Good Fruit Grower

April 1

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T he proper angle of shoots or branches is an aspect of tree training which encourages forma- tion of flower buds and fruit, allows for sunlight to penetrate into the canopy, gives structural strength to the tree, and directs growth within the young apple tree. Shoots or new shoots refer to current growth up to one year old. These shoots are long and vigorous, and their tip buds often remain vegetative. Branches are older than one year and have formed new shoots and laterals. A branch can have wood of different ages. Crotch angles and strength Branches with narrow crotch angles are structurally weak and can easily break in the wind or from heavy crop loads. Branches with wide crotch angles have greater con- nective strength and will not break because supporting wood is formed on the underside. Effects of branch spreading The way that shoots or branches are spread and posi- tioned, and the time of year when this is done, affect growth and cropping. Regardless of the growth habit of the apple variety, when shoots or branches are spread or repositioned from an upright position to a more horizontal position, termi- nal extension growth and apical dominance are reduced, and development of lateral branches is increased. Spreading the entire shoot or branch in a straight line results in uniform growth along the length of the shoot or branch and creates a strong crotch angle. The closer to the horizontal a branch is spread, the more uniform are buds that develop along the branch, especially those towards the base. Arch bending Arch bending is where the tip of the shoot or branch is bent below some point along the shoot or branch to form an arch or curve. This creates a different response than does straight- line spreading. Arch bending has little effect on crotch angle. At the highest point along the arch, one or more upright shoots will typically form. Other shoots are shorter in both directions from the arch. Apical dominance The gravitational force responses to spreading or bending are the result of a phenomenon called apical dominance. Apical dominance regulates growth via two plant hormones. Firstly, a plant hor- mone called cytokinin, which is produced in root tips, moves up the tree and breaks dormancy at the highest points (tip buds) in the tree. When the tip buds start to move, another plant hor- mone called auxin is produced in the actively growing shoot tip. As auxin moves down the shoot by gravity, it stops lateral buds below the tip from developing and branching. Apical dominance is strongest in vertical shoots where auxin moves downwards and accumulates uniformly in buds, stopping them from developing lateral shoots. The effect becomes weaker with increasing distance from the tip. As buds become more distant from the tip, they are released from the auxin influence and may develop lateral shoots. Apical dominance varies between species and varieties, and is very strong in apple and pear. Bending and arching affect apical dominance differently As shoots or branches are bent away from a vertical position, auxin and cytokinin are no longer concentrated in the tip, and the number and length of lateral shoots are increased while extension growth is decreased. If shoots or branches are arched below the horizontal, apical dominance by the bud at the tip is lost. Auxin accumulates in the lower side of the shoot or branch, and cytokinin accumulates in the uppermost buds on the curve of the shoot or branch. These buds, which are no longer under any apical control, develop into new upright shoots as the tree seeks to reestablish apical control in the shoot or branch. Thus, spreading or arch bending can have a pro- found effect on shoot growth, which can then later affect precocity and yield. What is a good angle? The best production comes from branches trained to grow at about a 45-degree angle from vertical. This angle is wide enough to prevent the branch ever competing with the tree's leader. The angle is also narrow enough to allow for the apical bud to be strong enough to impose 46 APRIL 1, 2014 GOOD FRUIT GROWER Proper spreading of shoots and branches is an important step in developing a productive orchard. by Bas van den Ende A perfectly trained two-year-old Gala tree (on MM.106 rootstock) on the Open Tatura system. Scaffold branches obey the 3-to-1 rule and have been spread at the correct angle. This method of spreading ensures that the space on the trellis is quickly filled with fruiting wood. It also allows renewal of fruiting wood while keeping enough vigor in the tree to grow an optimum crop of apples of good size and quality annually. Only a few vigorous shoots on the back of the leaders are removed in summer. The crop keeps vigor in check. These trees produced 10.5 tons per hectare (10.5 bins per acre) of packed fruit 30 months after planting. The planting density is 1,818 trees per hectare (about 700 trees per acre). Removing upright shoots on bent branches in midsummer can cause a massive sunburn problem in hot climates like Australia's. Shoots and branches that you spread must obey the 3-to-1 rule. The 3-to-1 rule ensures that a dominant leader with a balanced structure of branches develops. PHOTOS COURTESY OF BAS VAN DEN ENDE Spreading shoots of YOUNG APPLE TREES

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