As stated by Czinege et al. (2012), for example, in Hungary, 95 to 99% of plum orchards are grafted on strongly growing myrobalan seedlings. The main reason is the generally established mechanized harvesting (shakers), which requires massive, strong trees planted at wider spacings.

Fresh Market Demands

On the other hand, the fresh fruit market demands large, hand-picked fruits of high quality. These requirements are best met by relatively small trees with moderate growth vigor, which are highly productive and allow cultivation in high-density planting systems (Czinege et al., 2012).

Category 1: The Giants of the Orchards – Myrobalan (Prunus cerasifera)

Myrobalan (cherry plum) is historically the most widely used rootstock for the European plum. Its main advantage is its enormous adaptability. It is recommended for dry locations but is also often used in wet sites, where the grafts are protected from early frosts due to later vegetation (Czinege et al., 2012).

Disadvantages: Most P. domestica varieties grow vigorously to very vigorously on myrobalan, leading to a later onset of bearing. Moreover, the seedling population is genetically very heterogeneous, causing unevenness in the orchard (Czinege et al., 2012).

Clonal selection: To overcome heterogeneity, specific clones were selected. Well-known clones include 'C 162', 'C 174', or 'C 359'. The clone 'Myrobalan 29 C' (widely used in the USA and Italy) is characterized by strong growth and excellent adaptability to various soils. The 'Myrocal' clone from France is less susceptible to chlorosis and root asphyxia (suffocation) (Czinege et al., 2012).

Category 2: The Golden Mean – The Prunus domestica subsp. insititia Group

The transition to intensive orchards required rootstocks that tame the vigorous growth of noble varieties. This role is excellently fulfilled by rootstocks derived from the damson/bullace (Prunus domestica subsp. insititia). They are adapted to cooler and wetter sites (Czinege et al., 2012).

• 'St Julien A': Selected in East-Malling, England. It produces medium-sized trees that come into bearing very early. However, it is more sensitive to drought and winter frosts (Czinege et al., 2012).
• 'INRA St Julien GF 655/2': One of the most widely used rootstocks. It reduces growth vigor by about 10% compared to myrobalan. Grafts on it start bearing early, it produces almost no root suckers, and it tolerates both calcareous and dry soils well (Czinege et al., 2012).
• 'Pixy': An extremely weak-growing (dwarfing) rootstock from East-Malling. Although the trees start bearing very early, its major disadvantage is that as the tree ages, the fruit size also decreases (Czinege et al., 2012).

Category 3: Noble Roots – P. domestica ssp. domestica Rootstocks

Using the European plum itself as a rootstock for other European plum varieties is very popular in some countries. These rootstocks are valued for their excellent affinity (compatibility) and positive impact on fruit quality.

• 'Wangenheim' (Wangenheim's Early Plum): An old German variety propagated generatively (by seeds). It produces medium-strong to semi-dwarfing trees. The grafts bear early, are productive, and show excellent cold tolerance (Czinege et al., 2012).
• 'Wavit': This is a modern, vegetatively propagated clone selected from 'Wangenheim' seedlings in Austria. It combines the advantages of the original variety with absolute stand uniformity. Trees on 'Wavit' bear early and abundantly, and the fruits are larger and ripen slightly earlier (Czinege et al., 2012).
• 'Belošljiva': An autochthonous (indigenous) Serbian rootstock. A study conducted by Milošević et al. (2013) showed that this rootstock induces relatively strong growth. When evaluated in a high-density planting (HDP) system, trees on 'Belošljiva' achieved a large trunk cross-sectional area (TCSA), which in some varieties (e.g., 'Čačanska Najbolja') led to lower specific yield efficiency (YE).

Category 4: Modern Interspecific Hybrids and Extreme Dwarfs

Scientific research is constantly looking for the ideal rootstock by crossing different species of the Prunus genus to achieve maximum growth reduction without losing fruit size.

• 'Krymsk 1' (VVA-1): A Russian hybrid (P. tomentosa x P. cerasifera). It is one of the most dwarfing rootstocks today, reducing tree size by 40 to 50% compared to standards. It is ideal for super-intensive orchards with high planting density, but it requires irrigation in dry locations (Czinege et al., 2012).
• 'Ishtara': A French hybrid of myrobalan and Japanese plum. It acts as a semi-dwarfing rootstock (weaker than myrobalan). However, on wet soils, it is at risk of root asphyxia (Czinege et al., 2012).