Central leader-trained trees are adaptable to a wide range of environmental and socioeconomic conditions. The general tree shape is conical, with the largest diameter branches at the bottom and the smallest at the top. D. R. Heinicke popularized the system in the United States, with a "head and spread" training concept. This system of heading the central leader and scaffolds to encourage branching is particularly effective on spur-type 'Delicious' apple trees. D. W. McKenzie developed a similar system in New Zealand, a difference being that bays are created for ladder placement and ease of harvesting. With standard or semidwarf rootstocks, trees are freestanding, and there are three tiers of scaffolds on the leader with gaps in between for sunlight penetration. In recent years, growers have modified the system for use with staked dwarf trees. Trees grow together in some systems and are managed as hedgerows. Individual identity is lost, but from the ends of the rows trees should appear conical. Although pyramid training has been most widely tested on apples, central leader and modified central leader systems also are used on peaches, nectarines, cherries, plums, and pears. Due to growth habit, some stone fruit cultivars are better adapted to a central leader system than others. A disadvantage with pear trees is the potential for losing the leader to a fire blight strike. A successful alternative is to train trees to a four-leader system.
Various forms of the slender spindle system are highly productive in Europe and other fruit-growing areas. The system was developed in Holland and Belgium in the 1950s and described by S. J. Wertheim in 1970 in a Wilhelminadorp Research Station publication. It was quickly adopted in Germany and other countries with limited land for agriculture. Tree height is 2.0 to 2.5 meters, and only the bottom whorl (or table) of branches is permanent. Higher, fruiting branches are kept weak by bending and are regularly renewed by strategic pruning. Trees are individually staked, and branches are tied down to induce fruiting and tied up to support crop loads. Slender spindle trees are 1.0 to 1.5 meters at the base and taper to a point at the top. Super slender spindle trees are even narrower. Slender spindle-trained trees can be grown in multiple-row (bed) systems; however, most growers prefer single rows. Economic success of a slender spindle orchard depends on proper selection of a precocious, size-restricting rootstock, well-feathered nursery stock, and detailed training and pruning. Slender spindle-type systems for stone fruit trees are under test in some regions. The fusetto is an Italian hedgerow system that utilizes slender spindle-trained peach trees.
The vertical axis is the system of choice for many apple growers who are making a transition from low-density, central leader-trained trees to higher-density production systems. J.-M. Lespinasse developed the system in the 1970s and described it in a French apple tree management bulletin published in 1980. The system is designed to encourage equilibrium between fruit production and vegetative growth. Trees receive minimal pruning and training, which results in early production and natural growth control. A training pole attached to one or two trellis wires supports the leader of each tree. The bottom whorl of limbs is permanent, and upper branches are periodically renewed. Widely used rootstocks are M.9 and M.26, and tree height ranges from 3.5 to 5.0 meters, depending on stion (scion/rootstock) and site. Although ladders must be used for picking, harvest is more efficient due to narrow tree widths. A recent modification of the vertical axis is the solaxe, in which renewal pruning is replaced by bending.
A number of New Zealand growers train apple trees to a slender pyramid, which is a hybrid of the central leader and the vertical axis system. D. S. Tustin is one of the originators. Rapid canopy development and early fruiting are encouraged by avoiding dormant heading cuts and by strategic summer pinching. Basic tree form is pyramidal with a strong basal tier of scaffolds and a slender upper canopy. Commonly used rootstocks are M.26 and MM.106. Trees are only slightly narrower than central leader-trained trees, but due to New Zealand's ideal growing environment, production is equal to that from more intensive plantings in other regions.
The hybrid tree cone (HYTEC) combines some of the best characteristics of the vertical axis and the slender spindle. In the 1990s, B. H. Barritt developed the hybrid conical tree form for central Washington State cultivars and climatic conditions. Various modifications of the HYTEC have been successfully applied in other fruitgrowing regions. Trees are staked and are intermediate in height between the slender spindle and the vertical axis. To reduce excessive vigor in the top of the tree, the central leader is zig-zagged by annual bending or removal to a side branch. Otherwise, training is similar to that used with the vertical axis system.
352 CONCISE ENCYCLOPEDIA OF TEMPERATE TREE FRUIT OPEN CENTER SYSTEMS
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