The Impact of New Blossom Thinners and Hand-thinned on Fruit Quality and Quantity in Peach cv. Alberta and Nectarine cv. Sun King

ABSTRACT

Thinning is essential and important for the quality of peach and nectarine crops because of its impact on fruit size and next season’s flower bud initiation, but hand-thinning is a costly practice and an unacceptable alternative. Researchers have been trying to introduce new thinners for almost thirty years, replacing hand-thinning. In this study, the quantitative and qualitative effects of some compounds used as thinning agents were examined on the ‘Alberta’ peaches and the ‘Sun King’ nectarines. This study has been carried out in a commercial orchard in Semirom, Isfahan, Iran during 2015–2016. Treatments included lime-sulfur (6%, 8% and double application of 6%), 10 mL L⁻¹ fish oil, ammonium thiosulfate (20, 25 mL L⁻¹and double application of 20 mL L⁻¹), the plant growth regulator Apogee (300, 450 mg L⁻¹ and double application of 300 mg L⁻¹), hand-thinning of flowers (at full bloom) and hand-thinning of fruits (at pit hardening). The temperature at the application time of treatments on peach and nectarine trees was above 15°C and the sky was sunny. Results showed that all of the treatments, except for 300 and 450 mg L⁻¹ Apogee significantly thinned the fruit to 61%. The treatments increased fruit weight (42%), volume (46%), total soluble solids (TSS) (28%), TSS/titratable acidity (TA) ratio (47%) and color (37%) as compared to control. However, there were no significant differences in TA of fruits among the samples. In terms of yield, various treatments differed significantly. The highest yield for Alberta peach was achieved by the control (123 kg/tree) and the lowest yield was obtained for the single application of 20 mL L⁻¹ ammonium thiosulfate (85.41 kg/tree). In ‘Alberta’ peaches, the biggest fruit (125 cm³) was observed with the double application of 20 mL L⁻¹ ammonium thiosulfate and fruit-thinning by 114 cm³. Double application of these chemicals often resulted in more blossom thinning than a single application. The blossoms and fruits were hand-thinned by 120 cm³ and 116 cm³, respectively. Double applications of 6% lime-sulfur and 10 mL L⁻¹ fish oil gave the largest fruit size (115 cm³) of ‘Sun King’ nectarines. Trees under fruit and blossom-thinning, ammonium thiosulfate and lime-sulfur treatments showed the good return blossom in the second year.

KEYWORDS:

• Fruit set
• yield
• fruit quality
• lime-Sulfur
• ammonium thiosulfate
• apogee

Introduction

Both peaches and nectarines originated from the same tree. The peaches whose botanical name is Prunus persica L. is native in China where it was depicted in old paintings and mentioned in Chinese mythology and folklore as far back as the 10th century BC (Janick, 2005). Peaches and nectarines are both temperate zone plants. The commercial production areas are confined to the latitudes of 30º to 40° in both northern and southern hemispheres. Peaches and nectarines have different growth requirements according to their original native districts. Hardiness, whether they are early or late blossoming, and the requirement of cold exposure in winter (chilling) are the most important factors (Childers, 1969).

The cultivation goal for fruit trees is to achieve large annual crops with good quality during the economic lifetime of the orchard. The production of a tree includes such items as the number of tree flowers, the amount of fruit set and the quality of fruit at harvest time. Childers (1969) reported that in the case of high levels of fruit production, the amount of nutrients absorbed by the roots and produced by photosynthesis are not sufficient for a large number of flowers to make high quality fruits. This creates some problems such as production of small and low-quality fruits, broken branches, reduction in tree reserves and reduction of the trees resistance to the coldness (Childers, 1969; Martin et al., 2010). Thinning is practiced only on trees carrying moderate to heavy crops of fruit. It accomplishes relatively more on mature trees making small annual growth or on trees of reduced vigor with weak leaf surfaces than on young vigorous trees of the same cultivar (Childers, 1969). Response on light soil deficient in moisture is better than heavy loams with adequate moisture (Childers, 1969). Considering commercial experience in thinning the apples for many years, it has become increasingly apparent that thinning is necessary for all small-fruited cultivars, which tend to retain an overload of fruit after the June drop (Auxt Baugher et al., 2010; Byers et al., 2004; Childers, 1969).

Fruit trees are thinned by three general methods including hand, mechanical and chemical thinning. However, due to the high cost of hand-thinning and damage caused by mechanical thinning, chemical thinning is often used (Miller et al., 2011). Nonetheless, the chemical compounds used for thinning often result in environmental damage and other consequences. Chemical thinning of apples has been one of the most important developments in controlling biennial bearing of apples. The practice has gained widespread use in commercial orchards since 1949 (Auxt Baugher et al., 2010; Byers et al., 2004; Childers, 1969).

Flower and fruit hand thinning of apples and stone fruits may be regarded as an important horticultural practice because of the effect on fruit size and next season’s flower bud initiation (Fallahi et al., 1992).The fruit thinning is performed during high fruit set to increase the fruit size (importance of fruit size economically) in plums and apricots. In some trees with smaller fruits such as almond, the number of fruiting branches is high and fruit thinning is not done commercially because trunk cross-sectional area (TCSA) is low as compared to peaches and apples (Greene and Costa, 2013). Thinning of peach tree flowers can increase fruit size as much as 7–30% (Byers and Lyons, 1985).

Pesticides and chemical compounds that destroy insects needed for pollination or disrupt the hormonal balance of the plant can lead to severe damage to the environment and reduce food safety. These harmful thinning compounds can be replaced by minerals and safer agents (Fallahi et al., 2006, 1992). Fruiting is an exhaustive process to the tree, especially if the crop is heavy. Hence, the chief goal is to permit the trees to mature and produce a large crop while conserving sufficient nutrients and carbohydrates to spur good shoot growth, leaf development, and flower-bud formation for the next year crop (Childers, 1969; Cooley, 2007; Miller and Tworkoski, 2010).

Many blossom thinners reduce fruit set by injuring and burning flower organs such as anthers, stigmas, styles and pollen tubes, thus preventing fertilization (Fallahi and Willemsen, 2002; Miller et al., 2011). Comparing ammonium thiosulfate (ATS), Wilthin (Sulfcarbamide), and Endothall (Endothalic acid) determined that ATS was the best blossom thinner at least under the conditions of the State of Washington (Warner, 1998). In that report, fruits from ATS-treated trees remained small. Blossom-thinning of sweet cherry (Prunus avium L.) with ATS and Crocker’s fish oil (CFO) is becoming extremely important. Whiting et al. (2006) reported that a double application of ATS or CFO at 10% blossom and again at 90% blossom significantly reduced the fruit set on ‘Bing’ cherry over a period of two years with respect to the weather conditions in central Washington (Whiting et al., 2006). Fallahi et al. (2006) reported that trees receiving a double application of ATS at 25 mL L⁻¹ had smaller fruits than those receiving a single application of this chemical at 15 mL L⁻¹. Also, they reported that fruits of trees receiving a single application of 30 mL L⁻¹ of ATS or a double application of 15 or 25 mL L⁻¹ had a reduced red color than the untreated control fruit. Trees that received double applications of ATS showed severe foliage burning so that the smaller fruit size and poorer color could be resulted from reduction of leaf surface/fruit ratio in some of these treatments (Fallahi et al., 2006; Miller and Tworkoski, 2010). Wertheim stated that ATS can produce the reduced fruit set in apples and pears (Wertheim, 2000). Zilkah and colleagues used different concentrations of urea for thinning the peach and nectarine trees and showed that urea caused thinning and improved fruit quality at harvest time (Zilkah et al., 1988). Research on the ability of lime-sulfur to reduce fruit set in apples and peaches has been performed (Greene and Costa, 2013; Guak et al., 2004; McArtney et al., 2006). The application of lime-sulfur at 5%, once at 75% blossom or twice at 30% and again at 75% blossom has shown lime-sulfur to be an effective thinner according to Fallahi et al. (2006). The application of fish oil and lime-sulfur in 14 days after full blossom (DAFB) reduced sweet cherry fruit set by 29% as compared to the control (Lenahan and Whiting, 2006). This reduction is about 15% to 20% less than that previously reported from two applications of the same concentration of FOLS during blossom (Stern and Ben-Arie, 2009; Whiting et al., 2006). The greatest differences between the treatments were in the soluble solids, which were about 5% greater from untreated trees, but excellent for both treatments (Lenahan and Whiting, 2006). These two chemicals have been used as organic blossom thinners. Apogee® (Prohexadione-calcium) applied to apple trees reduced vegetative shoot growth by inhibiting biosynthesis of gibberellins, which regulate cell elongation. The reduction in shoot growth can result in the reduced pruning time, improved fruit color, improved disease and insect resistance and reduced the severity of fire blight shoot infections. Application of Apogee results in more difficult thinning conditions. Thus, product rates may need to be increased where Apogee has been applied and Apogee rates are based upon tree vigor and size of tree (Byers et al., 2004; Greene, 1999; Medjdoub et al., 2005; Miller and Tworkoski, 2010; Stern and Ben-Arie, 2009). A more aggressive thinning program may be required to adequately reduce the crop load on Apogee treated trees (Cooley, 2007). These thinning strategies generally include an increased dosage of thinners or multiple applications of thinners (Greene, 2007).

The aim of this study was to evaluate the effects of inorganic and biological compounds for fruit thinning and to improve the quality and quantity characteristics of the ʻAlbertaʼ peache and ʻSun Kingʼ nectarine in the orchards.

Materials and Methods

‘Sun King’ nectarine and ‘Alberta’ peach orchards located in Semirom. Semirom city is located in Isfahan province, Iran, at 31°24′51″N latitude and 51°34′10″E longitude and 2400 m above sea level with mean annual temperature of 10.1ºC and mean annual precipitation of 400 mm. This area has been damaged by frost for some years, but it did not happen in this experiment. The temperature during the experiment was higher than 15°C and relatively warm. The sky was not cloudy and it was sunny during the days of spraying compounds. Trees grown by seedling rootstock were spaced at 3.7 × 4.5 m in orchards. At the beginning of the study, the trees were 6 years old and healthy, and had full crops. General orchard management including irrigation, nutrition, and pest and disease control was consistent with local commercial practices.

Treatments were applied to ‘Sun King’ nectarine and ‘Alberta’ peach orchards in 2015 and 2016. Treatments included; control (un-thinned), lime-sulfur (Merck. Germany) at 6 and 8%, and double application of 6%, fish oil at 10 mL L⁻¹, ammonium thiosulfate (ATS; Merck. Germany) at 20 and 25 mL L⁻¹, and double application of 20 mL L⁻¹, Apogee (BASF Corporation, Research Triangle Park. Company Hi Yield. USA) at 300 and 450 and double application of 300 mg L⁻¹, hand thinning of flowers and hand thinning of fruits.

Trees that received any blossom thinner treatments twice were sprayed once at 30% blossoms and again at 80% blossoms during both years. Trees treated by a single application of blossom thinner were sprayed at 80% of blossoms. Trees were sprayed to runoff with a hand-gun sprayer at 100 psi. The volume of liquid was 4 liter per tree at each application. Hand thinning of flower was done at full bloom as about 70%-80% of full blossom; on 14^th April for ‘Alberta’ peaches and 12^th April for ‘Sun King’ nectarines, and hand thinning of fruits was done in the pit hardening on 15^th May for ‘Alberta’ peaches and 18^th May for ‘Sun King’ nectarines. Fruits were hand-thinned to maintain a 10–15 cm space between fruits. To perform flower and fruit hand-thinned, the flowers and fruits every 10–15 cm remains along the branch and other flowers and fruits were removed. This distance was indicated by a ruler along the branch. The fruit set in this treatment was determined based on the number of fruits after hand thinning per 100 flowers.

The total number of flower buds was counted about two weeks before the blossoms (before any treatment application) on three branches of each tree during both years. Then, the number of buds before the blossoms and also the total number of fruits were counted on the tagged limbs after “June drop”. The number of fruits was calculated per 100 flowers to determine fruit set percent. During both years, fifty fruits were randomly sampled from each tree at harvest time and the average fruit weight was calculated by a 100-gm digital electronic scale (Sartious-PT120 made in Germany). To measure the volume of fruit, a five-liter graduated cylinder was selected and 2 liters of water were poured into the container and an increase in the volume of water obtained from the volume of fruit was determined. In this measurement, the volume of 10 fruits from each replicate was determined and then the average fruit volume in each treatment was calculated. Total soluble solids (TSS) were determined for each fruit sample in two replications using a digital refractometer (Bleeker-N 52436 made in the Netherlands) at 20°C and expressed as °Brix. Treatable acidity (TA) was obtained by titrating 5 ml of fruit juice with 0.1 N NaOH up to pH 8.1. The result was expressed as grams of citric acid per 100 g of fresh fruit weight. The TSS to TA ratio (ripening index) was also calculated. Fruit color was measured visually on a scale of 1 (least color) to 5 (most color); where, fruit surface covered with pink or red colors was rated visually on a scale of 1 to 5, with 1 = 20% red progressively to 5 = 100% red. Yield per tree was recorded at harvest time for each year (Fallahi et al., 1997).

The experimental design consisted of a completely randomized block design with three blocks in both years. Each block consisted of three adjacent rows with 24 trees with seven one-tree replicates per treatment (84 trees in each block) and there were 12 treatments and 7 replicates for each treatment. This study included 254 trees for each species. Means comparison was carried out by LSD method using SPSS software version 21.

Results and Discussion

Effect of Treatments on Fruit Weight and Size in ‘Alberta’ Peaches

In the ‘Alberta’ peaches, all treatments were statistically significant for all parameters except for fruit acidity. All treatment groups, except for Apogee, significantly reduced the fruit set in the ‘Alberta’ peaches as compared to control (Table 1). The treatments of fruits and blossoms were hand-thinned by 14% and 16%, respectively. The fruit set in trees treated with ATS 20 mL L⁻¹ once at 80% blossoms by 45% and all treatments of Apogee were not significant as compared to control. In the ‘Sun King’ nectarines, all treatments were significant for all parameters, except for acidity.

Table 1. Effects of blossom thinning treatments on ‘Alberta’ peach fruit set, weight, size and yield in 2015–2016^a

Treatment	Fruit Set (%)	Fruit wt (g)	Fruit size (cm^3)	Yield (kg/tree)
Control	47.43 a	109.46 f	70.58 f	123.29 a
Bloom Hand Thinned	15.65 c	188.30 a	124.57 a	100.21 c
Fruit Hand Thinned	13.88 c	166.85 bc	113.37 bc	101.31 c
L-s 6% and 10 ml.L^−1 Fish Oil once at 80% bloom	35.69 b	140. 28 de	102.09 c	109. 39 bc
L-s 8% and 10 ml.L^−1 Fish Oil once at 80% bloom	28.74 bc	158.33 cd	103.64 c	104.55 bc
L-s 6% and 10 ml.L^−1 Fish Oil at 30% and 80% bloom	20.79 c	144.47 de	109.44 c	100.25 c
ATS 20 ml.L^−1 once at 80% bloom	45.21 a	148.84 de	91.02 d	85.41 d
ATS 25 ml.L^−1 once at 80% bloom	31.74 b	156.68 cd	91.54 d	90.74 cd
ATS 20 ml.L^−1 at 30% and 80% bloom	21.67 c	174.26 ab	113.57 bc	112.85 b
Apogee 300 mg.L^−1 once at 80% bloom	51.33 a	110.60 f	80.58 e	120.52 a
Apogee 450 mg.L^−1 once at 80% bloom	55.28 a	118.17f	81.71 e	117.95 a
Apogee 300 mg.L^−1 at 30% and 80% bloom	43.58 a	134.42 e	83.72 e	118.76 a

^aMean followed by the same letter are not significantly different by the protected LSD, P ≤ 0.01.

The average fruit weight of ‘Alberta’ peaches was not significantly affected by single treatment of Apogee at 300 and 450 mg L⁻¹ (Table 1). The double treatment of Apogee at 300 mg L⁻¹ also had little impact. However, all treatments except the Apogee treatment increased fruit weight and size as compared to the control. The highest fruit weight for ‘Alberta’ peaches was found for the hand-thinned blossom treatment (188 g), followed by double application of ATS at 20 mL L⁻¹ (174 g). The lowest fruit weight (109 g) was observed in the untreated control. Trees receiving a double application of ATS at 20 mL L⁻¹ have larger (114 cm³) fruits than those receiving once application (91 cm³). In addition, fruits of trees receiving a double application of 6% lime-sulfur were larger (109 cm³) than those receiving only a single application (102 cm³). The biggest fruits were those in which the blossoms were hand-thinned (125 cm³), followed by those treated with ATS at 20 mL L⁻¹ or in which the fruit was hand-thinned (113 cm³). Apogee treatments and the untreated control produced the smallest “Alberta” peaches (71cm³). Peach trees treated with lime-sulfur, ATS and hand-thinned had lower yield as compared to control trees, because, fruit size, weight and quality was increased to compensate for the reduced fruit numbers.

Effect of Treatments on Fruit Weight and Size in ‘Sun King’ Nectarines

In the ‘Sun King’ nectarines, all treatments were significant for all parameters, except for acidity. All the treatments, except for the Apogee treatments at 300, 450 mg L⁻¹, reduced the fruit set as compared to the control (Table 4). The ‘Sun King’ nectarines, which were blooms and fruits hand thinned showed the lowest levels of fruit set with yields of 13% and 19% as compared to the control, respectively. The ‘Sun King’ nectarines hand-thinning resulted in the lowest fruit set and the largest (176 g) fruit size in comparison with other treatments (Table 4). Double application of 6% lime-sulfur and 10 mL L⁻¹ fish oil showed the next highest (168 gr) fruit weight. Compared with the control, the weight and size of ‘Sun King’ nectarines were not affected by double application of ATS at 20 mL L⁻¹, which resulted in severe foliar injury and over thinning. Hand-thinning of blossoms and fruits gave mature fruits of 120 cm³ and 116 cm³, respectively. Double application of 6% lime-sulfur and 10 mL L⁻¹ fish oil produced ‘Sun King’ nectarines with a fruit size of 115 cm³ indicating to have the highest fruit size. The effects were proportional to the concentration of this chemical matter. Additionally, the fruit size increased proportionally to the amount of lime-sulfur application on the nectarines.

Table 2. Effects of blossom thinning treatments on ‘Alberta’ peach yield in 2015 and 2016^a

Treatment	2015	2016
Control	131.7 a	114.88 a
Bloom Hand Thinned	105.49 c	94.93 b
Fruit Hand Thinned	104.95 c	97.67 b
L-s 6% and 10 ml.L^−1 Fish Oil once at 80% bloom	119.05 b	99.73 b
L-s 8% and 10 ml.L^−1 Fish Oil once at 80% bloom	111.56 b	97.54 b
L-s 6% and 10 ml.L^−1 Fish Oil at 30% and 80% bloom	102.77 c	97.73 b
ATS 20 ml.L^−1 once at 80% bloom	92.01 d	78.81 c
ATS 25 ml.L^−1 once at 80% bloom	94.85 cd	86.63 bc
ATS 20 ml.L^−1 at 30% and 80% bloom	121.07 ab	104.63 ab
Apogee 300 mg.L^−1 once at 80% bloom	126.02 a	115.02 a
Apogee 450 mg.L^−1 once at 80% bloom	127.12 a	108.78 a
Apogee 300 mg.L^−1 at 30% and 80% bloom	127.39 a	110.13 a

^aMean followed by the same letter are not significantly different by the protected LSD, P ≤ 0.01.

Table 3. Effects of blossom thinning treatments on ‘Alberta’ peach soluble solids, total acidity, TSS/TA and fruit color in 2015–2016^a

Treatment	Soluble Solids (%)	Total Acidity (mg/100ml)	TSS/TA	Fruit Color (1–5)
Control	10.33 b	4.56 a	2.26 b	2.99 b
Bloom Hand Thinned	14.23 a	3.39 a	4.19 a	4.77 a
Fruit Hand Thinned	13.84 a	3.59 a	3.85 a	4.12 a
L-s 6% and 10 ml.L^−1 Fish Oil once at 80% bloom	11.38 b	4.44 a	2.56 b	3.09 b
L-s 8% and 10 ml.L^−1 Fish Oil once at 80% bloom	13.89 a	3.57 a	3.89 a	4.01 ab
L-s 6% and 10 ml.L^−1 Fish Oil at 30% and 80% bloom	13.03 a	3.76 a	3.47 a	4.33 a
ATS 20 ml.L^−1 once at 80% bloom	10.29 b	3.65 a	2.82 ab	4.13 a
ATS 25 ml.L^−1 once at 80% bloom	12.84 ab	4.42 a	2.90 ab	4.25 a
ATS 20 ml.L^−1 at 30% and 80% bloom	13.99 a	3.31 a	4.22 a	3.27 a
Apogee 300 mg.L^−1 once at 80% bloom	10.37 b	4.53 a	2.29 b	3.17 b
Apogee 450 mg.L^−1 once at 80% bloom	11.21 b	4.49 a	2.49 b	3.05 b
Apogee 300 mg.L^−1 at 30% and 80% bloom	11.66 b	3.51 a	3.32 b	3.56 ab

^aMean followed by the same letter are not significantly different by the protected LSD, P ≤ 0.01.

Table 4. Effects of blossom thinning treatments on ‘Sun King’ nectarine fruit set, weight, size and yield in 2015–2016^a

Treatment	Fruit Set (%)	Fruit wt (g)	Fruit size (cm^3)	Yield (kg/tree)
Control	44.83 a	110.55 d	76.31 d	118.28 a
Bloom Hand Thinned	12.63 c	175.61 a	120.49 a	113.79 a
Fruit Hand Thinned	18.72 c	166.05 b	116.41 ab	111.07 ab
L-s 6% and 10 ml.L^−1 Fish Oil once at 80% bloom	42.01 ab	146.76 c	103.64 bc	99.92 ab
L-s 8% and 10 ml.L^−1 Fish Oil once at 80% bloom	31.16 b	159.03 b	109.09 b	103.55 ab
L-s 6% and 10 ml.L^−1 Fish Oil at 30% and 80% bloom	22.39 c	168.09 ab	114.84 ab	106.99 ab
ATS 20 ml.L^−1 once at 80% bloom	38.06 b	141.16 c	98.88 bc	93.68 b
ATS 25 ml.L^−1 once at 80% bloom	29.36 b	156.61 bc	102.02 b	97.39 ab
ATS 20 ml.L^−1 at 30% and 80% bloom	19.13 c	157.74 bc	107.16 b	100.26 ab
Apogee 300 mg.L^−1 once at 80% bloom	46.96 a	108.77 d	75.08 d	113.52 a
Apogee 450 mg.L^−1 once at 80% bloom	51.43 a	108.12 d	70.63 d	110.95 ab
Apogee 300 mg.L^−1 at 30% and 80% bloom	46.02 a	111.58 d	79.24 d	117.76 a

^aMean followed by the same letter are not significantly different by the protected LSD, P ≤ 0.01.

Effect of Treatments on Changes in the Fruit Weight and Size of ‘Sun King’ Nectarines

The control and Apogee-treated nectarine groups resulted in smaller fruits throughout the season (Figure 1). The changes in fruit growth before the pit hardening stage in the hand thinning treatment were similar to those in the control group. After thinning, the fruit development changes greatly (Figures 1 & 2). Flower hand-thinning produced the highest fruit size in both peaches and nectarines. In the nectarines, flower thinning gave fruits of 120 cm³, double application of 6% lime-sulfur of 115 cm³ and fruit thinning of 116 cm³. In Alberta peaches, the largest fruit after flower thinning (125 cm³) was achieved by double application of 20 mL L⁻¹ ATS (114 cm³) and fruit hand thinning (113 cm³) (Table 1 and Figure 2). In the case of both peaches and nectarines, it was found that double application (at 30 and 80% blossoms) of thinning chemicals were better than single ones (Tables 1 and 3). Apogee had the least effect on weight and size, but twice applications were better than once.

Figure 1. Change in fruit size of nectarine “Sun King” for various treatments as a function of time in 2015–2016

Figure 2. Change in fruit weight of nectarine “Sun King” for various treatments as a function of time in 2015–2016

Effect of Treatments on Changes in the Fruit Weight and Size of ‘Alberta’ Peaches

The results showed that in both species, flower thinning greatly affected the fruit weight. Flower thinning of “Alberta” peaches yielded fruits weighing 188 g whereas double application of 20 mL L⁻¹ ATS and fruit thinning gave weights of 174 g and 167 g, respectively (Figure 3 and Table 1). The control and Apogee treatments resulted in the lowest fruit weight. In both fruits, a double application of thinning compounds was better than a single application (Figures 3 and 4).

Figure 3. Change in fruit size of peach “Alberta” for various treatments as a function of time in 2015–2016

Figure 4. Change in fruit weight of peach “Alberta” for various treatments as a function of time in 2015–2016

Increase in fruit weight may be due to the reduction in the number of fruits per tree. Because after the thinning operation, the leaf to fruit ratio increased resulting in more photosynthesis and less nutritional competition among the developing fruits, resulting in the improved fruit weight. These results are similar to the findings of Fallahi and Willemsen (2002), Meiteil et al. (2013) and Auxt Baugher et al. (2010). Recently the price for small and medium-sized fruits on international markets has remained constant or declined. The most pronounced effect of thinning is an increase in fruit size, largely as a result of allotting more leaf surface for each fruit. Experimental evidence has shown that good size and quality can be obtained when fruits of most varieties are spaced to allow about 30 to 40 average size leaves in the vicinity of each fruit on standard-size trees (Cooley, 2007; Greene and Costa, 2013). If there are 50 or more leaves per fruit, there appears to be minimal increased size and quality of the fruit. The usual commercial practice of spacing the fruits from 15 to 20 cm apart on the branch allows about 30 average-size leaves per fruit for most cultivars (Byers et al., 2004; Childers, 1969; Whiting et al., 2006).

Effect of Treatments on Yield of ‘Alberta’ Peach and ‘Sun King’ Nectarines

All treatments resulted in significantly reduced total yields with the exception of the Apogee treatments (Table 1). The highest yield for ‘Alberta’ peaches was in control (123 kg/tree) and lowest of yield was for a single application of 20 mL L⁻¹ ATS (85.41 kg/tree). Trees treated with Apogee had lower yields as compared to the control trees since the fruit set was not reduced and the leaf/fruit ratio was not increased to compensate for the fruit size and weight. In the ‘Sun King’ nectarines, foliage injury symptoms disappeared after 2–3 weeks. Trees treated with Apogee had the same yield as the control trees presumably because Apogee reduced gibberellin production and reduced the leaf/fruit ratio. However, the use of Apogee did not reduce the fruit set. The treatments of lime-sulfur and ATS except for 20 mL L⁻¹ ATS were significant compared to the control (Table 4). Peach trees treated with lime-sulfur, ATS and hand-thinned had lower yield as compared to control trees, because, fruit size, weight and quality was increased to compensate for the reduced fruit numbers (Tables 2 and 5).

Table 5. Effects of blossom thinning treatments on ‘Sun King’ nectarine yield in 2015 and 2016^a

Treatment	2015	2016
Control	127.49 a	109.07 a
Bloom Hand Thinned	119.74 ab	107.84 a
Fruit Hand Thinned	118.69 ab	103.45 ab
L-s 6% and 10 ml.L^−1 Fish Oil once at 80% bloom	104.8 c	93.04 bc
L-s 8% and 10 ml.L^−1 Fish Oil once at 80% bloom	112.19 b	94.91 bc
L-s 6% and 10 ml.L^−1 Fish Oil at 30% and 80% bloom	113.19 b	100.79 b
ATS 20 ml.L^−1 once at 80% bloom	96.78 c	90.58 c
ATS 25 ml.L^−1 once at 80% bloom	102.53 c	92.25 c
ATS 20 ml.L^−1 at 30% and 80% bloom	108.69 bc	91.83 c
Apogee 300 mg.L^−1 once at 80% bloom	120.45 a	106.59 a
Apogee 450 mg.L^−1 once at 80% bloom	118.06 ab	103.84 ab
Apogee 300 mg.L^−1 at 30% and 80% bloom	125.89 a	109.63 a

^aMean followed by the same letter are not significantly different by the protected LSD, P ≤ 0.01.

The thinning results of fruit yields were variable, but the majority of the experiments indicated that thinning by standard methods does not or only slightly reduces the yield (Childers, 1969; Greene and Costa, 2013; Stern and Ben-Arie, 2009). This is more or less to be expected since the practice was designed to decrease the competition between fruits so that the remaining fruits will derive the benefits of increased water and food materials. The effect on yield depends upon the extent of reduction of the crop by thinning (Childers, 1969; Greene, 2007; Meiteil et al., 2013; Whiting et al., 2006).

Effect of Treatments on Fruit TSS, TA and Color of ‘Alberta’ Peaches and ‘Sun King’ Nectarines

For the ‘Alberta’ peaches, double applications of 20 mL L⁻¹ ATS caused foliage burning (data not shown) and resulted in lower fruit color than the control (Table 3). Hand-thinning, double application of 20 mL L⁻¹ ATS and 8% lime-sulfur significantly increased total soluble solids (TSS) to 14% in all cases compared to the control value of 10% (Table 3). The lowest TSS amount (10%) was seen for the 20 mL L⁻¹ ATS treatment. Compared to the control, the total acidity of ‘Alberta’ peaches was not greatly affected by any treatments. However, fruits that were hand-thinned and had a double application of 20 mL L⁻¹ ATS showed a slight, but insignificant reduction in the total acidity. In the ‘Sun King’ nectarines, all treatments, except for Apogee 300 and 450 mg L⁻¹ treatments increased the total soluble solids relative to the control. The highest (15%) TSS was seen for the 8% lime-sulfur whereas the lowest (8%) TSS was seen for 450 mg L⁻¹ Apogee (Table 6).

Table 6. Effects of blossom thinning treatments on ‘Sun King’ nectarine soluble solids, total acidity, TSS/TA and Fruit Color in 2015–2016^a

Treatment	Soluble Solids (%)	Total Acidity (mg/100ml)	TSS/TA	Fruit Color (1–5)
Control	9.17 b	5.02 a	1.83 b	3.01 b
Bloom Hand Thinned	13.71 a	4.29 a	3.19 a	4.27 a
Fruit Hand Thinned	14.68 a	3.99 a	3.68 a	4.03 a
L-s 6% and 10 ml.L^−1 Fish Oil once at 80% bloom	10.52 ab	5.11 a	2.06 ab	2.87 b
L-s 8% and 10 ml.L^−1 Fish Oil once at 80% bloom	14.99 a	5.09 a	2.94 a	3.09 b
L-s 6% and 10 ml.L^−1 Fish Oil at 30% and 80% bloom	13.03 a	4.26 a	3.06 a	3.77 ab
ATS 20 ml.L^−1 once at 80% bloom	8.98 b	3.87 a	2.32 ab	3.12 b
ATS 25 ml.L^−1 once at 80% bloom	13.62 a	4.69 a	2.90 a	3.01 b
ATS 20 ml.L^−1 at 30% and 80% bloom	13.04 a	5.46 a	2.39 ab	3.77 ab
Apogee 300 mg.L^−1 once at 80% bloom	8.85 b	5.34 a	1.66 b	3.55 b
Apogee 450 mg.L^−1 once at 80% bloom	8.44 b	4.91 a	1.72 b	3.35 b
Apogee 300 mg.L^−1 at 30% and 80% bloom	10.53 ab	4.08 a	2.58 ab	3.76 ab

^aMean followed by the same letter are not significantly different by the protected LSD, P ≤ 0.01.

With ripening, the starch in the fruit is converted to simple sugars, and a simultaneous decrease in organic acids and acidity is observed (Biale and Young, 1981;Nuncio-Jáuregui  et al., 2014). When a fruit is accompanied by adequate and efficient leaf surface and is well exposed to light, it is better supplied with carbohydrates and other materials which are needed for flavor and TSS/TA. This is largely due to an increase in the sugar, sucrose (Greene, 2007; Miller and Tworkoski, 2010). Similar increases in TSS content upon GA3 treatment postanthesis have been reported for Japanese plums (González-Rossia et al., 2006), peaches and nectarines (Coneva and Cline, 2006; Erogul and Sen, 2015; González-Rossia et al., 2007) and cherries (Lenahan and Whiting, 2006). As for the acidity of the fruit, different doses of metamitron in peach tree thinning (de Farias et al., 2017) and of ethephon in plum tree thinning (Pavanello and Ayub, 2012) did not lead to any difference in fruit pH. According to Alcobendas et al. (2013), pH of peach juice may be related to the location of the fruit in the plant.

All treatments, except for 300 and 450 mg L⁻¹ Apogee and 6% and 8% lime-sulfur treatments, significantly increased fruit color for ‘Alberta’ peaches as compared to the control (Table 3). But in the ‘Sun King’ nectarines, total acidity and fruit color were little affected by the treatments (Table 6).

The best results were observed with hand-thinning of blossoms, which resulted in 16% fruit set, 188 g fruit weight, 125 cm³ fruit size and 100 kg/tree total yield for ‘Alberta’ peaches. Treatment by double application of 20 mL L⁻¹ ATS gave 22% fruit set, 174 g fruit weight and 114 cm³ fruit size for peaches. In the case of ‘Sun King’ nectarines, hand-thinning of blossoms again was the best thinning method, giving 13% fruit set, 176 g fruit weight, 120 cm³, 114 kg/tree yield and 14% TSS. Lime-sulfur at all concentration and 25 mL L⁻¹ ATS as well as double application of 20 mL L⁻¹ ATS gave good results for the ‘Sun King’ nectarines. Apogee is one of the fruit and blossom thinners for stone fruits but in the conditions of this study did not effectively thin blossoms. Apogee proved to be relatively ineffective in producing quality peaches and nectarines in this study. The control trees with excessively heavy crop loads did not have good return blossoms but in trees under fruit and blossom-thinning, ATS and lime-sulfur treatments showed the good return blossoms in the second year.

Fruit thinning tends to increase the amount and intensity of red and yellow color of apples and stone fruits. C. G. Brown in Oregon Circular 76 showed that more highly colored apples were obtained when the fruits were spaced 15 cm or more apart. This is important for apple cultivars such as the “delicious” which needs considerable red color to fall into higher grades. The general effect of thinning on color, however, is usually not as striking as its effect on size. Limb spreading helps to expose fruit to the sun for better coloring. Also, fruit on dwarfed trees receives better light than large trees, resulting in better colors. The red color in fruits is due to production of the antioxidant anthocyanin. Anthocyanin production differs between peach and nectarine. Fruits anthocyanin synthesis is promoted by high light levels and low temperatures, and will increase toward maturation (Childers, 1969; Meiteil et al., 2013; Whiting et al., 2006).

The fruit size and weight were increased by hand-thinning treatments because the selection of fruits and flowers in the hand-thinned trees was more regular than other treatments, resulting in a faster increase in fruit size and weight. Thinning in the early stage led to more rapid growth of the fruit cells. Fruit to fruit competition in the hand-thinned trees was reduced several weeks before the control trees were thinned. Thinning is the removal of a part of the yield before it matures on the tree with the object of (1) increasing the marketability of the remaining fruit, and (2) reducing the alternate bearing tendency of the tree (Fallahi et al., 2006; Meiteil et al., 2013; Miller and Tworkoski, 2010; Stern and Ben-Arie, 2009). Manual thinning aims at selecting which fruits must be removed from the plant, i. e., damaged, small and badly located fruits are eliminated. Although this practice brings many benefits to peach cultures, it requires much man power, must be carried out in a short period and has high production costs (McArtney et al., 2012; Simões et al., 2013). Hand thinning treatment produces larger and higher quality fruits than other treatments because thinning is done very carefully. However, this treatment requires a large amount of time and labor costs, which were not measured in this study. In general, the use of any of the thinning compounds is less expensive than hand-thinning because hand-thinning has a very high labor cost (Childers, 1969; Fallahi et al., 2006).

Conclusions

Treatments reduced fruit set, and thus reduced the needs for hand thinning and labor cost (data not shown) and this is economically important for peach and nectarine industry. Under conditions of this study, ATS (especially in twice application) and lime-sulfur were effective blossom thinners for ‘Alberta’ peaches and ‘sun king’ nectarines when applied at full blossoms. More research is necessary to understand the action mode of thinning and to develop sound blossom thinning recommendations. This study showed the potential for a promising new strategy for thinning peach and nectarine blossoms. More research is necessary to understand the action mode of thinning and to develop sound blossom thinning recommendations.

Flower and fruit-thinning is one of the most effective techniques in peaches and nectarines growth because such thinning increases qualities such as size and color of the fruit, making them more marketable. In addition, the taste of the fruit, determined by sugar and acid content, will be increased. The potential of improving fruit quality is limited because fruit storability will be impaired when fruit-thinning is performed too heavily. This study demonstrated that ATS and lime-sulfur were effective blossom thinners for stone fruits.