Return bloom enhancement of low, medium and high flowering ‘Golden Delicious’ apple trees by the application of 1-naphthaleneacetic acid and ethephon

To overcome the biennial bearing pattern and make apple production more sustainable, chemical thinning and flowering promotion are commonly used. The study was performed to assess how these two measures influence fruit set and return bloom (RB) of ‘Golden Delicious’ trees from three different flowering intensity classes: low (L), medium (M) and high (H) flowering trees. Five flowering promotion summer applications of naphthaleneacetic acid (NAA; 5 mg L^‑1) or ethephon (100 mg L^‑1) were applied on L/M/H trees previously thinned with four different chemical thinners (ammonium thiosulphate – ATS 0.7%; or naphthaleneacetamide – NAD 100 mg L^‑1; or NAA 12 mg L^‑1; or 6-benzyladenine – BA 100 mg L^‑1). Selected L/M/H flowering intensity classes had a dominant effect on present year fruit set and on RB flowering intensity as well on responsiveness to applied chemicals. All used thinning chemicals (ATS, NAD, NAA and BA) thinned L, M and H flowering trees, which in turn means unnecessary and harmful yield reduction of L trees. Non-thinned control trees had abundant RB only on L trees, but not on M and especially not on H trees. Application of thinning agents on H trees enhanced RB, but mostly were not significant or not sufficient for the commercial fruit production in the next year. Five summer NAA or ethephon applications successfully improved RB of M and H flowering trees on all experiments. Results confirm the effectiveness of five summer applications of NAA or ethephon as flowering promotion measure on H trees to prevent biennial bearing behaviour of trees within the orchard row. Selective, on individual tree based crop load management system is necessary to overcome alternate bearing and enhance fruit quality of apple trees.