Studies on the use of 2-chloroethylphosphonic acid (ethephon) as a thinning agent for Jonathan apples

Abstract

In several experiments in Victoria, a single spray of the growth regulator 2-chloroethylphosphonic acid at 200 p.p.m., applied at full bloom, at 7 days and at 35 days after full bloom reduced the fruit density highly significantly on Jonathan apples and resulted in adequate fruit thinning. In one season, 2-chloroethylphosphonic acid at 200 p.p.m., applied at full bloom, reduced the fruit density more effectively than a single spray of N.A.A. at 4 p.p.m. or carbaryl at 800 p.p.m., applied 14 days after full bloom, The following year all three spray treatments applied at the same concentrations and the same timings were equally effective in reducing fruit set. 2-chloroethylphosphonic acid thins Jonathans over a long period and it seems that there are two very sensitive periods for fruit thinning, one at about full bloom, and a second about 35 days after. No differences have been observed on fruit finish, fruit shape, flesh firmness or incidence of storage disorders of Jonathan apples following thinning sprays of 2-chloroethylphosphonic acid compared with N.A.A. or carbaryl.