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

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.