Effect of Kaolin Clay on Post-Bloom Thinning Efficacy, Cropping, and Fruit Quality in ‘Gala Vill’ Apple (Malus × domestica) Cultivation

Abstract

Effective thinning methods that balance yield, fruit quality, and ripening dynamics are essential to ensure efficient and sustainable apple production. This study examined the effects of various thinning treatments on ‘Gala Vill’ apples (Malus × domestica Borkh.) to assess their impacts on fruit set, yield, and quality parameters. The experiment was conducted in 2020 at the experimental orchard of WULS located in Wilanów, Warsaw, Poland. The treatments included chemical thinning using different doses of metamitron (Brevis 150 SG) alone and in combination with kaolin clay at two concentrations (50 and 100 kg∙ha⁻¹), and, as alternatives to the chemical method, using kaolin clay alone (50, 100, and 200 kg∙ha⁻¹) and artificial shading. The results highlight the effectiveness of thinning treatments in modulating key agronomic traits. Artificial shading significantly reduced the number of fruitlets, demonstrating its utility as a non-chemical thinning option. Metamitron application effectively reduced the number of fruitlets in a dose-dependent manner. Combining metamitron with kaolin clay did not enhance the thinning effect compared with metamitron alone. However, kaolin clay applied independently, particularly at higher concentrations, was associated with improved fruit setting and yield. For instance, kaolin clay at 200 kg∙ha⁻¹ (KC200) resulted in the highest fruit set (85.2) and yield (10.1 kg·tree⁻¹), suggesting its adverse effect on thinning to promote fruit retention under certain conditions.

1. Introduction

Apple fruitlet thinning is a key agronomic practice in apple cultivation, significantly influencing both the yield quality and quantity [1]. This process involves reducing the number of flowers or fruitlets on trees to ensure optimal conditions for the growth of the remaining fruits and to maintain a balance between fruiting and the trees’ health. The primary aim of thinning is to improve fruit quality parameters, such as size, color, and sugar content, while preventing undesirable alternate bearing [2,3,4]. Excess flowers or fruitlets can be removed by hand, chemically, or mechanically. Chemical thinning remains the primary approach for early thinning due to the high costs of manual thinning, the limited availability of mechanical equipment, and the lack of selectivity in mechanical methods [5,6,7,8].

The effectiveness of chemical thinning depends on various factors. The level of thinning and its impact on flowering in the subsequent seasons are influenced by the apple variety, rootstock, tree age, growth vigor, health status, flowering intensity, pollination success, and weather conditions before, during, and after the treatment [9,10,11]. In apple cultivation, thinning during the flowering stage is achieved mainly by preventing fertilization by blocking pollen germination. Desiccants are commonly used, targeting the highly hydrated pistil and its stigma and causing them to dry out. This treatment limits the deposition, germination, and fertilization of pollen [12,13,14,15]. Research indicates that desiccants, apart from their direct physical effects, may also influence physiological processes, such as increasing ethylene production in damaged flowers, which intensifies plant stress and enhances thinning efficiency [16,17].

The use of ATS (ammonium thiosulphate) or S-Ca (calcium polysulfide) often reduces photosynthesis efficiency, with calcium polysulfide potentially lowering photosynthesis by up to 50% for several weeks [18]. A similar effect on trees’ energy balance is observed with metamitron, a triazine-based compound used in horticulture for weed control. Metamitron disrupts photosynthesis by blocking electron transport in PSII, reducing the production of ATP and NADPH, which are essential for carbohydrate synthesis. This leads to plant stress caused by a negative energy balance, which results in the abscission of excessive fruitlets. Low light interception enhances its thinning effect by further decreasing carbohydrate availability, which promotes fruitlet abscission. High temperatures, on the other hand, can accelerate the plant’s metabolic processes, potentially intensifying metamitron’s impact, but may also reduce its persistence due to faster degradation [11]. The effects of metamitron are comparable to those of plant shading, whose beneficial impact on thinning and treatment efficiency has been confirmed in numerous studies [19,20,21,22]. Some researchers suggest that artificial shading may be an intriguing alternative to chemical thinning [23]. However, the practical application of this method is challenging due to its high material and labor costs [24].

For this reason, methods that achieve similar effects to shading while minimizing its drawbacks are worth exploring. One potentially effective method is covering trees with kaolin clay, a natural material belonging to the group of clay minerals that is widely used in apple and other fruit production [25,26,27]. Thanks to its light-reflecting properties, kaolin clay spraying helps mitigate stress caused by excessive sunlight and decreases losses from sunburn on developing fruitlets [28,29]. Additionally, kaolin has been shown to improve fruit quality in terms of better coloration and solid soluble content and promote better growth [25]. Typical doses range from 2 to 5% in solution, depending on environmental conditions and tree size. Applications are usually made 3–5 times during the growing season at 7–14-day intervals, starting before the onset of high temperatures [25,28,29]. Meanwhile, several studies have shown that kaolin clay application can reduce the leaf photosynthetic rate by decreasing the amount of light reaching the photosynthetic apparatus due to the improved reflective properties of the leaf surface and decreased energy absorption [30,31].

In our study, we hypothesized that applying high doses of kaolin clay by spraying trees after flowering will result in the formation of a light-reflecting layer on leaves, producing a similar effect to artificial shading or metamitron application, which reduces photosynthetic efficiency and leads to fruitlet thinning. The potential additive effect of combining kaolin clay with metamitron was also tested. The expected result was enhancing fruitlet abscission via dual mechanisms: kaolin clay reduces light interception by reflecting solar radiation as a physical barrier, while metamitron acts as a physiological photosynthetic inhibitor. This combined approach could optimize thinning efficacy and reduce the use of synthetic agents in fruit production. Providing new potential approaches for apple thinning strategies is of a great importance for improving fruit quality, ensuring consistent yields, and meeting the growing demand for more sustainable orchard management practices. Exploring alternative methods such as kaolin clay application offers a promising tool to reduce dependence on synthetic chemical agents while maintaining thinning efficiency and minimizing environmental impact.

2. Materials and Methods

2.1. Location, Plant Material, and Experimental Design

The experiment was conducted in the experimental orchard of Warsaw University of Life Sciences (WULS) in Wilanów, Warsaw, Poland (N 52°9′36.1″, E 21°5′58.2) in the growing season of 2020. The weather data on the plot were gathered using the Davis Vantage Pro weather station installed in the experimental plot and are presented in Figure 1.

The plant material consisted of apple (Malus × domestica Borkh.) trees of the ‘Gala Vill’ cultivar that were planted in the spring of 2016 on silty loam alluvial soil with 3.2 × 0.8 m spacing between them. The trees were trained as a standard spindle bush canopy. The floor management system in the experimental block maintained herbicide fallow within the rows of trees and turf grass in the inter-rows. Anti-hail nets were used to protect the fruits and trees against damage during hail storms. No irrigation was used on the experimental plots during the experiment.

Other agrotechnical treatments were applied following the local standards for the integrated production strategy and were the same for all trees in the experimental plot, excluding the thinning strategy.

During the trial, 10 different thinning treatments were tested as follows:

(1)

The control (control), where no thinning treatments were applied;

(2)

Artificial shading (AS), where a black agro-textile (density: 50 g∙m⁻²), commonly used for mulching the soil to avoid weed competition, was applied over the trees for 5 days at a 6–8 mm fruitlet size (Figure 2a);

(3)

Brevis 2.2 (B2), where single applications of Brevis (containing metamitron as an active ingredient) at a dose of 2.2 kg∙ha⁻¹ were applied at a 6–8 mm fruitlet size;

(4)

Kaolin Clay 50 (KC50), where a single application of kaolin clay was applied at a 50 kg∙ha⁻¹ dose at a 6–8 mm fruitlet size (Figure 2b,c);

(5)

Kaolin Clay 100 (KC100), where a single application of kaolin clay was applied at a 100 kg∙ha⁻¹ dose at a 6–8 mm fruitlet size (Figure 2d);

(6)

Kaolin Clay 200 (KC200), where a single application of kaolin clay was applied at a 200 kg∙ha⁻¹ dose at a 6–8 mm fruitlet size (Figure 2e);

(7)

Brevis 1.1 + Kaolin Clay 50 (B1 + KC50), where a single application of Brevis at a 1.1 kg∙ha⁻¹ dose was applied, followed by kaolin clay application at a 50 kg∙ha⁻¹ dose at a 6–8 mm fruitlet size;

(8)

Brevis 1.1 + Kaolin Clay 100 (B1 + KC100), where a single application of Brevis at a 1.1 kg∙ha⁻¹ dose was applied, followed by kaolin clay application at a 100 kg∙ha⁻¹ dose at a 6–8 mm fruitlet size;

(9)

Brevis 2.2 + Kaolin Clay 50 (B2 + KC50), where a single application of Brevis at a 2.2 kg∙ha⁻¹ dose was applied, followed by kaolin clay application at a 50 kg∙ha⁻¹ dose at a 6–8 mm fruitlet size;

(10)

Brevis 2.2 + Kaolin Clay 100 (B2 + KC100), where a single application of Brevis at a 2.2 kg∙ha⁻¹ dose was applied, followed by kaolin clay application at a 100 kg∙ha⁻¹ dose at a 6–8 mm fruitlet size.

The spraying of trees was carried out using a Stihl 430 motor sprayer (Andreas Stihl AG & Co., Waiblingen, Germany), applying the working liquid at a rate of 1000 L·ha⁻¹. All treatments were performed in three replications, consisting of 8 experimental trees in each (24 trees per treatment). Four buffer trees separated the plots. A complete randomized block design was used to perform the trial. Its layout is presented in Figure 3.

2.2. Data Collection and Analysis

2.2.1. Flowering, Fruit Setting, and Yield

The number of inflorescences per tree was recorded pre-flowering and is given in pieces per tree. The fruitlets post June drop were counted, and the setting rate was expressed as fruitlets per 100 inflorescences. The tree size was assessed as a trunk cross-sectional area (TCSA) for each plot. This parameter was calculated as a function of the tree trunk diameter measured with a hand caliper at a height of 30 cm above the soil level in a permanently marked spot. Trunk diameter measurements were performed twice during the trial in the beginning and at the end of the vegetation season. The results are presented in cm².

Fruits were harvested at the beginning of September according to a starch index test made for apples grown in a control plot. The yield was assessed during the harvest by weighing apples picked from each separate tree in a plot. The blooming efficiency index (BEI) was calculated and given in pcs∙cm⁻² based on the number of inflorescences per tree and the TCSA measured at the beginning of the season. The cropping efficiency index (CEI) was computed as a quotient of the fruit mass per tree, and the TCSA was measured at the end of the season. These values are presented in kg∙cm⁻².

2.2.2. Physiological Status and Inner and Outer Fruit Quality Determined Directly After Harvest

All physiological and quality fruit parameters, except for the mean fruit mass, were determined in a representative sample of ten apples from each replication.

The starch index was measured in the cross-sections of apples by dipping their surface in an iodine solution in potassium iodide. The color reaction of the fruit flesh was assessed from 1 to 10 according to standard tables of starch degradation.

The internal ethylene concentration (IEC) was measured using a gas chromatograph (Clarus 690, PerkinElmer, Inc., Waltham, MA, USA). A sample of air containing 1 cm³ was taken from the seed socket for analysis using a syringe. The results are expressed in µL·L⁻¹.

The fruit firmness was measured using an Instron 5540 penetrometer (Instron, Norwood, MA, USA). A narrow strip of skin was removed from both the blush side and the opposite side of the fruit to expose the flesh. The measurement was performed in the peeled areas directly in the flesh using a probe with a diameter of 11 mm, penetrating the flesh to a depth of 10 mm. The measurement results are expressed in Newtons (N).

The fruits’ soluble solid content and acidity were determined in the juice extracted from the tested fruit using a juicer for each replication. The soluble solid content (SSC) was measured directly in the juice using an ATAGO PAL-1 (Atago Co. Ltd., Tokyo, Japan) refractometer and expressed in °Brix.

For titratable acidity (TA) measurements, the apple juice was diluted with distilled water in a 1:10 ratio, and the resulting solution was titrated with 0.1 M NaOH to a pH of 8.1 using a semi-automated titrator (TitroLine 5000, Xylem Analytics Germany GmbH, Weilheim, Germany). The TA was calculated and expressed as a percentage of malic acid based on the amount of NaOH used for each sample. Additionally, the SSC–TA ratio was calculated.

Considering the flesh firmness, SSC, and starch index value, the Streif index was calculated using the following formula:

SI = F/(R × S)

where SI denotes the Streif index, F denotes the firmness (expressed in kG), R denotes the soluble solid content (°Brix), and S denotes the starch test result (scale: 1–10).

The apple skin coloration was measured with a Minolta CR-400 colorimeter (Konica Minolta Inc., Tokyo, Japan) and presented in a CIE Lab system as the values of three parameters: ‘L’ (lightness), ‘a’ (red–green axis), and ‘b’ (yellow–blue). Additionally, the hue and chroma values were computed.

The mean fruit mass was measured in a sample of 50 apples per replication using a simple scale and was given in grams.

2.3. Statistical Analysis of Data

One-way ANOVA was used to analyze the data gathered during the trial. The normality of the data distribution of results was verified using the Shapiro–Wilk test. The means were separated according to the Newman–Keuls post hoc test at a significance level of p ≤ 0.05. All tests were performed with the Statistica 13.3 software package (Statsoft Inc., Tulsa, OK, USA).

3. Results

The results presented in

Table 1. Blooming, fruit setting, and cropping of the tested apple trees depending on the treatment applied during the trial.

Treatment	No. of Flower Clusters [pcs·Tree−1]	Blooming Efficiency Index (BEI) [pcs·cm−2]	No. of Fruits [pcs·Tree−1]	Fruit Set [pcs·100−1 Flower Buds]	Yield [kg·Tree−1]	Cropping Efficiency Index (CEI) [kg·cm−2]
Control	89.6 a 1	11.9 a	66.3 cd	74.6 cd	6.35 d	0.59 de
AS	88.2 a	8.80 a	7.06 a	7.92 a	1.17 a	0.10 a
B2	90.0 a	12.0 a	10.7 a	11.9 a	2.71 ab	0.29 abc
KC50	92.5 a	11.4 a	57.4 c	63.1 c	6.67 d	0.60 de
KC100	87.2 a	9.72 a	58.7 c	67.6 c	6.93 d	0.65 e
KC200	88.1 a	9.20 a	75.4 cd	85.2 cd	10.1 e	0.85 f
B1 + KC50	91.1 a	9.51 a	28.5 b	31.2 b	4.79 cd	0.40 bcde
B1 + KC100	98.0 a	10.0 a	33.8 b	34.6 b	6.21 d	0.52 cde
B2 + KC50	94.8 a	12.1 a	18.5 ab	19.9 ab	2.62 ab	0.26 ab
B2 + KC100	88.9 a	10.1 a	18.3 ab	20.7 ab	3.93 bc	0.35 bcd

¹ Note: Means followed by the same letter in a column do not differ statistically according to the Newman–Keuls post hoc test at p ≤ 0.05.

demonstrate that the treatment applied to the trees had a significant impact on their flowering, fruiting, and yield characteristics.

The number of flower clusters per tree did not significantly vary across treatments, with all values falling in a similar range. The highest number was observed for the B1 + KC100 treatment (98.0 pcs·tree⁻¹), followed closely by B2 + KC50 (94.8 pcs·tree⁻¹) and KC50 (92.5 pcs·tree⁻¹). The control group recorded 89.6 pcs·tree⁻¹, comparable to the other treatments. The blooming efficiency index (BEI) values also showed no statistically significant variation among treatments. The highest BEI value was recorded for B2 + KC50 (12.1 pcs·cm⁻²), which was similar to the values observed for B2 (12.0 pcs·cm⁻²) and the control group (11.9 pcs·cm⁻²). The KC100, KC200, and B1 + KC50 treatments exhibited slightly lower BEI values, but the differences were not significant.

The number of fruits per tree significantly varied between treatments. The KC200 treatment resulted in the highest number of fruits per tree (75.4 pcs·tree⁻¹), followed by the control group (66.3 pcs·tree⁻¹) and KC50 (57.4 pcs·tree⁻¹). Lower values were noted for B1 + KC50 (28.5 pcs·tree⁻¹) and B1 + KC100 (33.8 pcs·tree⁻¹). The lowest numbers were recorded for AS (7.06 pcs·tree⁻¹) and B2 + KC100 (18.3 pcs·tree⁻¹). Based on the results of the calculation and analysis of fruit setting, fruit set significantly varied between treatments, with KC200 achieving the highest value (85.2 pcs·100⁻¹ flower buds). The control group and KC100 showed moderate fruit set values (74.6 and 67.6 pcs·100⁻¹ flower buds, respectively). The AS (7.92 pcs·100⁻¹ flower buds) and B2 + KC100 (20.7 pcs·100⁻¹ flower buds) treatments exhibited the lowest fruit set values.

The yield per tree displayed notable variations across treatments. The KC200 treatment achieved the highest yield (10.1 kg·tree⁻¹), while the control group recorded a significantly lower yield (6.35 kg·tree⁻¹), similar to KC50 (6.67 kg·tree⁻¹). AS produced the lowest yield (1.17 kg·tree⁻¹). Combined treatments such as B1 + KC50 and B2 + KC50 recorded intermediate yields of 4.79 and 2.62 kg·tree⁻¹, respectively. This also had a significant impact on the cropping efficiency index (CEI), which also significantly varied across treatments. KC200 exhibited the highest CEI value (0.85 kg·cm⁻²), followed by KC100 (0.65 kg·cm⁻²). The control group and KC50 had similar CEI values (0.59 and 0.60 kg·cm⁻², respectively). The lowest CEI was recorded for AS (0.10 kg·cm⁻²), while intermediate values were noted for treatments such as B1 + KC50 (0.40 kg·cm⁻²) and B2 + KC50 (0.26 kg·cm⁻²).

According to the results presented in

Table 2. Physiological parameters and skin coloration of fruits measured directly after harvest depending on the treatment applied during the trial.

Treatment	Internal Ethylene Concentration [μL·L−1]	Starch Index [-]	Streif Index [-]	Apple Skin Coloration
				L [-]	a [-]	b [-]	Hue (h°)	Chroma (C)
Control	0.143 ab 1	4.60 a	0.143 c	42.1 a	23.8 a	7.93 a	1.25 a	25.1 a
AS	0.094 bcd	6.73 ab	0.094 abc	42.2 a	21.8 a	6.73 a	1.27 a	22.9 a
B2	0.096 abc	6.30 ab	0.096 abc	39.6 a	20.4 a	6.53 a	1.26 a	21.4 a
KC50	0.135 a	4.96 a	0.135 bc	41.6 a	23.4 a	7.86 a	1.25 a	24.7 a
KC100	0.140 a	4.70 a	0.140 c	42.7 a	22.5 a	7.70 a	1.24 a	23.8 a
KC200	0.130 a	5.60 ab	0.130 abc	41.2 a	23.0 a	7.16 a	1.27 a	24.1 a
B1 + KC50	0.101 ab	6.23 ab	0.101 abc	41.1 a	22.0 a	7.13 a	1.26 a	23.1 a
B1 + KC100	0.088 abc	6.56 ab	0.088 ab	40.5 a	20.7 a	6.33 a	1.28 a	21.7 a
B2 + KC50	0.082 cd	7.10 b	0.082 a	41.0 a	20.5 a	6.50 a	1.27 a	21.6 a
B2 + KC100	0.096 d	6.73 ab	0.096 abc	40.1 a	20.1 a	6.33 a	1.27 a	21.1 a

¹ Note: Means followed by the same letter in a column do not differ statistically according to the Newman–Keuls post hoc test at p ≤ 0.05.

, the physiological status of fruits depended on the treatments tested during the trial, while there was no variation in fruit coloration.

The internal ethylene concentration significantly varied across treatments. The highest ethylene concentration was recorded for the KC50 treatment (0.135 μL·L⁻¹), followed closely by the KC100 treatment (0.140 μL·L⁻¹) and the control group (0.143 μL·L⁻¹). Conversely, the lowest concentration was observed in the B2 + KC50 treatment (0.082 μL·L⁻¹), with similarly low values for B1 + KC100 (0.088 μL·L⁻¹). Treatments such as AS and B2 recorded intermediate values (0.094 and 0.096 μL·L⁻¹, respectively).

The analysis of the starch index values showed noticeable variations across treatments, indicating differences in fruit ripening. The highest starch index was observed in the B2 + KC50 treatment (7.10), followed by B2 + KC100 (6.73) and AS (6.73). Lower starch index values were recorded for KC50 (4.96), KC100 (4.70), and the control group (4.60).

The Streif index values, which are inversely related to fruit maturity, displayed a range of results. The control group and KC100 treatment had the highest Streif index values (0.143 and 0.140, respectively), indicating lower fruit maturity. The B2 + KC50 treatment recorded the lowest Streif index value (0.082), reflecting advanced fruit ripening. Intermediate values were observed for treatments such as AS (0.094) and B2 (0.096).

The parameters related to apple skin coloration, including L (lightness), a (red–green axis), b (yellow–blue axis), hue, and chroma, showed no significant differences across treatments.

The inner fruit quality and fruits were also affected by treatments used in the trial (

Table 3. Fruit quality parameters measured directly after harvest depending on the treatment applied during the trial.

Treatment	Flesh Firmness [N]	Soluble Solid Content [°Brix]	Titratable Acidity [% Malic Acid]	SSC–Acidity Ratio [-]	Fruit Mass [g]
Control	72.4 a 1	11.2 a	0.39 a	28.6 ab	171 a
AS	74.7 ab	12.1 ab	0.44 ab	27.2 ab	197 b
B2	73.9 ab	12.6 ab	0.47 bc	26.8 ab	192 b
KC50	73.1 a	11.3 a	0.39 a	28.8 ab	196 b
KC100	75.4 ab	11.9 ab	0.39 a	29.9 b	188 b
KC200	73.8 ab	11.4 a	0.41 a	27.6 ab	169 a
B1 + KC50	73.6 ab	12.0 ab	0.40 a	29.5 b	186 b
B1 + KC100	73.7 ab	13.3 b	0.38 a	34.6 c	189 b
B2 + KC50	75.9 ab	13.2 b	0.51 c	26.0 ab	186 b
B2 + KC100	79.5 b	12.7 ab	0.51 c	24.7 a	183 b

¹ Note: Means followed by the same letter in a column do not differ statistically according to the Newman–Keuls post hoc test at p ≤ 0.05.

). Flesh firmness varied across treatments but to a lesser extent. Relatively high fruit firmness was observed in the B2 + KC100 treatment (79.5 N), followed by B2 + KC50 (75.9 N). These values were significantly higher than those recorded for the control plots, KC50. Intermediate firmness values were noted for KC100 (75.4 N) and AS (74.7 N). The soluble solid content (SSC) was higher for B1 + KC100 and B2 + KC50 (13.3 °Brix and 13.2 °Brix, respectively) compared with the control, KC50, and KC200 treatments, while the AS, B2, and B1 + KC50 combinations showed intermediate levels of the tested parameter. Similar variations between treatments were observed for TA, with B2 + KC50 and B2 + KC100 exhibiting higher values (0.51% malic acid) than the control, KC50, and KC100 (0.39%). Intermediate acidity levels were observed for the AS and B2 treatments.

The SSC–TA ratio showed distinct differentiation. B1 + KC100 showed the highest ratio (34.6), indicating a sweeter taste profile, while B2 + KC100 had the lowest one (24.7) due to its high acidity. Most of the other treatments, such as KC50, KC100, and the control, recorded intermediate values of the described parameter (27.2–29.9).

The fruit mass was the highest in the AS and KC50 treatments (197 g and 196 g, respectively). The smallest fruits were observed in the control and KC200 treatments (171 g and 169 g, respectively), while intermediate mass values were recorded for the B2, KC100, and B1 + KC50 treatments.

4. Discussion

The experiment conducted to complete this study on ‘Gala Vill’ aimed to identify an alternative method to using chemical thinning agents or enable a reduction in metamitron dosages during the post-bloom thinning of apple fruitlets. The tested combinations in the experiment were designed to limit, or potentially limit, the photosynthetic process and act as a factor increasing competition among fruitlets and causing weaker fruitlets to drop, which is a well-known phenomenon contributing to crop load management techniques [32].

One of the thinning methods examined in this study was the use of a black agro-textile for artificial shading. Shading trees for 5 days in combination with AS resulted in excessive thinning, with an almost 80% reduction in fruitlet retention compared with the control, leading to a large decrease in yield in our experiment. Previous studies reported good thinning effects when shading was applied for up to 5 days, with shorter periods being insufficient and longer periods, such as 7 days, causing overthinning [33,34,35]. The use of kaolin clay, in theory, was expected to produce similar results to shading by limiting the production of assimilates during the photosynthetic process. However, this study showed that kaolin clay had no significant effect on fruitlet thinning. Higher doses of kaolin clay positively influenced fruit set and led to a significantly increased yield. This may be attributed to several reasons related to kaolin clay’s impact on environmental conditions and its indirect effect on tree physiology.

The kaolin layer that reflects sunlight can reduce the temperature on the plant’s surface. This can be particularly beneficial during heat stress, which is responsible for improper fruit development. The light reflection by kaolin clay also reduces transpiration, enabling plants to cope better with water deficits. As a result, trees can allocate more resources to the development of fruitlets. Reducing heat and water stress can enhance photosynthetic activity and tree metabolism, allowing more energy to be directed toward fruit development. While this is probably not the case in the present experiment, because of the mild temperatures which are commonly present in Polish conditions during flowering and in the post bloom period (average temperature for May on the experimental site was 12.4 °C), other factors may have played a role. According to Wünsche [31], while a thin film of kaolin clay on a leaf can reduce the individual leaf’s photosynthesis, it does not affect the global canopy due to better light distribution. Redistribution of light reflected from leaves covered with kaolin helps illuminate shaded areas within the canopy, as a result enhances the overall photosynthetic efficiency of the plant. This improved light distribution could support more balanced photosynthesis across the canopy [28], and might explain kaolin clay’s improvement of fruit setting and cropping observed in our experiment.

Spraying trees with metamitron significantly reduced fruit set, confirming its strong thinning efficacy, which is well documented in several authors’ reports [35,36,37,38,39]. The effect of metamitron sprayings followed by kaolin clay application did not enhance its thinning effect. In cases where lower doses of metamitron were used with kaolin clay, the results were comparable to those using the same lower doses of Brevis (metamitron) alone and were less effective than the standard dose of 2.2 kg·ha⁻¹ used in the presented experiment.

The lack of an additive effect of metamitron and kaolin clay may be attributed to the different mode of action of both agents but mostly to the failure of kaolin clay as a thinning agent at all. In light of the revealed potential of kaolin clay to improve fruit setting, this seems to be a reasonable outcome.

The thinning of apple fruitlets is crucial for increasing the fruit size because it reduces competition between the fruit for limited tree resources. According to Robinson and Lakso’s research [40], properly performed thinning increases the size of apples by up to 20–30% compared with unthinned trees, which makes this procedure an essential element of modern fruit-growing technology. Other authors have confirmed similar relationships. Our study demonstrated comparable findings, where an increase in fruit size was observed for all thinning treatments. Basak reported similar findings [41], where thinning resulted in a larger proportion of fruits exceeding 70 mm in diameter compared with control treatments.

Thinning treatments positively affected fruit firmness, which has been proven by the results provided by several authors [2,6,42,43]. While significantly higher firmness values were observed only with the B2 + KC100 combination, the tendency was observed among the other thinning treatments in our experiment. This can be attributed to the better conditions for growing young fruitlets and more dynamic cell division in the early stage of development, which improved their size.

The thinning of apple fruitlets has a positive effect on the sugar content and titratable acidity of the fruits. Several authors’ studies [4,44,45] showed that fruits from thinned trees had a significantly higher soluble solid content compared with fruits from unthinned trees. Similar results were confirmed by Robinson’s observations [40], indicating a clear relationship between the thinning intensity and the increase in fruit quality, including sweetness. Some combinations in the presented trial also exhibited such effects. Fruits from trees treated with B1 + KC100 and B2 + KC50 exhibited higher soluble solid contents. These findings may be linked to the lower number of fruits on the trees, allowing the remaining fruits to accumulate more assimilates. It is also widely known that well-developed fruits typically have higher sugar content, which contributes to better flavor and overall quality [2,3,24,44,45]. The highest levels of titratable acidity were observed in fruits from combinations involving higher metamitron doses, namely, B2, B2 + KC50, and B2 + KC100. This suggests that these combinations may enhance the fruits’ acidity levels, contributing to a better flavor balance, which is an essential quality trait for apples.

One of the limitations of this study is the lack of assessment of the long-term effects of kaolin clay applications in terms of overall tree health and orchard ecosystems. While kaolin clay has not shown effectiveness in thinning, other benefits appeared such as improved fruit setting. Taking into account fruit crops is essential to investigate potential cumulative effects that may include changes in soil composition due to runoff and accumulation and the effect on pests and disease development. Such observations should be performed over multiple growing seasons.

5. Conclusions

The findings indicate that the application of kaolin clay does not show a thinning effect or enhance the thinning efficacy of metamitron, suggesting that its use as an additive for chemical thinning agents may not bring additional benefits. However, high doses of kaolin clay sprays positively affected fruit set and yield in trees where chemical thinning was not applied. This demonstrates the promising potential of kaolin clay usage to improve orchard productivity.

Kaolin clay application in the early stages of fruitlet development (6–8 mm diameter) does not exhibit a clear impact on internal fruit quality or skin coloration. This highlights its potential as a neutral treatment option that can improve yields without unwanted side effects.

Since the present study was an initial one-year investigation, future research will focus on proving the observed effects, revealing the exact mode of action, and evaluating its long-term effects on fruit yield, quality, and tree health under different management conditions. Special interest should be focused on kaolin clay’s potential impact on non-target organisms, particularly beneficial insects. While the effect of kaolin clay is based more on its physical properties and influencing physiological processes in plants, it may also affect the activity or foraging efficiency of beneficial insects on coated surfaces. Taking this into account, it is crucial to investigate these effects more comprehensively to develop best practices that help mitigating any potential unwanted impact.