Ammonium thiosulphate as a blossom thinner of 'Delicious' apple, 'Winter Cole' pear and 'Hunter' apricot

The efficacy of ammonium thiosulphate as a blossom thinner of 'Delicious' apple, 'Winter Cole' pear and 'Hunter' apricot in southern Tasmania was assessed. Ammonium thiosulphate at concentrations of 0.3, 1.5 and 4% (v/v) was applied to both 'Delicious' apple and 'Winter Cole' pear trees at 20% bloom, 50% bloom or full bloom. At 4%, ammonium thiosulphate proved to be phytotoxic to both crops, resulting in the severe burning of foliage and flowers and also bud death. At 1.5%, ammonium thiosulphate reduced the crop load to an acceptable level without excessive damage. The 0.3% concentration was ineffective regardless of time of application in 'Delicious' and at 20% bloom in 'Winter Cole'. Russet incidence was increased on 'Delicious' by the highest concentration applied at 20% bloom. Shoot number increased in some treatments, however there was no effect on return bloom. When ammonium thiosulphate at 2.0% was applied to 'Hunter' apricot at 20, 40, 60, 80 or 100% bloom, fruit set was reduced regardless of time of application. Ammonium thiosulphate also improved return bloom of 'Hunter' apricot.