Sweet cherry (Prunus avium) production is increasing in many areas in both the Northern (United States, Italy, Spain, Hungary, and Turkey) and Southern (Chile, Australia) hemispheres. Sweet cherry production has advanced in the last decade as a result of new dwarfing rootstock selections, improvements in crop protectants to reduce rain-induced cracking, and better methods in postharvest handling and storage. New cultivars have also been released, characterized by novel traits such as precocity, self-fertility, improved mechanical harvest, and improved fruit appearance and quality (Bassi, 2010; Lugli, 2003).
Semi-dwarfing and dwarfing rootstocks improve the potential for developing HDP systems for sweet cherry (Balmer, 2001; Hrotkó, 2010; Weber, 2001). The dwarfing rootstock series Gisela®, developed at the Liebig University of Giessen in Germany, induce early bearing of sweet cherry. Gisela® 5 and Gisela® 6, in particular, show promise for use in high-density cherry orchards (Robinson et al., 2004; Sitarek et al., 2005). Several years of experimentation have led to a more specialized and intensive cherry orchard cultivation with densities of 5000 trees/ha (Balmer, 2001; Lugli and Musacchi, 2010).
The current trend toward higher densities in pome and stone fruit orchards, including sweet cherry, requires adopting more efficient training systems. Novel architectures that enhance light interception and distribution into the canopy have been developed, ensuring early cropping, high yield, improved cropping efficiency, and fruit quality (Lauri and Claverie, 2005; Long et al., 2005; Whiting, 2006).
Worldwide research has focused on comparing rootstocks and training systems suitable for different sweet cherry orchard models. The choice of training systems should account for cultivar–rootstock, growing environment, and labor force availability. Rootstock vigor can determine if low–medium, medium–high, and high-density plantings are possible. For low–medium density, the principal training systems are vase, palmette, and Drapeau Marchand (Savini et al., 2007). For medium–high density, the spindle system, with various modifications (Zahn, Vogel, and Modified Brunner spindle) (Hrotkó, 2005; Hrotkó et al., 1997; Long, 2001), the Spanish Bush (Negueroles Pérez, 2005), its Australian variant, the KGB (Kym Green Bush; Green, 2005), the innovative UFO system (Upright Fruiting Offshoots) developed at Washington State University (Ampatzidis and Whiting, 2013), and the Solaxe, which was initially adopted for apple orchards (Lauri, 2005), are possible. Inclined shapes, including the Tatura trellis (Y-shape) and V-system, are suitable for high-density plantings (Musacchi and Lugli, 2014).
There are major differences of vigor and fruiting habit among cherry cultivars. It is important to consider cultivar growth habit in orchard management activities, particularly in regard to pruning, to ensure that proper distribution between vegetative and reproductive output is realized (Bargioni, 1994). Depending on the cultivar, cherries are mainly produced on spurs of 2-year-old wood or older. The fruiting density on these spurs varies from one to 20 fruits per cluster. Flower buds, residing on 1-year-old wood, are typically solitary and produce one to three fruits. On wood 2 or more years old, the number of flower buds per spur typically increases from base to apex of the branch (Lang, 2005). In Italy, new tree pruning and training systems have been tested for HDP of sweet cherry including the spindle, the intensive V-system, and the SSA (Lugli and Musacchi, 2010; Musacchi et al., 2012; Musacchi and Lugli, 2014). Planting densities range from 2000 trees/ha to almost 6000 trees/ha. In the past, other pruning and training systems tested never exceeded 1500 to 2000 trees/ha and were typically 3.5 m (11.5 ft) between rows and 0.5 m between trees.
The spindle is mainly used for apple (Robinson et al., 1991), pear (Musacchi et al., 2005), and peach (Caruso et al., 2001), but has recently been adopted for cherry production. The spindle is a central axis with a strong scaffold of four to five permanent structural basal branches. The fruiting branches are renewed frequently to improve fruit size. The fruiting spurs are positioned on basal branches or branches connected directly to the central axis. The planting distance for spindle-trained trees is typically 3.5 to 4.0 m between rows and 1.0 to 2.5 m within the row. Harvesting from these trees is carried out mainly from the ground and with platforms. The conical habit improves light interception, whereas the reduced size of the plant can improve the efficiency of chemical treatments as well as facilitate the use of protective nets to protect the orchard from birds, hail, and rain (Hrotkó et al., 1998).
The management-intensive V-system is an angled training system characterized by a double fruiting wall in each row. Trees are planted at a 20° angle from the vertical, alternating the direction each tree faces from the center of the row, allowing a high planting density ranging from 3000 to 5000 trees/ha (Sansavini et al., 2001). The V-system requires a support structure with three wires on each side (Sansavini et al., 2001). The planting distance initially adopted for this type of training system was 5.5 to 6.0 m between the rows and 1 m within the row, but recently this system has been used to increase planting density up to 5000 trees/ha by using planting distances of 3.5 × 0.5 m combined with intensive pruning.
The SSA system has been developed as a modification of the spindle to have better control over tree growth. The training system is a central axis with short limbs and the ability to fruit on 1-year-old wood; winter pruning promotes renewal (Musacchi and Lugli, 2014). Cultivars with relatively high numbers of flowers and good fertility in the basal portion of 1-year-old wood are preferred when developing a SSA orchard.
Low vigor or self-fertile varieties may also be used in combination with Gisela® 6. All of these rootstocks, trained to SSA, require relatively higher levels of water and nutrients and have limitations resulting from their sensitivity to certain soil conditions and environmental factors (De Salvador et al., 2005).
The SSA and V-system do not require a particular kind of feathering or other training from the nursery such as “knip” trees that are used in other high-density planting systems like the spindle or the solaxe. The SSA system presents a very positive fruit/leaf ratio that helps to increase fruit size and quality. Production can range from 10 to 15 Mt·ha–1 and in some cases 20 Mt·ha–1 is possible. A weakness of the SSA system is that not all cultivars have fertile basal buds; therefore, suitable cultivars must be selected (Musacchi and Lugli. 2014).
In all the training systems described, the key feature is that the cropping occurs mainly on basal floral buds of 1-year-old shoots, which represents an innovative management approach. However, cherry cultivars vary in their potential to produce flower buds on 1-year-old wood as well as disposition toward tree architecture suitable to management-intensive HDP training systems. The objective of this research was to compare training systems suited for HDP in combination with different rootstock–cultivar combinations to test vegetative control and yield performance.
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