Fruit drop and bloom return according to time of metyamitron application for chemical thinning in 'Red Chief' apple

Abstract

Early removal of some of the fruits in apple trees improves the quality of remaining fruits at harvest and ensures adequate bloom return. An excess of fruits inhibits the flower bud induction in apple trees, a process that occurs between three and eight weeks after bloom. The objective of this work was to describe fruit drop during spring according to different chemical thinning and its relationship with bloom return in ‘Red Chief’ apple trees. Metamitron (MM) applications at a concentration of 200 ppm were done when mean fruit diameter were 8, 12 and 16 mm (between 15 and 28 days after full bloom) in three seasons: 2018-19, 2019-20 and 2020-21. Thinning effectiveness was evaluated by fruit set two months after bloom and fruit number tree‑1 at harvest. Fruit drop was recorded weekly, by counting the number of persisting fruits in six clusters tree‑1 during spring. Bloom return was determined by the number of flower clusters tree‑1 in the following season. Bloom return presented a strong negative correlation with fruit number per tree at harvest. Generally, MM treatments presented a higher percentage of dropped fruits around 20 days after application. The percentage of dropped fruits in the trees sprayed with MM at 16 mm was only higher with respect to control after ≈40 days after full bloom. Even so this late thinning treatment improved bloom return, indicating that floral induction may also occur later in the season.