The experimental orchards for this study were located in Fruitland, Sunny Slope, and Parma, Idaho, USA. Trees were irrigated with a micro-jet system and trained into a central leader vertical axis. In general, the cultural practices, other than applications of post-bloom thinners were according to the local recommendations [18]. These practices are summarized by application of about 50 to 90 kg nitrogen/ha, calcium sprays throughout the season, and spraying the most environmentally sound non-organic commercial chemicals and/or biological tools for pest, weed, and disease control [18].

In each experimental orchard, 2-butoxyethanol, poloxalene, monopropylene glycol (Regulaid; nonionic surfactant; KALO, Inc. Overland Park, KS) at 500 ul·L⁻¹ was mixed with ACC and/or Ethrel treatments. Trees in Untreated Control treatments in all experimental orchards of this study only received Regulaid at 500 ul·L⁻¹ spray at the same time that other post-bloom treatments were applied. Trees were sprayed with a 189 L motorized pump-driven sprayer to deliver solution at the rate of 1870 L·ha^-1 when fruitlet diameter was about 20 mm. All orchards were sprayed with lime sulfur at 5% during 80% to 100% bloom. No post-bloom thinner other than ACC or Ethrel treatments was applied in these orchards.

Four branches per tree, each 50- to 65-cm in length with about 1.0 to 2.0 cm diameter were selected and number of flower cluster in each of these tagged branches was counted before blooms were open. At the time of hand thinning in each year (about 20 to 30 days after applications of post bloom thinners, as described for each cultivar), fruit set was calculated as number of fruits in the selected branches/number of flower clusters × 100. After fruit set measurements, fruits in all trees including Untreated Control were hand-thinned to create a 15-cm spacing among fruits, if they were not already at that spacing after chemical thinning. The reason for hand thinning according to this protocol was that even Untreated Control trees must be hand-thinned to a 15-cm spacing among fruit to follow the existing commercial practice. If fruit spacing was more than 15 cm, they were not hand-thinned any further.

To measure return bloom, four 50 to 65-cm branches per tree on each tree were tagged, and the diameter of each branch at the 50 to 65-cm mark was measured with a digital caliper in mid-March each year. Number of flower cluster within that area in each tagged branch was counted before blooms were open on March 22, 2014 (the year after application of treatments). Return bloom was calculated as the number of flower buds/ branch cross sectional area at the 50 to 65-cm mark.

In 2013 and 2014, fruit were harvested at commercial maturity dates as follows: “Buckeye Gala” on Aug. 20, 2013; “Sun Fuji” in Sunnyslope on Oct. 12, 2013; “Top Export Fuji” in Parma on Oct. 14, 2013, and “Aztec Fuji” in Parma on Oct. 16, 2014. Total fruit yield per tree was harvested and recorded in each orchard. Twenty-five fruits per tree were sampled randomly from all sides of the tree and used for quality attributes measurements as described by Fallahi et al. [19].

The maximum and minimum temperature fluctuations in the region during fruit thinner application and fruit set measurements (April and May) were recorded during 2013 and 2014 (Figure 1). The following apple orchards and treatments were used in the 2013 experiments:

1) “Buckeye Gala”: For this experiment, a seven-year-old orchard, where “Buckeye Gala” trees on RN-29 rootstock were planted at 1.8 × 4.2 m spacing at a north-south row orientation in Fruitland, Idaho was used. Number of mixed buds on each of the four tagged branches was counted and recorded on an attached plastic tag, and branch cross sectional area was calculated on April 23, 2013.

These trees were sprayed with ACC at three concentrations (150 mg·L⁻¹, 250 mg·L⁻¹, and 350 mg·L⁻¹) and Ethrel (300 mg·L⁻¹) on May 21, 2013 when the fruitlet diameter was about 20 mm (Figure 2). Number of fruits per branch was counted on June 10, 2013. A sub-sample of 16 fruit per tree was taken on June 24, 2013 and fruit weight, diameter (D) and length (L) of each fruit were measured using a digital caliper and L/D ratio of each fruit was calculated.

2) “Sun Fuji”: In this experiment, an eight-year-old “Sun Fuji” apple orchard, where trees on Bud. 9 rootstock were planted at 1.5 × 3.0 m spacing at a north-south row orientation in Sunny Slope, Idaho was used. Mixed buds (flower clusters) were counted on April 23, 2013. These trees were sprayed with ACC at two concentrations (150 mg·L⁻¹, 300 mg·L⁻¹) on May 20, 2013, and fruits were counted on June 10, 2013.

3) “Top Export Fuji”: In this study, five-year-old “Top Export Fuji” apple trees on RN 29 rootstock were planted at 1.5 × 4.2 m spacing at a north-south row

orientation at the University of Idaho Pomology and Viticulture Orchards near Parma, Idaho. Trees in this study were sprayed with ACC at three concentrations (150 mg·L⁻¹, 300 mg·L⁻¹, 450 mg·L⁻¹) on May 21, 2013 and fruits were counted on June 18, 2013.

“Aztec Fuji”. For this study, a five-year old “Aztec Fuji” orchard with trees on Bud. 9 rootstock, planted at 1 × 3.9 m spacing on a north-south row orientation at the University of Idaho Pomology and Viticulture Orchards near Parma, Idaho was used. Trees were sprayed with ACC at the rates of 300 mg·L⁻¹, 450 mg·L⁻¹, and 600 mg·L⁻¹ when average fruit size was about 20 mm on May 13, 2014 and fruit was counted on June 14, 2014.

The experimental design for all experiments in 2013 and 2014 was a randomized complete block with six blocks, each with six trees. The middle two trees of each segment were used for measurements. The assumption of normal data distribution was checked by computing univariate analyses for all responses in this study. Analyses of variance were conducted by General Linear Model, using SAS. Means separation was analyzed by least significant difference (LSD) at 5%.

In 2013, application of Ethrel at 300 mg·L⁻¹ did not affect fruit set at hand thinning time, or fruit weight, fruit diameter (D), fruit length (L), or fruit L/D ratio in the fruit sampled 34 days after applications (Table 1), or yield, fruit weight, color, russet, and SDP at harvest in “Buckeye Gala” apple (Table 2). Nevertheless, application of ACC between 150 to 350 mg·L⁻¹ had a linear dose response in fruit set and reduced fruit set by 19% to 34% (Table 1). Return bloom of “Buckeye Gala” in 2014 was inversely proportional to the concentrations of ACC applied in the 2013 season, so that trees receiving 350 mg·L⁻¹ had the highest return bloom and those with 150 mg·L⁻¹ had the least (Table 1).

“Buckeye Gala” trees of Untreated Control and those receiving Ethrel at 300 mg·L⁻¹ in 2013 had similar levels of return bloom and these levels were significantly lower than trees receiving 250 mg·L⁻¹ and 360 mg·L⁻¹ ACC in 2014 (Table 1). This observation suggests that contrary to other post bloom thinners that need to be applied shortly after petal fall to assure return bloom, late application of ACC effectively thins the current seasons’ fruit without adversely affecting next year’s fruit induction in “Buckeye Gala” apple.

Our fruit set results in “Buckeye Gala” apple are in general agreement with a study with ACC in “Golden Delicious” apple by Shupp et al. [15] who found a linear relation between rates of ACC applied at 300 - 500 mg·L^–1 and level of

^z 2014 return bloom = number of fruits per length of a branch, divided by the branch cross sectional area in cm² (Bcm²); Un. Control = Untreated Control. ^y Mean separation within columns by LSD at 5% level.

^z Abbreviations: Fruit color rating ranged from 1 = up to 20% of fruit skin was red, progressively to 5 = the entire skin was red; SDP = starch degradation pattern, ranging from 1 = very starchy to 6 = starch mostly hydrolyzed; Un. Control = Untreated Control. ^y Mean separation within columns by LSD at 5% levels.

fruitlet thinning when fruitlets were at about 20 mm. Studying the effects of various rates of ACC on late post bloom thinning in apple, McArtney (2011) found a linear dose response in fruit set of individual spurs of “GoldRush” from 50 to 200 mg·L^–1 ACC, and reported that application of 200 mg·L^–1 ACC when the fruitlet diameter was at 10 mm was a useful chemical thinning treatment for apple fruit. In that study, they suggested that concentrations between 200 and 500 mg·L^–1 should be investigated further, focusing on the 20-mm timing. McArtney’s result in “GoldRush” apple [14] generally agrees with our results in “Buckeye Gala” apples.

Application of ACC at 250 mg·L⁻¹ or higher, increased fruit weight, diameter and length, and application of ACC at 150 mg·L⁻¹ significantly increased fruit L/D ratio as compared to Untreated Control when fruitlets were sampled 34 days after application (Table 1). Application of ACC at all rates reduced total yield per tree in “Buckeye Gala” (Table 2). Our result is in agreement with McArtney [14] who found that ACC at a rate of 200 mg·L⁻¹ to be an optimum rage for late fruitlet thinning when fruitlet size was at about 10 mm in “GoldRush” apple. Our report in “Buckeye Gala” is also consistent with McArtney and Obermiller’s finding in “Gale Gala” apple [13].

Starch degradation pattern increased with every incremental increase in the ACC rate, although differences were not always significant. This effect is indirectly related to crop load, as reduced crop load on ACC-treated trees resulted in advanced maturity. Since we harvested trees of all treatments on the same day, fruit from trees thinned with ACC had advanced maturity. Even if all of the trees were hand thinned to a similar crop load, the fact that ACC reduced crop load much earlier compared to hand thinning could result in advanced fruit maturity. Application of ACC only at the rate of 350 mg·L⁻¹ significantly increased fruit size, color, and SDP as compared to Untreated Control in “Buckeye Gala” apple in 2013 (Table 2).

Based on our research, although ACC at the range of 150 to 350 mg·L⁻¹ effectively thinned “Buckeye Gala” apple, application of ACC at 350 mg·L⁻¹ provided sufficient thinning so that fruitlet size between application time and 34 days later was higher (Table 1) and fruit size at harvest (Table 2) was larger (heavier) than those from trees of Un-treated Control (Table 1 and Table 2). With an ever-increasing cost of hand thinning and labor availability challenges, application of ACC at 350 mg·L⁻¹ may be advisable for “Buckeye Gala” apple.

Application of ACC at 300 mg·L⁻¹ significantly reduced fruit set in “Sun Fuji” apple in Sunny Slope in 2013 as compared to Untreated Control (Table 3). Spray of ACC at 150 mg·L⁻¹ tended to reduce fruit set as compared to Untreated Control but the difference was not significant (Table 3). Return bloom of “Sun Fuji” apples trees receiving ACC tended to be higher than that of Untreated Control, but differences were not significant (Table 3).

^z Abbreviations: Bcm² = branch cross sectional area in cm²; return bloom was calculated as the number of half-opened mixed buds/branch cross sectional area in cm²; fruit color rating ranged from 1= up to 20% of fruit skin was red, progressively to 5 = the entire skin was red; SSC = soluble solids concentration in degree Brix; SDP = starch degradation pattern, ranging from 1 = very starchy to 6 = starch mostly hydrolyzed; Un. Control = Untreated Control. ^y Mean separation within columns by LSD at 5% levels.

Applications of ACC at 150 mg·L⁻¹ or 300 mg·L⁻¹ did not affect yield or quality attributes in 2013. The reason could be that trees in all treatments, including in Untreated Control, were hand-thinned to establish a 15-cm spacing, if fruits were not already at that spacing after chemical thinning.

Application of ACC reduced fruit set and increased fruit size in “Top Export Fuji”, although differences were not always significant (Table 4). The fruit set reduction in “Top Export Fuji” had a linear relation with the rate of applied ACC (Table 4). This observation is in agreement with Schupp et al. [15] who applied ACC at concentrations of 0, 100, 300, or 500 mg·L⁻¹ to “Golden Delicious”/ Bud.9 apple trees at 20 mm fruit diameter and found a linear dose relationship between concentration of ACC and fruit thinning.

Reduction of fruit set between trees of Untreated Control and those receiving 150 mg·L⁻¹ ACC, and between those with 300 mg·L⁻¹ and 450 mg·L⁻¹ were not significant in “Top Export Fuji” apple, but application at or greater than 300 mg·L⁻¹ ACC significantly reduced fruit set (Table 4). ACC applications in 2013 did not affect the return bloom of trees in 2014 in “Top Export Fuji” apple (Table 4), which is another excellent feature of late ACC application in “Fuji” apples.

Other than the case of fruit weight, applications of ACC did not affect yield or fruit quality attributes. The reason could be that trees in all treatments were hand-thinned at the time of hand thinning.

Application of ACC at 300 mg·L⁻¹ or greater significantly reduced fruit set in “Aztec Fuji” apple and the reduction was proportional to the dose of applied (Table 5).

General limitations in this study is the presence of a number of factors that could

^z Abbreviations: Bcm² = branch cross sectional area in cm²; return bloom was calculated as the number of half-opened mixed buds/branch cross sectional area in cm²; fruit color rating ranged from 1 = up to 20% of fruit skin was red, progressively to 5 = the entire skin was red; SSC = soluble solids concentration in degree Brix; SDP = starch degradation pattern, ranging from 1 = very starchy to 6 = starch mostly hydrolyzed; Un. Control = Untreated Control. ^y Mean separation within columns by LSD at 5% levels.

^z Un. Control = Untreated Control. ^Y Mean separation within columns by LSD at 5% levels.

collectively affect the success or failure of any fruit thinning practice in a given region. Stage of bloom, temperature, bee activity, varietal differences, wind, tree vigor, and spray volume are among factors influencing the effectiveness of blossom thinning in apples [4]. For post bloom thinning, stage of fruit development (size of fruit), carbohydrate reserves in the leaf, the rate of transportation of carbohydrates from leaf into fruit tissue, and all factors that contribute to the rate of this transportation such as temperature would affect strength of fruit attachment to the tree and thus impact the degree of fruitlet thinning [4]. In our study, the temperature fluctuations in both 2013 and 2014 were almost similar. Average minimum and maximum temperatures during months of April and May were 5.82˚C and 21.10˚C, respectively in 2013 and 5.70˚C and 21.40˚C, respectively in 2014 (Figure 1). We also found that ACC at the rate of 300 mg·L⁻¹ was an effective rate for post-harvest thinning in all three strains of “Fuji” (“Sun Fuji”, “Top Export Fuji” in 2013 and “Aztec Fuji” in 2014). Schupp at al. [15] reported that the impact of ACC on apple fruitlet thinning was consistent from year to year, while Jones and Koen [20] noted that the impact of Ethrel on fruit thinning varied between seasons, depending on the temperature. Relative independency of seasonal temperature effect is an outstanding feather for ACC. Modern apple growers over the past three decades have been spraying post bloom thinners when fruit let diameter is between 5 mm (shortly after petal fall) and 10 mm. Temperatures before, during and after post-bloom applications can have major impact on apple fruit thinning. This makes application of most available post-bloom thinners risky, as growers must take a chance by applying these thinners with little or no control on weather conditions.

Application of excessively high or low rates of post bloom thinners may result in overthinning or under thinning of apples, respectively. At the same time, preexposure of fruitlets to even a moderate freeze may result in overthinning after application of a post-bloom thinner, even at an optimum rate. Also, applications of a blossom thinner or a post bloom thinner at an early stage of fruitlet development may result in serious reduction of yield if a freeze event occurs after applications of these thinners. These risks underscore the importance of ACC application which can thin fruitlet even when their diameter is about 20 mm.

Registration has been set for ACC and will be marketed as a naturally accruing compound, marketed under the brand name Accede, utilizing the active ingredient 1-aminocy clopropane-1-carboxylic acid [17].

Late application of ACC will have the following advantages to an apple grower:

1) The risk a freeze event is minimized at a later time in the growing season when fruit-lets are at 20 mm.

2) Natural fruit abortion is less, and growers will have a better idea about their final crop, and thus can make a better hand thinning strategy.

3) As we noticed in our study, the size of fruit in trees receiving ACC at higher rates were already larger than those of Untreated Control at about the hand-thinning time, often resulting in larger fruit size at harvest.

4) In general, ACC gives growers a tool to have better predictability about their final crop load and fruit quality.