Preharvest fruit drop can reduce production of some modern apple cultivars in commercial orchards by as much as 30% (Marini et al., 1993). A recent determination of natural variation in fruit abscission-related traits in apple suggested that preharvest fruit drop can occur independently of fruit ethylene production (Sun et al., 2009). Losses resulting from preharvest fruit drop can be mitigated by applying either naphthaleneacetic acid (NAA; Fruitone L; AMVAC Chemical, Newport Beach, CA) or aminoethoxyvinlyglycine (AVG; Retain; Valent BioSciences, Libertyville, IL) to delay drop. Holding fruit on the tree beyond the normal harvest date may also increase crop value due to an increase in fruit size (yield) and improvement in red color development. Byers and Eno (2002) estimated that delaying harvest for an additional 3 weeks increased crop value by as much as 20% due to an increase in fruit size and improvement in fruit color; provided fruit maturity was not also advanced during this period.
Gardner et al. (1939) provided the first report that NAA delayed fruit drop. NAA delayed fruit drop most effectively when applied at concentrations of 5 to 20 mg·L−1 3 to 4 weeks before the optimum harvest date and then again 14 to 21 d later (Marini et al., 1993). Many growers remain cautious about using preharvest NAA sprays for drop control because of the risk of accelerated fruit softening that they occasionally observe and that has been demonstrated in some studies (Batjer and Moon, 1945; Byers, 1997a; Greene and Schupp, 2004; Li and Yuan, 2008; Marini et al., 1993; Schupp and Greene, 2004; Yuan and Li, 2008). Growers in the southeastern United States commonly apply NAA as four sprays of 5 mg·L−1 at weekly intervals beginning 4 weeks before the normal harvest date in a program referred to as “preload NAA” (PL-NAA). The supposed advantage of a PL-NAA program is that it provides equivalent drop control compared with a standard NAA program without the risks of accelerated fruit softening associated with higher rates of NAA. The reduction in fruit drop following preharvest NAA sprays was associated with reduced expression of genes involved in cell wall degradation (MdPG2 and MdEG1) in the fruit abscission zone (Li and Yuan, 2008). However, preharvest NAA sprays also increased ethylene production and softening in the fruit cortex, associated with increased expression of genes involved in ethylene biosynthesis (MdACS1 and MdACO1) and perception (MdERS1) and cell wall degradation (MdPG1) (Li and Yuan, 2008).
The control of preharvest drop in apples with AVG was first described by Bangerth (1978). AVG is typically applied 28 d before the anticipated harvest date and reduces fruit drop by suppressing expression of genes involved in ethylene biosynthesis (MdACS5A, MdACS5B, and MdACO1) and cell wall degradation (MdPG2 and MdEG1) in the fruit abscission zone (Li and Yuan, 2008; Yuan and Li, 2008). AVG also inhibited ethylene production and delayed softening in fruit by suppressing the expression of MdACS1, MdACO1, MdERS1, and MdPG1 in the fruit cortex (Li and Yuan, 2008).
Numerous studies have compared the relative efficacy of AVG and standard NAA programs for fruit drop control in apple (Byers, 1997a, 1997b; Dal Cin et al., 2008; Greene, 2005; Greene and Schupp, 2004; Li and Yuan, 2008; Schupp and Greene, 2004; Yuan and Carbaugh, 2007; Yuan and Li, 2008). However, with one exception, these comparisons were restricted to different groups of trees in individual years. In only one instance was the efficacy of AVG and standard NAA treatments for drop control directly compared in the same orchard over two consecutive seasons (Greene, 2005). To our knowledge, there have been no studies comparing the effects of AVG and NAA treatments on fruit drop or fruit firmness at normal and delayed harvests within the same apple orchard over a prolonged period. This article describes natural patterns of fruit drop within a commercial orchard of ‘Scarletspur Delicious’ apples over a period of 11 years (consecutive) and compares the effects of AVG, standard NAA, and PL-NAA treatments on fruit drop and fruit firmness within the same orchard.
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