of the crop is sold as fresh fruit, and 10% is exported.
3. Propagation of Apricot and the Role of Rootstocks
Apricots can be propagated by seed but will not be true to type. Rootstock is only needed for budding or grafting to maintain and propagate the outstandingly worthy genotypes supervened during domestication. Budding and grafting are the most common vegetative propagation methods, while rooting of apricot cultivars (by cutting, layering or micropropagation) is difficult and inefficient. The importance of rootstocks is considerably due to spread of grafted trees
[9][10]. Evaluation of apricot rootstocks is mainly based on the compatibility of the graft and the expansion of adaptability to soil conditions of the orchard, followed by other cultivating aspects such as vigor and resistance, etc.
Several traits of rootstocks (tolerance against pests, diseases, high lime content of soil) enable cultivation in locations that are not optimal for the needs of modern apricot cultivars. Rootstocks affect growth, vigor and phenology of the scion; quantity and quality of fruit; and tolerance to soil biotic and abiotic factors
[11][12][13][14][15]. In addition, Nyujtó and Kovács
[16] noted that some rootstock and scion combinations show higher resistance to bacterial canker.
4. Rootstock Breeding
During the evaluation of their largescale rootstock experiment, Southwick and Weis
[17] identified that some Myrobalan rootstocks cause greater mortality in apricot orchards than in other species. Such incompatibility may not be exhibited for many years
[18][19][20] but can be seen soon after grafting
[21]. Several studies have focused on apricot rootstock incompatibility in recent decades, but the results cannot be generalized
[22][23][24]. Compatibility tests are needed for every rootstock and scion combination. In addition, rootstock–scion combinations must be determined for geographical suitability
[25].
New apricot rootstocks are required to combine resistance or tolerance against nematodes, diseases, pests and edaphic conditions, while good performance in the nursery and exceptional rooting is also essential. To capitalize on the potential of rootstock–scion combinations, the interdependence of their vegetative and generative traits must be determined accurately.
In Hungary, where seedling rootstocks are still dominant in nursery production, a seedling rootstock selection program was operated at the Cegléd Research Station
[26]. Subsequently, a clonal rootstock evaluation trial was established in Cegléd over the last decade
[27].
5. Grafting Compatibility of Apricots
Although commonly used rootstocks are compatible with most of the apricot cultivars, this trait is not always evident
[28]. Grafting compatibility of the apricots with apricot seedlings (
P. armeniaca L.) is excellent; no exceptions can be found in the literature. Only chlorotic leaf spot virus (CLSV) can cause compatibility problems on apricot seedling rootstocks
[29]. First signs of this phenomenon are the poor graft union formation in the nursery and the death of shoots. Symptoms are more significant when only one of the grafting partners is infected, mostly when the virus-free rootstocks are budded with infected scions.
Phytoplasma contamination of a component results in similar symptoms.
Apricot cultivars usually have good-to-moderate compatibility with plum species (
P. cerasifera Ehrh.,
P. salicina Lindl.,
P. ×
mariana,
P. insititia Jusl.,
P. domestica L.). The incidence of incompatibility with plum and Myrobalan species occurs more often when grafting, rather than during the budding of an apricot scion. In these cases, parenchymal cells are stuck between the tracheas and the stereomes
[18][30][31], and mechanical forces (such as strong winds) can separate the smooth surfaces between the scion and the rootstock
[32]. This sign of incompatibility is often partly on the surface of the graft union or occurs in the later life of the graft (
Figure 1).
Figure 1. Effect of incompatibility of apricot cultivar and plum trunk (Photo: Hrotkó).
6. Environmental Adaptation of Rootstocks
6.1. Soil Requirements
In areas of lower altitudes, with colder, wet and heavy soil, apricot seedlings and plum rootstocks are more common in apricot production. Suitability of the environment is the main factor in the location of apricot orchards; therefore, the adaptability of the rootstock to soil conditions is the main priority. Heavy, moist and cold soils require rootstocks with good tolerance against waterlogging. Stagnant waters are lethal to apricot seedlings
[33]; clonal Myrobalan and ‘Brompton’ rootstocks are moderately tolerant, while bullace, Damascene and Marianna plum have full tolerance against these soil conditions.
6.2. Cold Tolerance and Winter Hardiness
In the northern part of Europe, efficient apricot production is more dependent on the winter hardiness of the rootstock–scion combination. Apricot seedlings have a wide range of tolerance to freezing winter conditions, associated with origin of the rootstock. In northwest China, apricot is produced in regions with long winter periods, with minimum temperatures below −20 °C for several days
[34]. Layne and Harrison
[35] stated that the ‘Haggith’ cultivar, in addition to providing a uniform seedling progeny, has solid hardiness in Canada, and based on the experiments of Kappel
[36], using this rootstock provided the best apricot yields. Select
P. armeniaca rootstocks possess winter hardiness of both roots and trunk. However, where scions are damaged by freezing winter temperatures, budding at height is often employed.
6.3. Susceptibility to Pests and Diseases
The most adverse pests of rootstocks are nematodes, with
Meloidogyne spp. being the most prevalent in the Mediterranean apricot-producing region. Apricot seedlings tolerate these species well, whereas the most common rootstocks in southern Europe (peach and almond species) are susceptible. Myrobalan B, Marianna GF 8-1, and GF 31 plum rootstocks are somewhat tolerant to
Meloidogyne spp. In northern regions,
Pratilenchus species are more widespread; apricot seedlings GF 31 Myrobalan and GF 2038 hybrids show tolerance; other rootstocks were proved susceptible.
Xiphinema spp. plays a unique role in the spread of viruses; thus, nurseries and virus-free plantations should only be established at nematode-free locations.
7. Rootstock Effects on Vigor and Productivity
Ten percent of Hungarian apricot orchards are intensively planted (distance between and within rows is less than 5 m and 3 m, respectively), and 60% of them are irrigated. Trees in these orchards are typically trained to open vase (45%) systems, or have a natural round shape, but intensive compact vase (10%) and spindle (5%) training are also practiced
[37].
Increasing planting density from medium (600 to 750 trees/ha) to high density (1.000 to 1.250 trees/ha) results in substantial changes to orchard management for orchardists
[38]. Upright axes training systems are more productive; therefore, where possible, canopies should be created with at least one or more axes in commercial orchards
[39][40][41][42][43]. Using these canopy forms, a planting density of 1.100 to 1.600 trees/ha can be achieved.
Wild apricot and Myrobalan are widely used as rootstocks in more countries
[6][44]. In orchards with these rootstocks, pruning is important in managing the size of the canopy. Pruning during winter and summer needs to moderate the canopy growth of the grafted tree to encourage more flower buds and enhance light exposure of fruit to ensure high quality
[45]. In Hungary, experimental orchards with spindle canopy were established at the predecessor of the Hungarian University of Agriculture and Life Science in the 1990s (
Figure 2).
Figure 2. An example of a 6-year-old ‘Bergeron’ apricot grafted on Marianna GF 8-1, trained to a spindle canopy (Photo: Hrotkó).
8. Rootstocks in Hungary and around the World
In Hungary, 73% of apricot trees are on Myrobalan (seedling and clonal combined), another 10% are on apricot rootstocks. The ratio of the vegetative plum rootstocks is 17%
[37]. In Hungary and its neighboring countries, old orchards are typically on apricot seedlings, but in recent decades, clonal Myrobalan rootstocks have predominated
[46][47]. Similar to Hungary, there were rootstock breeding programs in the Czech Republic (M-VA) and Romania (Constanza 14, Constanza 16)
[48][49]. There are many countries around the world where Myrobalan and its vegetatively propagated hybrids (Myrobalan B, Myrobalan 29C) are used as rootstocks. The Myrobalan 29C used in larger quantities during the past decades provides this rootstock an early bearing period and good adaptation, especially on soils with high lime content
[50]. Some rootstocks increase the yield of grafted trees, such as ‘Marianna GF 8-1’, ‘Greengage CD-4’, and ‘Damas1869’ rootstocks
[51]. ‘Marianna GF 8-1’ provides long life for the grafted cultivars. The ‘Pollizo’ plum rootstock (
Prunus institia L.), derived from Spain, has good tolerance to anaerobic soil conditions, which is typical for Mediterranean regions
[33].
The vigorous ‘Monclar’ (
Prunus persica L.) rootstock supports the early turning to bear of grafted apricot and peach cultivars
[52]. ‘Rootpac R’ has characteristics similar to those of ‘Monclar’, with good adaptability to suboptimal soil and climatic conditions and strong vigor. It is derived from a
Prunus cerasifera myrobalana L. ×
Prunus dulcis Mill. cross, and it is mainly used for replanting
[53].
8.1. Wild Apricot (Prunus armeniaca L.)
This rootstock is propagated by seeds only. There are currently no known apricot clones that can be propagated at an economic scale using either cuttings or in vitro methods. Apricot seedlings have good compatibility with apricot cultivars producing high- or very high-vigor trees. Rootstocks named wild apricot can include seedlings of cultivars with small fruit size (e.g., ‘Korai piros’), local cultivars (e.g., in North Africa ‘Balady’ and ‘Mech-Mech’), and also the mostly wild, so-called sea apricot rootstocks. In almost all central European countries, a selection of sea apricot mother trees for seedling production exist, selected because their seed to fruit ratio is good, their germination ratio is high, and their seedling plants are very uniform in the nursery.
8.2. Plum Species as Rootstocks for Apricots
European plum species as rootstocks for apricots have become very popular among growers in recent decades, as they have a good grafting rate
[54], although there are differences between the various species and groups. The taxonomical questions of the European plum species are very difficult; therefore, it is necessary to create some groups to orientate among them. The most important plum rootstocks and hybrids can be put into four groups:
-
Myrobalan plum and its hybrids;
-
Bullace and Damascena plum;
-
Local European plum and the local selected plum cultivars;
-
Other species and interspecific hybrids
[55].
8.3. Myrobalan (Prunus cerasifera Ehrh. var. cerasifera Schneid. cv. myrobalana)
Myrobalan is native to Europe and Asia, with a widespread range, and it is planted as both a rootstock and ornamental tree. Hungarian nurseries graft 70% of apricot cultivars on Myrobalan
[37]. Myrobalan is a very diverse subspecies; it survives high soil water levels and waterlogging but does not like dry, stony soils. Apricot cultivars grafted on it are susceptible to late spring frost damage and to
Verticillium. Incompatibility depends on their origin; among the seedling Myrobalan rootstocks derived from Cegléd, only ‘C. 174’ exhibits incompatibility and only in the case of apricot. Scions grafted on Myrobalan produce strong vigor, grow faster, turn to bear early and their yield is higher compared to trees grafted on wild apricot seedlings. Among the Myrobalan rootstocks derived from the breeding program running at the Research Station of Cegléd, ‘C. 162’ and ‘C. 359’ are recommended as rootstocks for apricot
[46][55][56].
8.4. Vegetatively Propagated Myrobalan Species
‘Myrobalan B’ is one the oldest clonal Myrobalan rootstocks, and it was selected in East Malling. The cultivars grafted on it produce very strong vigor and begin to bear late. This rootstock grows well in all types of soil. In orchards, it produces few suckers. Its compatibility with Hungarian-bred cultivars is unknown at present; according to Magyar
[54], ‘Bergeron’ and ‘Ceglédi óriás’ grafted on it had a low survival rate in the nursery. In long-term trials under Hungarian climatic conditions, this rootstock produces good results. It has tolerance to
Meloidogyne sp. when used as interstock and is known to be resistant to
Pseudomonas sp.
[21][57][58].
8.5. Bullace, or St. Julien Plum (Prunus insititia Jusl.)
These penta- or hexaploid species, which have round-shaped fruits, are native to Hungary. They have strong vigor but are considered a medium-vigor rootstock. It is important to mention that there are dwarf rootstocks within this group. According to the newest results, bullace is a natural cross of sloe (
Prunus spinosa L.) and cherry plum (
Prunus cerasifera Ehrh.) that may have occurred in the overlapping area of both species in Europe
[59][60].
The oldest St Julien clone is ‘St. Julien A’, which was selected in East Malling. Initially, this rootstock was selected for plum, but it has good compatibility with apricot scions, inducing medium-vigor trees. As a young standalone tree, it produces upright shoots with few laterals. Cultivars grafted on this rootstock produce strong, vigorous trees; however, the canopy volume of the grafted trees is only 75% of ‘Myrobalan B’
[21][58][61][62]. The ‘INRA Saint Julien GF 655/2’ clone is used as an apricot rootstock outside of France.
Hungarian fruit growers often graft or bud plum and apricot cultivars on different local plum varieties, seedlings and their suckers. Some of these rootstock genotypes, derived from the local plum cultivars, are used in foreign nurseries. In Hungary, ‘Fehér besztercei’ and ‘Kisnánai lószemű’ were released as state-approved cultivars, which had been selected for apricot from local plum varieties. Other plum seedlings such as ‘Vörös szilva’ and ‘Bódi szilva’ are older selections. There is a lack in Hungarian fruit research, as the previously mentioned two rootstocks are not introduced in production.
‘Fehér besztercei’, was bred by Dr. Pál Nagy in the 1960s from a selection of the local population at the Research Institute for Fruit Growing (Budapest, Hungary)
[63]. It is a state-approved cultivar that grafts well to ‘Magyar kajszi’, but this rootstock has good compatibility with other cultivars. According to the results derived from trials set up at the late University of Horticulture and Food Industry (Budapest, Hungary), this rootstock is considered ideal for apricot. It can be propagated using hardwood cuttings, with cuttings collected in December producing the best rooting rate
[64][65].
8.7. Interspecific Hybrids
Among Hungarian climatic conditions, ‘INRA Marianna GF8-1’ is a suitable rootstock for apricot. This rootstock is derived from a cross between Marianna plum and Myrobalan from the research station in Grande Ferrade (France). As rootstock for apricot, this stock produces a strong-vigor tree, and according to the French experience, this rootstock has good compatibility with ‘Bergeron’, but its compatibility with ‘Canino’ and ‘Rouge de Roussillon’ cultivars is not satisfactory. Apricot cultivars grafted on ‘INRA Marianna GF 8-1’ had strong vigor in the trial planted in Szigetcsép (central Hungary); ‘Magyar kajszi C235’, ‘Bergeron’ as well as ‘Ceglédi óriás’ were grafted onto this rootstock, which began cropping early.
9. Suckering
Suckering of apricot rootstocks have high correlation with plum pox virus infection, as new shoots attract aphids. In addition, the cost of orchard management increases, as the removal of suckers is labor intensive
[17]. Knowles et al.
[66] found that ‘Marianna 9.52’ produced an unmanageable number of suckers, while the propensity of ‘Marianna 6.64’, ‘Marianna GF 8-1’, ‘Marianna 2624’, ‘Pixy’, ‘Ishtara’ and ‘St. Julien GF 655/2’ to sucker was moderate.
Prunus armeniaca,
Prunus persica and
Prunus dulcis produce no suckers, whereas species of
Prunus sect.
Prunus (
P. cerasifera,
P. domestica,
P. insiticia,
P. salicina,
P. spinosa) have high suckering potential
[17][67][68].
P. pumila and
P. besseyi also sucker
[69][70]. It seems to be a hereditary dominance effect of suckering, as most of the hybrids of suckering species have this trait. Every plum, Myrobalan, bullace, greengage and blackthorn rootstock produces suckers.
10. Conclusions
Apricot is a significant stone fruit species cultivated worldwide. Most of the 560,000 hectares of orchard is located in warmer climates. In modern fruit growing, the importance of rootstocks has expanded considerably, as grafted trees are used to achieve production goals. The lack of a wide range of adaptability (which is typical of apricot) means that the rootstock is extremely important for plantation establishment. During the last 50 years, more than 100 rootstocks have been developed for apricots, each with different growth vigor and tolerance against biotic and abiotic factors, and they originate from several species. Virus-free, homogenous and uniform planting material is required for a good orchard, and this only can be ensured using micropropagated rootstocks. Interspecific rootstocks can provide added value. In practice, only 6–8 rootstocks are widely used by producers, even though new selections arguably offer better performance. This low number is due to a lack of practical experience. Largescale comparative experiments could help to select appropriate rootstocks. Available rootstocks could be mentioned for specific growing areas based on the generated data to help apricot producers.
Most of the commercial apricot plantations in Hungary are established on Myrobalan rootstocks. Based on experiments and experience so far, vegetatively propagated Myrobalan rootstocks are well suited to Hungarian climatic and soil conditions, providing that tree water requirements can be met. However, in calcareous soils with poor water management, rootstock with strong root growth helps maintain the performance of the orchard. A greater adoption of rootstocks of interspecific hybrid origin with strong vigor can be expected in the future.
This entry is adapted from the peer-reviewed paper 10.3390/horticulturae9060720