The vigorous ‘Monclar’ (
Prunus persica L.) rootstock supports the early turning to bear of grafted apricot and peach cultivars
[52][77]. ‘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][78].
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][51], 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][86].
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][56]. 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][69,83,86].
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][51], ‘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][27,89,91].
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][93,94].
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][27,91,95,96]. 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][103]. 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][104,105].
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][23]. Knowles et al.
[66][118] 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][23,119,120].
P. pumila and
P. besseyi also sucker
[69][70][121,122]. 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
rootstocks29C’. 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.