Good Fruit Grower

May 1

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GOOD TO KNOW A research report from Rachel Elkins, Kitren Glozer, and Michael Devencenzi, California. P Preventing premature fruit drop in pears This is the first part of an article comparing NAA and AVG (ReTain) for delaying preharvest drop in pears. remature fruit drop can cause 20 percent or more yield loss in pears without intervention. Natural drop signals fruit maturity and the onset of tree dormancy. Losses are incurred directly via fruit drop and indirectly due to the need to harvest before attaining maximum potential fruit size and soluble solids. NAA (napthaleneacetic acid) has been used for many years as a stop- drop in both pears and apples. In recent years, AVG (aminoethoxyvinyl- glycine hydrochloride) has largely replaced NAA in some major apple-growing areas. If efficacy issues can be worked out for pears, AVG could become a valuable new tool, along with NAA, for both conven- tional and organic pear growers as the market demands for large fruit sizes and higher firmness levels increase, and labor supply at harvest tightens. Abscission Fruit stems loosen from the spur about 7 to 10 days prior to harvest maturity when the cell walls of the separation cell layer in the abscission zone between the stem and spur dissolve when the amount of endoge- nous (natural) auxin produced by the fruit declines. Auxin decrease Figure 1 Pattern of growth in pears The graph shows the patterns of change in Bartlett pear diameter and fruit weight after full bloom. Delaying or reducing preharvest drop improves yield and allows fruit to size optimally, increasing the economic return. 250 - Weight Diameter 200 - 150 - 100 - 50 - 0 - 0 - 20 - 15 - 10 - 5 20 40 60 80 100 120 140 DAYS AFTER FULL BLOOM - 0 SOURCE: "Vegetative growth and fruit development" by Rachel Elkins, Bas van den Ende, and James Beutel in Pear Production and Handling Manual, University of California, 2007. corresponds to the increasing level of the ripening hormone ethylene. Wind and/or mechanical forces can then facilitate final separation of the fruit from the spur. Delaying or reducing preharvest drop improves yield and allows fruit to size optimally, increasing the economic return (Figure 1). Some major factors associated with the need to delay drop in pears are: • Susceptible cultivar—Bartlett and Bosc • Tree vigor/fertility—High vigor, heavily nitrogen-fertilized, high boron, high potassium, low magnesium, iron chlorosis • Stress—Very wet or very dry (water stressed), injured foliage due to insect or mite feeding, disease • Weather—Cool (45°F) nights four to five weeks prior to harvest, leading to premature ripening. NAA NAA is a synthetic auxin that mimics the endogenous plant growth regulator indoleacetic acid. As early as 1939, U.S. Department of Agriculture researchers found that auxins could delay abscission in apples; University of California research followed soon after with data on Bartlett pears. NAA acts directly to temporarily prevent ethylene-induced enzymes from dissolving the linkages that bind the abscission layer cell walls. This process occurs independently of ethylene build-up in the fruit; in fact, when applied early for fruit thinning, auxins actually trigger ethylene production, subsequently accelerating the onset of fruit matu- rity. Applied preharvest, NAA delays fruit loosening and drop, but it hastens fruit soften- ing, which is its main drawback as a stop-drop. For abscission to occur, ethylene must be present at the abscission zone, and the abscission zone cells must perceive and respond to that ethylene. Auxin inhibits abscission by delaying the abscission tissue's sensitivity, and hence response time, to ethylene. As long as auxin is transported from the fruit through the abcission zone, fruit separation is inhibited. NAA is formulated as ammonium, potassium, or sodium salts for drop control, and as an ester for sucker control. Use in California is largely confined to the ammonium salt. There are three registered, tradenamed products. One or two applications are made, depending on use; timing of those applications is critical for the desired effect. Product 22 MAY 1, 2012 GOOD FRUIT GROWER - 25 WEIGHT (gm) - - - - - - DIAMETER (cm)

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