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Trials were conducted in 2009 and 2010 to evaluate the use of a hand-thinning gauge [Equilifruit; Institut National de la Recherche Agronomique (INRA), Montpelier, France] on three cultivars of apple (Malus ×domestica) trees trained to tall spindle. Hand-thinning treatments were applied after June drop to trees with supra-optimal crop loads. Three hand-thinning treatments were applied using the hand-thinning gauge: 1) thinning to ≈6 fruit/cm² branch cross-sectional area (BCSA) (F value), 2) subtracting the delta value [Δ (an adjustment factor to increase or decrease the number of fruit per BCSA] from the F value (F − Δ), and 3) F − 2Δ. These treatments were compared with a control and a traditional hand-thinning heuristic of spacing a solitary fruit every 7 to 8 inches of branch length. Use of the hand-thinning gauge generally improved fruit weight and maintained whole tree yields when compared with the control. Hand-thinning based upon traditional fruit-spacing heuristics reduced crop density and increased final fruit weight of apple, but significant reductions in yield were observed in two of four studies when compared with the control. We find the hand-thinning gauge a useful tool in adjusting final crop load of apple.

The objective of these studies was to evaluate the efficacy of several concentrations of 1-aminocyclopropane carboxylic acid (ACC) for thinning apple at the standard growth stage for chemical thinning timing and a late thinning growth stage. ACC was applied at concentrations of 0, 100, 300, or 500 mg·L⁻¹ to ‘Golden Delicious’/Bud.9 apple trees at 10 mm or 20 mm fruit diameter. Treatments were applied to the point of drip to individual whole trees in a completely randomized design with five (2010) and six (2011) replications. When ACC was applied at 20 mm, there was a linear dose relationship between concentration and fruit thinning in both years. ACC was ineffective at 10 mm. The naturally occurring compound ACC shows potential for use as a reliable late chemical thinner for apple.

Blossom thinning can confer significant benefits to apple growers, including increased fruit size and annual bearing. However, current blossom thinning practices can damage spur leaves and/or fruit. We evaluated the use of short duration forced heated air treatments [thermal shock (TS)] as a blossom thinning strategy for ‘York Imperial’. Using a variable-temperature heat gun, TS treatments were applied to solitary blossoms 24 hours after pollination. Effects of output temperature (five levels) and treatment duration (four levels) were evaluated using a completely randomized design with a factorial treatment structure. Short duration treatments (0.5 and 1.0 seconds) were ineffective for arresting pollen tube growth in vivo. TS temperature required to inhibit stylar pollen tube growth was inconsistent across years. In 2014, TS temperatures ≥56 °C inhibited pollen tubes from reaching the style base at 2.0 and 4.0 second durations. However, in 2015, TS temperatures ≥81 °C at 4.0 seconds prevented pollen tubes from reaching the style base. Inconsistent effects of TS across years were attributed to treatments being applied too late due to optimal conditions for pollen tube growth during the intervening 24-hour period after pollination. Excessive injury to spur leaf tissue was observed at temperatures higher than 84 °C and 70 °C (2.0 and 4.0 seconds, respectively). Pollen tube growth was reduced or arrested at temperature and duration combinations that caused minimal visible injury to spur leaves. Identifying and exploiting structural differences between apple blossoms and vegetative spur leaves may provide insight for the future development of TS or other attempts at developing selective thinning technologies.

The objectives of this experiment were to test the efficacy of a mechanical string thinner (Darwin PT-250; Fruit-Tec, Deggenhauserertal, Germany) on apple and to identify an optimal range of thinning severity as influenced by spindle rotation speed. Trials were conducted in 2010 and 2011 at the Pennsylvania State University Fruit Research and Extension Center in Biglerville, PA, on five-year-old ‘Buckeye Gala’/M.9 apple trees that were trained to tall spindle. A preliminary trail on five-year-old ‘Cripps Pink’/M.9 was conducted to determine the relationship between string number and thinning severity. As the number of strings increased, the level of thinning severity increased. A range of spindle speeds (0 to 300 rpm) was applied to the same trees for two consecutive years. As spindle speed increased, blossom density (blossom clusters per limb cross-sectional area) was reduced as was the number of blossoms per spur. In 2010, leaf area per spur was reduced 9% to 45%. In 2011, the fastest spindle speed reduced leaf area per spur 20%. Although increased spindle speed reduced cropload, injury to spur leaves may have inhibited increases in fruit size. The largest gain in fruit weight was 28 g (300 rpm) compared with the control. In both years, the most severe thinning treatments reduced yield by more than 50%. There was no relationship between spindle speed and return bloom. Severe thinning treatments (240 to 300 rpm) caused significant reductions in spur leaf area, yield, and fruit calcium and did not improve fruit size or return bloom. Spindle speeds of 180 and 210 rpm provided the best overall thinning response and minimized injury to spur leaves, but cropload reduction was insufficient in years of heavy fruit set. Therefore, mechanical blossom thinning treatments should be supplemented with other thinning methods. Mechanical string thinning may be a viable treatment in organic apple production, where use of chemical thinners is limited.

Reducing apple crop load during bloom can increase fruit size and promote annual bearing when compared with crop reduction at later timings. In this study, we compared the efficacy of chemical blossom-thinning strategies on ‘Golden Delicious’ and ‘Gala’ apple trees. Several blossom-thinning treatments were evaluated, including 1) unthinned control (control), 2) hand-thinned (HT) at bloom, 3) liquid lime sulfur + Stylet-Oil (LS + SO), 4) ammonium thiosulfate (ATS), 5) endothall (ET), and 6) naphthaleneacetamide (NAD). Chemical treatments were applied twice during bloom, using a predictive model to determine application timings. Blossom thinner effects on pollen tube growth, fruit set, and yield responses were evaluated. LS + SO and ATS reduced the number of pollen tubes that entered the style for ‘Golden Delicious’ by 75% and 63%, respectively. ET and NAD did not affect the number of pollen tubes that entered the style. In one of 2 years, LS + SO resulted in a near-ideal crop load and increased fruit weight. ATS was effective in reducing initial fruit set in ‘Golden Delicious’ and ‘Gala’, but did not reduce whole-tree crop density. ET reduced crop load in all experiments but caused excessive spur leaf injury and negatively affected fruit size of ‘Gala’ but not ‘Golden Delicious’. NAD had limited efficacy on ‘Golden Delicious’ at the concentrations and application timings used in this trial. When used as the sole method of crop load management, none of the chemistries evaluated over-thinned or increased fruit injury. However, ET caused excessive thinning when evaluated as part of a commercial crop load management program on ‘Gala’. Of the products evaluated, LS + SO provided the best overall thinning response.

The use of short-duration applications of thermal energy (thermal shock; TS) as an apple blossom thinning strategy was investigated. Effects of TS temperature and timing on stigmatic receptivity, pollen tube growth in vivo, and visible leaf injury were evaluated in multiple experiments on ‘Crimson Gala’. TS treatments were applied to blossoms and spur leaves using a variable temperature heat gun. TS temperatures ≥86 °C had a strong inhibitory effect on pollen tube growth on the stigmatic surface and in the style. TS temperatures >79 °C reduced average pollen tube length to less than the average style length. Timing of TS treatment (0 or 24 hours after pollination) was not an influential factor, indicating that effective TS temperatures reduced pollen tube growth up to 24 hours after the pollination event. The onset of thermal injury to vegetative tissues occurred at similar TS temperatures that inhibited pollen tube growth in vivo. Excessive leaf injury (>33%) was observed at 95 °C, suggesting relatively narrow differences in thermal sensitivity between reproductive and vegetative tissues. Inconsistent TS temperatures and/or responses were observed in some experiments. Ambient air temperature may have influenced heat gun output temperatures and/or plant susceptibility. While results suggest some promise, additional work is required to validate and further develop this concept.

Pruning is the cutting away of vegetation from plants for horticultural purposes. Pruning is known to reduce apple tree size, increase fruit size and quality, and decrease yield. Methods for studying the effects of varying degrees of severity of pruning on a whole-tree basis have used qualitative descriptions of treatments rather than repeatable whole-tree quantitative metrics. In this study, we introduce a pruning severity index calculated from the sum of the cross-sectional area of all branches on a tree at 2.5 cm from their union to the central leader divided by the cross-sectional area of its central leader at 30 cm from the graft union. This limb to trunk ratio (LTR) was then modified by successively removing the largest branches of ‘Buckeye Gala’ to achieve six severity levels ranging from LTR 0.5 to LTR 1.75, with lower values representing more extreme pruning with less whole-tree limb area relative to trunk area. Pruning treatments were applied for three consecutive years and tree growth and cropping responses were observed for the first 2 years. With increasing pruning severity the following characteristics increased after seasonal growth: number of renewal limbs, number of shoots, shoot length, number of shoot leaves, shoot leaf area, final fruit set, fruit size, yield of large fruit, crop value from large fruit, soluble solids, and titratable acidity. The following characteristics decreased: limb age, number of secondary limbs, number of spurs, number of spur leaves, spur leaf area, the ratio of spur leaf area to shoot leaf area, fruit count per tree, yield, yield efficiency, crop value from small fruit, overall crop value, and sugar:acid ratio. The LTR provides a measurable way to define and create different levels of pruning severity and achieve consistent outcomes. This allows a greater degree of accuracy and precision to dormant pruning of tall spindle apple trees. The use of the LTR to establish the level of pruning severity allows the orchard manager to set crop load potential through regulation of the canopy bearing surface. This metric is also a necessary step in the development of autonomous pruning systems.