Fruitlet shedding is affected by the position in the apple cluster

Abstract

In apple trees, natural fruitlet shedding is often insufficient to achieve optimal fruit size and yields. Natural fruitlet abscission can be stimulated by several technological measures and its knowledge is crucial for the implementation of adequate measures. The natural phenomenon of fruitlet shedding was monitored in apple trees in two consecutive years. Abscission started 4 weeks after full bloom and lasted for 6 weeks. During this time, more than 70% of all fruitlets shed in one or more peaks, depending on the year. The ‘Golden Delicious’ cultivar predominantly developed clusters with 5 flowers (39%), followed by clusters with 4 flowers (34%) and 6 flowers (24%); clusters with three or seven flowers were rare. Each flower/fruitlet position in the cluster was separately monitored and the link between specific position and abscission was determined. Fruitlets rarely abscised from the central position (less than 40% incidence), while the lowest positions (positions 5 and 6) shed more than 70% of fruitlets. Interestingly, high abscission potential was also observed at the 2nd position (nearest to the central position), from which fruitlets abscised more frequently than from lower positions. Results confirm at least partly the carbohydrate theory of apple fruitlet shedding and describe the shedding process as a type of stress. However, sugar and/or phenolic contents are not the only reasons for the initiation of the shedding process.