229 Alternate Thinning Chemicals for Apples S.A. Bound Tasmanian Institute of Agricultural Research University of Tasmania 13 St. Johns Avenue, New Town, TAS 7008 Australia Keywords: Malus × domestica, blossom thinning, desiccants, lime sulphur, fish emulsion, potassium bicarbonate, sodium chloride Abstract The removal of excess or unwanted flowers and/or fruit from trees is one of the most important orchard practices in pome fruit production, ensuring regular cropping and good fruit quality. Increased environmental and public health concerns have led to the banning or phasing out of many thinning chemicals and has encouraged the development and introduction of non-persistent chemicals with low toxicity levels. Many desiccating chemicals have been trialled but most have been unsuccessful because of lack of thinning at low concentrations and phytotoxic side effects at higher concentrations. However, new alternatives show potential. Two trials examined the potential of lime sulphur, fish emulsion, fish oil, potassium bicarbonate and sodium chloride as blossom thinners under Australian conditions. Lime sulphur reduced crop load in ‘Gala’ apples by 40%. Addition of an adjuvant increased the thinning effect up to 80%, depending on the adjuvant. Fruit size and shape were also improved where significant thinning was achieved. A combination of fish emulsion and fish oil reduced crop load by 70%, but fish emulsion combined with Tween 20 had no thinning effect. All fish emulsion treatments resulted in fruit russet. Ecocarb (potassium bicarbonate, Organic Crop Protectants, Australia) resulted in similar thinning levels to ammonium thiosulphate, reducing crop load of ‘Jonagold’ by 50% when applied as a 2% solution with a wetting agent. However a higher concentration of 5% Ecocarb resulted in over-thinning. Sodium chloride applied at either 2 or 5% resulted in over-thinning. Neither Ecocarb nor sodium chloride caused any fruit russeting. INTRODUCTION Most apple cultivars have a strong biennial bearing tendency, hence regulation of crop load (thinning) is an important component of orchard management in pome fruit production (Jones et al., 1998; Greene, 2002). Thinning can be undertaken either by hand, which is very time consuming and expensive, mechanically, or through the application of chemical thinning agents. Chemical thinning uses either hormonal-type growth regulators or caustic (desiccating) materials to reduce the amount of flowers and/or fruit and to overcome biennial bearing. Adjuvants are often added to chemical thinners to increase their thinning effectiveness (Miller, 1985; Jones et al., 1998). Increased environmental and public health concerns have led to the banning or phasing out of many thinning chemicals and encouraged the development and introduction of non-persistent chemicals with low toxicity levels. Many desiccating chemicals were trialled during the 1980s and 1990s, but most were unsuccessful due to a lack of thinning at low concentrations and/or phytotoxic side effects at higher concentrations. However, a few have been shown to be capable of thinning without excessive phytotoxicity. Sulfcarbamide (1-aminomethanamide dihydrogen tetraoxosulphate) is now used in the USA (Fallahi and Willemsen, 2002) although it is ineffective under Australian conditions (Bound and Jones, unpublished data). Ammonium thiosulphate (ATS) has been developed for both pome and stone fruits in Australia, USA and Europe, while chemicals such as endothal (dipotassium 7-oxoabicyclo (2,2,1) heptane-2,3,-dicarboxylate) and pelargonic acid have also shown promise. The aim of this Proc. XI th IS on Plant Bioregulators in Fruit Production Ed.: G. Costa Acta Hort. 884, ISHS 2010