Bound, S.A., Tasmanian Institute of Agricultural Research, University of Tasmania, 13 St Johns Avenue, New Town, TAS 7008, Australia
Klein, J.D., Institute of Plant Sciences, ARO-Volcani Center, Bet Dagan 50250, Israel

Chemical thinning is commonly practiced in perennial fruit crops and uses either hormonal-type growth regulators or caustic (desiccating) materials to reduce the amount of flowers and/or fruit and to overcome biennial bearing. Surfactants are commonly used with chemical thinning agents, and some surfactants can also have a thinning action in their own right. The trisiloxane-based agricultural adjuvant Silwet 408 has previously been investigated for its thinning effect in stone fruit. In the current work Silwet 408 was applied to two apple cultivars to determine its potential as a thinning agent. Treatments included 0.5% Silwet 408 and 100 ppm ethephon applied alone and in combination at 80% bloom stage to 'Jonagold' apple trees grown in tubs. The same treatments were applied to 'Fuji' apple trees with an additional two treatments consisting of Silwet or Silwet plus ethephon applied at 80% bloom and again 7 days later (5 days after full bloom). An untreated control was included in both trials. In both cultivars, ethephon alone had no thinning effect. In the 'Jonagold' trial, Silwet reduced crop load by 50% and the combination of ethephon plus Silwet reduced crop load by over 90%. In the 'Fuji' trial, crop load was reduced by over 85% in both the Silwet and the ethephon plus Silwet treatments. There was no additional thinning effect with two applications of Silwet, but two applications of ethephon plus Silwet produced more thinning than a single application of this combination. These results merit further investigation of Silwet 408 as a potential thinning agent for apple.