Good Fruit Grower

February 1

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Horticulture Lower costs and better vigor control make them attractive for high-density plantings. scan to watch the interview Multileader TREES PLAY by Richard Lehnert lberto Dorigoni's vision of apple orchards of the future is one where high yields of high-quality fruit grow on thin fruiting walls, with much less labor devoted to pruning, training, and harvesting. If he could lower the cost of establishment, he'd like that, too. Dorigoni, a researcher from Trento, in Italy's most northeastern province, thinks multileader trees with two, three, or four main leaders growing from one trunk will, in fact, result in fruiting walls that look just like super spindle or tall spindle orchards, except there will be fewer trunks. Instead of 2,000 trees per acre as in super spindle systems or 1,300 in tall spindle systems, the number of trees could be reduced to 800 to 1,000. Dorigoni explained the concepts of fruiting walls and multileader trees to International Fruit Tree Association members meeting in Boston a year ago. After researchers and growers in New York and Michigan tried them out last summer, Dorigoni was invited to "tell more." He gave two presentations at the Great Lakes Fruit, Vegetable, and Farm Market Expo in Grand Rapids, Michigan, in December. "The main drawbacks of traditional high density lie not just in the high economic investment, especially for the club varieties, but even more in the difficult management, which often requires limb bending, precise crop load control, and skilled pruning," he said. "It takes just small mistakes, like one year of low crop or bad pruning, to turn high-density super spindles into uncontrollable trees that only sophisticated techniques based on heavy hormonal treatments and severe root pruning can recover." High density is essential for low-input management, he said. The problem is that traditional high density is only good for shallow soils; on fertile sites, excellent early yields diminish because tree vigor is too difficult to control. Splitting a tree's vigor by putting several axes on a single trunk can help growers control vigorous varieties like Red Delicious and Golden Delicious. And for weak varieties like Honeycrisp and Gala, it would allow them to use more vigorous rootstocks and spread the increased vigor across more leaders. "Increasing the number of leaders by splitting the canopy in two, three or four mini-spindles results in significant weakening of trees and a remarkable change in their architecture," he said. "The only structural wood left is given by the vertical leaders, while all remaining wood is short, does not need bending, and is covered with flower buds near the trunk." This concept of multiple, columnar leaders is also being developed for growers of sweet cherries by researchers in the United States. A systems with vigorous Malling or seedling rootstocks, he explained. Trials he started in 2008 confirmed that not only can trees with two axes replace high-density super spindles, but that three- and four-leader trees can be obtained after planting on M.9 in strong soils, Dorigoni said. There are several ways to achieve multileader trees. In young trees, virtually any limb can be promoted to be a leader, provided it has enough room and light, is put vertically, is defruited if necessary for a year or two, and there is enough horsepower in the root system. Heading back a whip is not the best option, he said. Start with a feathered tree and leave the central stem plus two laterals to get a threeleader tree. "The bigger diameter of the central trunk compared to the two laterals will not be a problem if we encourage their growth by keeping them vertical and suppressing competitive wood," he said. "Such trees can bear crop already in the second leaf, mostly on the center, while the laterals are kept more vegetative." Similarly for the trees with four axes, two new leaders in the middle of the tree naturally rise when spreading a Bibaum, he said. Spacing between leaders in the row should be kept to 18 to 24 inches, but alleyways can be 15 to 20 percent narrower compared to standard spindle, since trees protrude less into the alleyway. While it takes some time and care during the early years to develop the axes, limb bending is not needed for —Alberto Dorigoni Bibaum and multileader trees, he said. "Depending on soil fertility, it may take multileader trees from four to six years to achieve a fully formed canopy, usually one or two more than for spindle or Bibaum," he said. But the result is a different kind of tree. "In all cultivars, multileader trees had many more shoots than spindle, particularly in the medium and upper part. The length of the scaffold shoots was inversely related to the number of leaders (i.e., more leaders, shorter branches). The percentage of branches longer than 60 centimeters (24 inches) was reduced from 11 percent on spindle to 5 percent, 4 percent, and 3 percent on two, three, and four leaders, respectively." As a result, he said, average width of the lower part of trees was dramatically reduced, in the case of Golden Delicious, from 144 centimeters (57 inches) in spindle to 76 centimeters (30 inches) with four leaders. "Once the concept of 'more than one leader is possible' was accepted, the next question was whether the number of leaders could be further increased." Making multileader trees The idea of using a double-leader training system was promoted in Italy a decade ago by Vivai Mazzoni, a company that produces double-budded trees in its nurseries. The Bibaum tree process is patented. "Once the concept of 'more than one leader is possible' was accepted, the next question was whether the number of leaders could be further increased to get a natural fruiting wall in fertile soils and encourage mechanization," Dorigoni said. Again, the principle was that increasing the number of axes has a similar effect as shifting to a more dwarfing rootstock, a well-known concept in the old training 16 FEBRUARY 1, 2014 GOOD FRUIT GROWER Pros and cons The main benefit of the multileader system and fruiting walls comes from the shortening of limbs, making the trees more suitable to any kind of mechanization—thinning, pruning, weed control, and harvest, he said. In addition, trees get better spray coverage. Shorter, narrower trees would be suited to tunnel spraying, which reduces drift and uses less spray material, he said. The main drawbacks of multiple leaders involve poorer yields in the first three or four years and the need to complete tree formation in the field. Once established, the multileader system generally performs as well as or better than spindle, Dorigoni said. Uneven axis formation can result if trees are not properly trained, he said. Since leaders are weak, a strong trellis is needed to bear a crop, which is uniformly distributed throughout the tree and, to a large extent, hangs on the wires. •

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