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The normal window for application of thinning chemicals in apple extends from bloom until 3 weeks after bloom, when the fruit reach a mean diameter of ≈16 mm. After this time fruit are generally insensitive to standard chemical thinning sprays. The potential for the photosystem II (PSII) inhibitor metamitron and the ethylene precursor 1-aminocyclopropane carboxylic acid (ACC) to thin apple fruit after the traditional thinning window was investigated in field experiments over three years. A standard rescue thinning spray of carbaryl plus ethephon plus naphthaleneacetic acid (NAA) reduced fruit set of Gale ‘Gala’ if applied when the mean fruit diameter was 18, 20, and 27 mm in 2010, 2011, and 2012, respectively. The thinning activity of 400 mg·L⁻¹ ACC was equivalent to the standard rescue thinning spray in 2010, whereas 350 mg·L⁻¹ metamitron reduced fruit set more effectively than either the standard or ACC in 2010. Application of 400 mg·L⁻¹ ACC plus 350 mg·L⁻¹ metamitron when the mean fruit diameter was 18 mm reduced fruit set to almost no crop in 2010. The combination of metamitron plus ACC exhibited thinning activity after application at 25 and 33 mm mean fruit diameter in 2011 and 2012, respectively. Increased ethylene evolution was found in detached ‘GoldRush’ fruit 24 h after applications of ACC from 11 mm to 27 mm mean fruit diameter, but not when ACC was applied at 31 mm mean fruit diameter. Ethylene evolution was much higher after application of ACC at the 11 mm or 17 mm mean fruit diameter stage compared with application when fruit diameter was 23 mm or 27 mm. The thinning activity of ACC was related to the period of maximum ethylene response. The effects of delayed applications of ACC and metamitron on fruit set tended to be greater when these two chemicals were combined, suggesting that the creation of a carbohydrate stress and the capacity to convert ACC to ethylene are both required to trigger abscission of apple fruit larger than 18 mm in diameter.

Notching is a technique to stimulate shoot growth from paradormant buds with the long-term objective of increasing the cropping potential of fruit trees by developing new fruiting scaffolds. Notching is not commonly practiced by apple (Malus ×domestica) growers in the southeastern United States because results can be inconsistent; notched buds frequently develop into weak spurs producing a few leaves rather than strong extension shoot growth. Therefore, a study was initiated to investigate the potential of 6-benzyladenine (6-BA) to enhance the shoot growth response from notched buds on 2- or 3-year-old wood. Notching paradormant buds on the leader of young ‘Granny Smith’/‘M.26 EMLA’ (‘M.26’) trees at the pink bud stage resulted in new growth developing on 59% of notched buds; however, fewer than 4% of these extended beyond 1.0 cm in length. Spray application of a solution of 1.5 g·L⁻¹ 6-BA into the notch increased budbreak to 95% and the proportion of new growth that developed into shoots to 89%, with a mean new shoot length of 7.9 cm. In a timing study on ‘Granny Smith’/‘M.26’, the optimum budbreak and shoot growth responses to notching plus 6-BA treatment were observed between budbreak and 2 weeks after budbreak. Notching paradormant buds on the leader of ‘Fuji’/‘M.9 T337’ (‘M.9’) trees increased budbreak, with 83% of the new growth developing into extension shoots. Spray application of 0.75 g·L⁻¹ 6-BA into the notch did not increase budbreak or extension growth of ‘Fuji’/‘M.9’ compared with notching alone. Spray application of a solution of 6-BA directly into a notch made above a paradormant bud may greatly enhance budbreak and shoot extension responses in cultivars or situations where a poor response to notching alone is expected.

The effects of foliar applications of the photosystem II (PSII) inhibitor metamitron on chlorophyll fluorescence and fruit set were compared in peach and apple trees. Metamitron increased dark-adapted chlorophyll fluorescence, measured as a reduction in Fv/Fm values, in both peaches and apples. Maximum suppression of the normalized ratio of variable fluorescence to maximum fluorescence (Fv/Fm) in peaches occurred 1 to 2 days after application and Fv/Fm values recovered by 7 days after treatment. The effects of metamitron on chlorophyll fluorescence were more persistent in apples compared with peaches. Fv/Fm values in apple declined within 2 days of treatment and did not start recovering until 5 days after treatment or longer. Concentrations of metamitron greater than 200 mg·L⁻¹ were phytotoxic to peach leaves, reducing the leaf chlorophyll concentration as determined by SPAD measurements. At 300 mg·L⁻¹, metamitron reduced fruit set in apple but not in peach. Inclusion of a non-ionic surfactant (Silwett L-77) with metamitron greatly increased its negative effect on Fv/Fm, quantum photosynthetic yield of PSII (ΦPSII), and relative electron transport rate (ETR). These results suggest that metamitron may be a useful thinner in apple but not in peach. Additional information is needed to understand how combining metamitron with existing thinning chemicals might enhance their activity. In particular, caution may be necessary if metamitron is applied as a tank mixture with commercial thinning products that have been formulated with a wetting agent.

Experiments were conducted to compare the effects of different preharvest and postharvest 1-methylcyclopropene (1-MCP) treatment combinations on ‘Law Rome’ and ‘Golden Delicious’ apple fruit. Preharvest 1-MCP sprays had minimal effects on maturity as determined by flesh firmness, starch index, internal ethylene concentration, and soluble solids concentration. Fruit internal ethylene concentration and firmness loss after 30- to 40-days storage at 0 °C plus 7 days at 20 °C were reduced by preharvest and postharvest 1-MCP treatments. The positive effects of preharvest 1-MCP on postharvest quality of ‘Law Rome’ declined in fruit that were harvested 3 days or more after spraying, whereas preharvest 1-MCP continued to have a positive effect on postharvest fruit quality of ‘Golden Delicious’ that were harvested up to 9 days after spraying. The loss in postharvest effects of preharvest 1-MCP treatment on ‘Law Rome’ at delayed harvests was reinstated by exposing fruit to gaseous 1-MCP on the day of harvest. These findings suggest that attached apple fruit of some cultivars may be capable of rapidly generating new ethylene receptors.

Three experiments were undertaken to evaluate the effects of different preharvest 1-methylcyclopropene (1-MCP) spray treatments on apple (Malus × domestica Borkh.) fruit maturity at harvest and quality after long-term storage in a regular atmosphere or controlled atmosphere (CA). Trees were sprayed within 7 days of the anticipated harvest date (H) and fruit for long-term storage were sampled at either H in the case of ‘Law Rome’ or at harvest dates that were delayed by up to 21 days (H + 21) in the case of ‘Golden Delicious’ and ‘Law Rome’. Preharvest 1-MCP sprays within 7 days of H reduced fruit drop, internal ethylene concentration, and starch index and reduced firmness loss during long-term storage of fruit at delayed harvest dates but had only minor effects on fruit maturity at H. Preharvest 1-MCP sprays reduced the incidence of superficial scald on ‘Law Rome’ apples more effectively than either diphenylamine or CA storage. Application of 1-MCP within 7 days of H may be used to delay harvest date, thereby allowing continued fruit growth without a concomitant advance in fruit maturity and to reduce firmness loss and superficial scald during long-term storage both for normal and delayed harvests.

A series of experiments were undertaken to compare the effects of individual and combined applications of GA₄₊₇ and prohexadione-Ca (P-Ca) on scarf skin and fruit quality parameters on red strains of `Rome Beauty' and `Gala' apples. Three applications of GA₄₊₇ at 10-day intervals beginning at petal fall (PF) significantly reduced scarf skin severity in all experiments. A single application of P-Ca at PF had no effect on scarf skin in one experiment but reduced scarf skin severity in two further experiments. Combining P-Ca with the first of three GA₄₊₇ sprays as a tank mix reduced the severity of scarf skin more effectively than either material alone in two of three experiments at P < 0.05 and in all three experiments at P < 0.10. Combining P-Ca with the first application of GA₄₊₇ as a tank mix generally reduced scarf skin as effectively as applying P-Ca and the first GA₄₊₇ spray two days apart, although in one experiment, greater scarf skin control was achieved when P-Ca was applied 2 days after the first GA₄₊₇ spray. A single application of P-Ca at PF consistently reduced, and three applications of GA₄₊₇ consistently increased, mean fruit weight at harvest compared with the control. The economic benefits as a result of reducing scarf skin severity with P-Ca and GA₄₊₇ sprays will need to be balanced against the negative effect of P-Ca on mean fruit weight. There is no antagonism between early season P-Ca and GA₄₊₇ sprays for scarf skin control, and P-Ca may increase the efficacy of GA₄₊₇ sprays for scarf skin control in apple.