Estimation of the Relationship between the Natural Frequency and Fruit Drop during the Fruit Enlargement Stage of Apple

This study was conducted to investigate changes in the adhesivity of ‘Fuji’ apple (Malus domestica Borkh) fruits during the period when typhoons mainly occur and to obtain agricultural engineering information related to fruit drop through a simulation using a wind tunnel and shaking table. Fruit adhesivity was assessed by measuring the tensile force at the time of fruit drop when pulling the fruit from the tree. Fruit adhesivity during the growing season (July to September) tended to decrease as maturity approached, but this was not statistically significant. On the other hand, fruit adhesivity when measured at artificial drop angles of 0°, 30°, and 60° based on the fruit stalk decreased significantly as the angle increased. Using an acceleration sensor, the natural frequencies of the inner, middle, and outer parts of the scaffold branch (in the direction of the main stem) were found to be 52.0 cycles·min^-1, 332.2 cycles·min^-1, and 346.5 cycles·min^-1, respectively. The natural frequency according to the movement direction of the scaffold branch was higher for horizontal displacement (874.2 cycles·min^-1) than for vertical displacement (443 cycles·min^-1). The natural frequency of the fruit was measured and found to be 420.5 cycles·min^-1. Additionally, as a result of measuring the natural frequency after binding 60 cm parts (from the main stem) of the scaffold branches, the natural frequency was found to be lower than the control (no binding), and the vertical displacement was lower than the horizontal displacement. Using a wind tunnel, fruits did not fall at wind velocity of 0 to 9 m·s^-1, and the average displacement (up to 17 mm) tended to increase as the wind velocity was increased. In an experiment using a shaking table, displacement values in the range of 5 to 30 mm were applied based on the displacement coefficient calculated in the wind tunnel experiment, and the bearing branch of the apple tree was shaken using a ‘pendulum’ type of vibration method. The fruit drop began with a displacement coefficient of 10 mm, and the time required for fruit drop shortened as the displacement coefficient was increased. In the relationship between the displacement coefficient and the wind velocity, a displacement coefficient of 10 mm indicated a wind velocity of 6 to 9 m·s^-1, but this result may lead to fruit drop even at lower wind velocities because certain cultivation characteristics (e.g., tree form, number of leaves, fruit yield, abscission layer maturity of fruit) and natural wind characteristics (e.g., gusts, eddies, wind direction) were not taken into account. Wind can cause primary displacement in apple trees containing fruits and can then induce secondary displacement due to the weight of the fruit and the elasticity of the branches, and it is presumed that these primary and secondary displacements cause fruit drop.