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  • Author or Editor: Max W. Williams x
  • HortTechnology x
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In fruit-growing areas where pollination and fruit set conditions are optimum, biennial bearing is a regular occurrence unless steps are taken to reduce fruit set in the heavy bloom year. In the past, a two- or three-spray thinning program was used in the state of Washington on difficult cultivars, consisting of Elgetol at bloom time, followed by a petal fall spray of Amide-thin, followed by a later postbloom spray of NAA or Sevin. In 1989, Elgetol was removed from the market by the manufacturer because of the high cost of re-registration. In 1990, a search for an Elgetol replacement was initiated. Elgetol was a contact spray that damaged flower parts and prevented fertilization of the ovule. Thus, it was logical to look for other products that had a similar mode of action. Sulfcarbamide (monocarbamide dihydrogen sulfate) (D-88) (Wilthin), a foliage desiccant used on potatoes, onions, and alfalfa, and tested as a dilute full-volume spray on apple blossoms, was found to be an effective blossom thinner. Chemical rates of 0.25% to 0.5% (v/v) applied at 80% of bloom open reduced fruit set by 25% to 50% compared to controls.

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Biennial bearing of apple trees can be overcome either by the use of a blossom chemical thinner or by early application of a postbloom thinner. Carbaryl (Sevin) is a post-bloom fruit-thinning chemical with an effective thinning period of 4 to 5 weeks after bloom. Sevin was compared in 1992 and 1993 with NAA as an early petal-fall spray. Sevin treatments reduced fruit set to one fruit per cluster with no adverse side effects on the foliage. NAA inconsistently reduced fruit set and the remaining fruit were in clusters, The NAA-treated foliage was adversely affected; having small curled leaves. NAA at 10 ppm under-thinned in 1992 and seriously over-thinned in 1993, whereas Sevin treatments were consistent for fruit thinning in both years. Sevin applied at petal-fall or at petal-fall + 7 days effectively reduced fruit set and reduced fruit competition.

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Endothall [7, oxabicyclo (2,2,1) heptane-2-3 dicarboxylic acid] is an aquatic herbicide with potential use as a blossom thinner for apples (Malus domestics Borkh.). Trials conducted in Washington, New Zealand, and Australia on several apple cultivars indicate that endothall is a safe, consistent blossom thinner. Cultivars treated were `Golden Delicious', `Delicious', `Royal Gala', and `Granny Smith'. Single and repeat applications were used in the New Zealand and Washington tests. With multiple applications of endothall, no fruit marking occurred on any of the test cultivars. In temperate fruit zones with extended apple bloom periods, multiple applications of endothall at a low rate may be beneficial for reducing fruit set and biennial bearing.

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