Effects of Mechanical Flower Thinning on the Fruit Set and Fruit Quality Attributes of ‘Hongro’ Apples

‘Hongro’ apple flowers were thinned with chemical (lime sulfur), low mechanical (200 rpm rotor speed, 4 km·h^-1 tractor speed), high mechanical (300 rpm, 6 km·h^-1), low mechanical (200 rpm, 4 km·h^-1) + chemical, and high mechanical (300 rpm, 6 km·h^-1) + chemical thinning treatments. Manual (hand) thinning served as a control. Most terminal and lateral flowers per cluster were removed by high mechanical thinning and the combination of this method with chemical thinning, compared to low mechanical thinning combined with chemical thinning. Additionally, the fruit set rate for both terminal and lateral flowers was mostly reduced when using the high mechanical + chemical thinning combination. However, a higher rotor speed during mechanical thinning increased the leaf damage rate compared to a lower rotor speed. Moreover, the high mechanical + chemical thinning combination improved flesh firmness, titratable acidity, and the soluble solid content in fruit at harvest compared to the control plants with low mechanical thinning combined with chemical thinning. The a* value (measured in fruit peel by CIE L* a* b* method) was highest when high mechanical thinning was used and lowest when chemical thinning was used. However, the fruit weight and size and the starch index were unaffected by all thinning treatments. Overall, the effects of mechanical thinning varied according to the rotor and tractor speed, and a high rotor speed (300 rpm) had a greater impact than a lower rotor speed (200 rpm). The highest thinning efficiency was achieved when using the high mechanical + chemical thinning combination, which improved the fruit quality of ‘Hongro’ apples.