Hampson, C. and Bedford, K. 2011. Efficacy of blossom thinning treatments to reduce fruit set and increase fruit size of Ambrosia and Aurora Golden Gala™ apples. Can. J. Plant Sci. 91: 983-990. The usefulness of several blossom thinning treatments to reduce fruit set and improve the final fruit size of two new Canadian apple (Malus×domestica Borkh.) cultivars was investigated. Ambrosia and 8S6923 (also called Aurora Golden Gala™) apple trees in a high density commercial superspindle planting were used. Ammonium thiosulphate (ATS) at 1.6% vol/vol or Crocker's fish oil and lime sulphur (FOLS) at 2 2% vol/vol were applied at 20 and 80% full bloom with an airblast sprayer. The chemical blossom thinning treatments were compared with a positive control [hand blossom thinning (HBT)], done at open cluster to king bloom stage, and a negative control (no blossom thinning). All trees received follow-up hand thinning as necessary to end up with a commercial crop load (single fruits about 15 cm apart). Hand blossom thinning resulted in about 95% single-fruit clusters and obviated the need for further thinning, but required considerable labour. Both ATS and FOLS reduced initial set (fruit number per tree, mean fruit number per cluster), but neither eliminated the need for follow-up fruitlet thinning. The two chemicals had similar efficacy in many respects, but FOLS was superior in certain specific measures of performance. In 2009, ATS negatively altered the fruit size distribution for 8S6923. For 8S6923, HBT improved fruit size without reducing yield. Ambrosia fruit size was less responsive to blossom thinning, and in the second year, yield was lower for trees receiving HBT. No fruit russet occurred on either cultivar. Return bloom was sufficient for a full crop for both cultivars after both years. 8S6923 was more productive than Ambrosia but was more difficult to thin.