Thinning response of 'Summerred' apple to Brevis® in a northern climate

F.M. Maas, M. Meland
The response of three-year-old 'Summerred' trees to crop load regulation by the chemical thinning agent Brevis® was tested in comparison with different levels of hand thinning. Brevis® was applied once or twice at a dosage of 1.1 and 2.2 kg ha-1 when fruitlets were approximately 15 and 19 mm in diameter at the time of the first and second application, respectively. All Brevis® applications significantly reduced the number of fruits tree-1. Both the single application of 2.2 kg ha-1 and the two-fold application of 1.1 kg ha-1 thinned the tree to the target crop load level of 30 apples tree-1. The degree of thinning by the single application of 1.1 kg ha-1 Brevis® did not significantly increase fruit weight and resulted in a very strong inhibition of return bloom similar to that observed in the untreated controls and trees hand thinned to 60 fruits tree-1. Return bloom was positively related to the level of thinning obtained. At the target fruit load of about 30 apples tree-1, return bloom was around 40% of the bloom in the year of the thinning trial. A further thinning to circa 15 fruits tree-1 increased return bloom to approximately 60%. In conclusion, 'Summerred' trees planted as two-year-old feathered trees should not bear more than 20 to 25 fruits tree-1 in their 3rd leaf in a northern climate to avoid the onset of biennial bearing and to ensure regular annual yields. Only then sufficient numbers of flower buds will develop to have the chance to obtain a good crop in the next year.
Maas, F.M. and Meland, M. (2016). Thinning response of 'Summerred' apple to Brevis® in a northern climate. Acta Hortic. 1138, 53-60
DOI: 10.17660/ActaHortic.2016.1138.7
Malus domestica Borkh., crop load, fruit quality, fruit set, fruit weight, metamitron, return bloom