Chemical Thinning of King Dominant and Non-King Dominant Apples: A New Approach for Assessing Thinner Efficacy

The efficacy of chemical thinning is often evaluated by means of fruit set or crop load, and little attention is paid to cluster composition, i.e. fruit number per cluster and fruit position on cluster. However, these traits may also play important role in fruit quality. To assess thinner efficacy, net photosynthesis, dark respiration and ethylene production of individual fruitlets in known positions of the cluster of king dominant (‘Fuji’/B.9) and non-king dominant apples (‘Buckeye Gala’/B.9) were measured after 6-BA, NAA and carbaryl applications at 12 mm fruitlet size. All chemical thinners reduced fruit set, crop load and altered cluster composition in both ‘Gala’ and ‘Fuji’ cultivars. None of the chemicals appeared to be selective for king vs. side fruitlets for ‘Gala’ resulting in a constant share of single, double and multiple fruited clusters in the control and any of the treatments. For ‘Fuji’, 6-BA and NAA seemed to be more selective than carbaryl and caused primarily side fruitlets to abscise over king fruitlets. NAA applications significantly reduced fruitlet photosynthesis regardless of cultivars and fruit position on the cluster. Clear alteration of fruitlet photosynthesis after 6-BA treatments was not observed; while photosynthesis of ‘Gala’ fruitlets were not significantly influenced, photosynthesis of ‘Fuji’ side fruitlets decreased in some positions of the cluster. Ethylene evolution of treated fruitlets was significantly increased by NAA or 6-BA applications (4 and 7 days after application) but not by carbaryl sprays.