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Premature 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 (aminoethoxyvinylglycine 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 conventional 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 endogenous (natural) auxin produced by the fruit declines. Auxin decrease ­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 maturity. Applied preharvest, NAA delays fruit loosening and drop, but it hastens fruit softening, 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 availability varies with geographic location and supplier. NAA is not allowed for organic use.

NAA rate—Main application factors are cultivar and fruit hanging time, which varies year to year and with target harvest maturity. The maximum rate for pears is 25 grams per acre (10 ppm) in one or two applications. University of California and industry trials in the 1960s and 1970s confirmed previous research and field observations that multiple applications and/or rates higher than 50 grams per acre could cause excessive and early ripening and core breakdown, as well as damaged or dead buds and shoots the following spring.

A rate of 25 grams applied only once failed to reduce drop of Bartlett pears in the cool districts, which are more prone to premature ripening. Based on these findings, the California Department of Agriculture authorized a rate of 37.5 grams (an above-label rate) applied to control drop, and one NAA product label continues to maintain a Section 24(C) for pear in ­California at that higher rate.

NAA timing—NAA is generally applied to pears 5 to 14 days prior to first anticipated harvest. Applied too early, it may not last in the tissues long enough to be of use, and applied too late may miss the drop control window. Once stem loosening begins, NAA only impedes further loosening; it cannot “retighten” the connection (reverse cellular changes that have already occurred).

Labels vary most on the timing aspect, ranging from 5 to 30 days prior to first estimated harvest of untreated fruit. Pears are treated later than apples, from 5 to14 days prior to first anticipated harvest. It is important not to treat prematurely (and hence, ineffectively) by mistaking fruit “pushing off” due to short stems or ­multiples in a cluster for actual loosening.

Physiological development should also be monitored to determine proper timing. In pear-growing regions prone to premature ripening, an option has been to apply NAA when the firmness of the largest fruit averages about 21.5 pounds of force, since ripening slows if it is warm in the month or so prior to harvest and ­hastens when preharvest temperatures are cooler.

NAA application—NAA penetrates foliage through the leaf cuticle; absorption at the stem abscission zone is not necessary as NAA is transported away from the leaf. Waxy leaves hinder uptake, thus uptake is said to be better in cloudy or moist, cool conditions. Soil moisture should be moderate, not too wet or dry, and trees should not be stressed, or uptake will be poor. Do not irrigate immediately before application or until after uptake and activity is evident. Absorption is best at between 70 and 85°F, followed by cooler, moist weather for several days to enhance and extend activity. Very hot weather reduces uptake and activity. NAA is applied at 100 to 250 gallons per acre, depending on tree size and spacing; good coverage is critical, and action is local to the absorption site.

Slightly acidic solutions may increase uptake, but surfactants are only needed if uptake conditions are poor.

Cultivar—Bartlett and Bosc must be treated, and other pear cultivars generally benefit as well, depending on anticipated hanging time.

Once active, NAA requires 2 to 4 days to “take;” fruit will drop in the meantime. Drop control should last at least 7 to 14 days, often longer, depending on cultivar, coverage, initial maturity, weather, tree vigor and stress level, and other factors. Drop will resume suddenly when the material ceases activity, and increase severely along with endogenous ethylene levels. Fruit softening should be monitored, and harvest timing adjusted. Fruit softening in storage is a possibility and should also be monitored.

Rachel Elkins is with University of California Cooperative Extension, Lake and Mendocino Counties; Kitren Glozer, Department of Plant Sciences, University of California, Davis; and Michael Devencenzi, Ag Pest Management and Research, Woodbridge, California.