Efficacy of lime sulphur alone and in combination with olive oil on blossom thinning in organically grown 'Cripps Pink' and 'Gala' apples

Organic apple constitutes 30% of the world's planted organic temperate tree fruit. Blossom thinning improves apple fruit quality, but plant growth regulators cannot be used in organic apple production. Hand thinning is tedious, laborious and not economical. We investigated the effects of spraying lime sulphur (LS) alone and in combination with organic olive oil on blossom thinning, fruit retention, size and quality in organically grown apples. An aqueous emulsion containing different concentrations (1, 2, 3 or 4%) of LS alone and in combination with olive oil (3%) and 0.05% synertrol oil as a surfactant was sprayed onto whole trees until run off at 75-80% bloom stage of 'Gala' and 'Cripps Pink' apple. Unsprayed trees were kept as control. The experimental lay out was a randomised block design with two-factor factorial, single tree as a treatment unit with four replicates. Fruit set was recorded 26 days after spraying. Fruit retention was recorded 2-weeks prior to harvest. All the treatments had significantly reduced fruit set. Spray applications of LS (3-4%) alone was most effective in reducing fruit set (7-11%) coupled with higher leaf scorch (13-14%), respectively. The combination of LS and olive oil reduced the leaf scorching compared to spraying LS alone. Spray application of LS (4%) in combination with olive oil (3%) seems to be most effective treatment in reducing fruit set (14.9 and 15.6%) with minimised leaf scorching (10.1 and 12.1%), less fruit retention and improved fruit size and quality in 'Gala' and 'Cripps Pink' apple.