Effect of flowering promotion with NAA or ethephon on ‘Golden Delicious’ apple trees with different flowering intensities

Abstract

Heavy cropping causes low-quality fruit and can lead to the biennial bearing pattern in the apple (Malus x domestica Borkh.). To make fruit production more sustainable, chemical thinning and flowering-promotion chemicals are used to reduce crop load and promote flower-bud formation in years when flowering is abundant. In this paper, the results of return bloom (RB) enhancement with five summer weekly applications of 5 mg L-1 naphthaleneacetic acid (NAA) or 100 mg L-1 ethephon to promote flowering are presented. Treated 'Golden Delicious' apple trees with a low, medium and high flowering intensity were previously thinned with ammonium thiosulphate (ATS), naphthalene-acetamide (NAD), NAA or 6-benzyladenine (BA), or left unthinned. Thinned and unthinned highly flowering trees had twice as much fruit at harvest and at least 6 times fewer flowers per tree in RB if trees were not treated with flowering-promotion chemicals. Five summer NAA or ethephon applications significantly improved RB in medium and highly flowering trees in all the experiments. However, in some experiments over-thinning and excessive RB flowering was observed on trees with a low flowering intensity, especially with five summer ethephon applications. Additionally, an NAA concentration-response study was performed on 'Fuji, 'Red Delicious, 'Golden Delicious' and 'Elstar' to assess NAA flowering-promotion and the fruitlet-retention effect on these cultivars. NAA applications were linearly correlated to RB enhancement on all the cultivars except 'Red Delicious'. However, NAA concentration was linearly correlated to the number of small fruits retained on 'Fuji, 'Elstar, and particularly 'Red Delicious' trees.