High-density tree training systems are important for overcoming some of the challenges of sweet cherry (P. avium L.) production. Cherry fruit are susceptible to many pests and diseases, rain-induced cracking, and bird damage, requiring multiple sprays for pests, rain covers, and nets to ensure marketable crops in locations prone to rain during ripening. High-density training systems can make sweet cherry production more efficient, by reducing pesticide and herbicide use and facilitating mechanization of orchards and the use of nets and covers for fruit protection. Recent developments in rootstocks have provided precocious fruiting and dwarfism (Lang, 2000), and allowed high-density training systems to be developed (Lang, 2005; Lang et al., 2014; Musacchi et al., 2015; Robinson, 2005).
There are several important factors to consider when designing a high-density training system to maximize yields, minimize disease, and facilitate easy pruning and harvesting. Training goals for producing high quality fruit include 1) good light interception and distribution by the canopy, 2) a balanced leaf to fruit ratio (Lang, 2005), and 3) renewable fruit-bearing sites on minimal permanent structure. A good high-density training system should address these principles and be efficient to prune, train, and harvest. The UFO training system develops a trellised, planar multiple leader tree to create a narrow fruiting wall with evenly distributed vertical fruiting branches (or “uprights”) along a cordon-like trunk (Long et al., 2015). This provides a tall, narrow fruiting canopy that is easy to train and prune for renewal of uprights. The UFO system’s planar architecture and pedestrian size also help increase harvest efficiency (Ampatzidis and Whiting, 2013). The efficient interception of light by UFO orchards has been described (Zhang et al., 2015); however, too few or uneven spacing of fruiting uprights creates gaps in the fruiting wall and reduces orchard efficiency by failing to optimize both interception and distribution of light throughout the canopy. Little work has been published to determine how to achieve the ideal canopy structure and maximize early shoot growth for UFO trees.
Sweet cherry trees exhibit strong apical dominance (the suppression of subtending buds by the shoot terminal) resulting in vigorous top growth and minimal branch development lower in the canopy. It can be difficult to redistribute that vigor during the first year of establishment into balanced secondary shoots along the trunk, whether oriented vertically or horizontally. The mechanism of apical dominance is not fully understood, but it is generally accepted that basipetal transport of auxin produced in the terminal meristem suppresses growth of lower buds and branches (Leyser, 2005). Different training techniques can alter shoot growth patterns. In apple [Malus ×domestica (Borkh.)], changing the orientation of vertical branches released lower buds from apical dominance (Ferree and Schupp, 2003). As deviation from vertical increased, more buds were released. Bending sweet cherry branches below horizontal reduced subsequent growth of the leader and subtending shoots, compared with unbent branches (Lauri et al., 1998). Bending also increased the number of flower buds and flowers per flower bud. Placing sweet cherry trunks horizontally caused a reduction in shoot growth, relative to upright trees, by reducing node number and internode spacing (Wareing and Nasr, 1961). The more horizontal orientation of the trunk in the UFO system may partially reduce the effects of apical dominance, but that alone will not ensure well-distributed uprights.
Various techniques have been used to promote precise placement of new branches, enabling efficient use of storage reserves during tree establishment. In sweet cherry, heading cuts can promote branching, but the branches are poorly distributed and have acute crotch angles (Hoying et al., 2001). Other techniques to alter meristem outgrowth include the topical use of Promalin® (Valent Biosciences, Libertyville, IL) (containing gibberellic acids 4 and 7, and 6-benzyladenine) to alter the hormone balance at a bud and cause it to elongate into a new shoot, but effectiveness can vary due to temperature (Lang, 2005). Notching (or scoring), by cutting through the bark and phloem just above a bud, facilitates branch placement by disrupting hormone flow and promoting elongation of the bud into a new shoot (Hoying et al., 2001). Another meristem management technique is bud selection and removal. When a portion of buds is removed, the remaining buds are more likely to grow into shoots (Lang, 2005). Selective bud removal (selecting buds to be retained for placement of branches and removing all others) has been more effective than Promalin® or notching for producing laterals from remaining buds (or nodes) in the lower portions of the trunk (Hoying et al., 2001). However, with bud selection or notching, caution should be taken to remove buds during warm, dry weather when risk for bacterial canker infection is low. Furthermore, gaps may be left in the canopy if any of the selected buds fail to grow.
Current recommendations for UFO tree training are to use precocious rootstocks, such as the Gisela series, to bring trees into production quickly (Long et al., 2015). Trees on precocious rootstocks enable earlier yields, but they can be susceptible to poor structural development and/or overcropping if poorly managed (Lang, 2001; Long, 2001). Understanding how fruiting branches develop can help growers make wise training and pruning decisions to maximize early yields. The year a shoot forms, a single leaf is produced at each node. The next year, that shoot usually forms a small number of nonspur flower buds at its basal nodes which then become blind wood after fruiting (Lang, 2005). The other nodes each form nonfruiting leafy spurs with 5–9 leaves. In the 3rd and subsequent years, those nodes will be the fruiting spurs that will bear fruit until they become damaged or diseased. This fruiting progression often brings trees on Gisela rootstocks into significant flowering in the 3rd or 4th year in the orchard. These different populations of fruit-bearing sites (onetime nonspur fruiting nodes and multiyear fruiting spurs) illustrate how shoot growth in the first year becomes minor fruiting area in Year 2 and significant fruiting area in Year 3. This underscores the importance of maximizing canopy shoot growth in the first year to optimize yield potential from fruiting spurs in Year 3 and beyond.
Successful development of the UFO fruiting wall canopy architecture requires several decisions to be made at planting or soon thereafter. First-year establishment of UFO sweet cherry trees was investigated to determine the effects of planting angle, height of cordon bending to horizontal, and selective bud removal on number of structural shoots, shoot growth and distribution, and early fruiting potential.
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