Response of 'Rubinstep' apple to flower and fruitlet thinning in a northern climate

F.M. Maas, M. Fotirić Akšić, M. Meland
The aim of this study was to evaluate the thinning efficacy of metamitron (Brevis®) as fruitlet thinner compared to ammonium thiosulphate (ATS) and ethephon (Cerone) as flower thinning agents to 'Rubinstep' apple. The thinning efficacy of the flower thinning agents was compared to different timings (8-10 and 12-14 mm fruitlet diameters) and dosages (165 and 330 ppm) of the fruitlet thinner metamitron at the experimental farm at Nibio Ullensvang, western Norway (60°19'8.03”N; 6°39'14.31”E) in 2018. Untreated trees and trees manually thinned after June drop were used as reference treatments. Fruit set, yield data and fruit quality parameters for each treatment were recorded. None of the chemical thinning treatments resulted in a significant reduction in the final number of fruits and yield tree‑1. Crop load of the untreated trees was almost twice the target crop load of the hand-thinned trees. Ammonium thiosulphate (ATS) was the only chemical thinning treatment that significantly increased fruit weight above the untreated trees, but to a lesser extent than achieved by hand thinning. Qualitative traits of 'Rubinstep' apples (ground and over colour, firmness, starch index and soluble solid contents were not correlated with the fruit set. Return bloom of the untreated trees was 50% of the bloom of the previous year. The ammonium thiosulphate-treated trees showed a significant higher percentage return bloom of 122%. Return bloom in all other treatments did not differ significantly from the untreated controls. However, the average values showed a clear trend of a decrease in return bloom in metamitron-treated trees in comparison with the untreated and hand-thinned trees. The exceptionally high levels of solar radiation in May 2018 and the excellent pollination conditions resulting in very high seed numbers fruit‑1 are likely the reasons for the lack of thinning efficacy of Brevis®.
Maas, F.M., Fotirić Akšić, M. and Meland, M. (2020). Response of 'Rubinstep' apple to flower and fruitlet thinning in a northern climate. Acta Hortic. 1295, 41-48
DOI: 10.17660/ActaHortic.2020.1295.5
Malus domestica L. Borkh., crop load, fruit quality, fruit set, fruit weight, metamitron, ethephon, ammonium thiosulphate, ATS