Effect of Blossom Thinning On Ambrosia and Aurora Golden Gala Apples

Ambrosia and Aurora Golden Gala are two new apple cultivars with outstanding consumer acceptance, but in some years, fruit size has been smaller than desired by wholesale fruit buyers. Our objective was to assess the effectiveness of blossom thinning treatments for reducing initial fruit set and increasing final fruit size. The treatments were: fish oil and lime sulfur (2 + 2% v/v) at 20% and 80% bloom; aminothiosulfate (ATS, 1.6% v/v) at 20% and 80% bloom; hand blossom thinning, leaving only the king flower in each cluster, with clusters 10-15 cm apart; no blossom thinning. The treatments were applied in two consecutive years to mature trees trained as superspindles in a commercial orchard in Summerland, BC, Canada. Follow-up thinning of fruitlets was done by hand at the time of June drop. The following data were recorded: time required to hand blossom thin; initial fruit set and cluster size distribution; time required for follow-up hand thinning of fruitlets; final crop load; yield per tree; average fruit weight; fruit box size distribution; return bloom. Return bloom in 2009 was satisfactory for all trees. Both chemical thinners were effective at reducing initial fruit set, but neither achieved the same reduction in set as blossom thinning by hand, and neither chemical eliminated the need for follow-up fruitlet thinning. Fruit box size distribution was best for the hand blossom thinning treatment in both cultivars. For Ambrosia, the ATS treatment produced a slightly more favorable fruit box size distribution than the fish oil + lime sulfur, but for Aurora Golden Gala, the opposite was true.