Prediction of Initial Set and the Thinning Response to Postbloom Chemical Thinners on Apples

Experiments were initiated to identify how and when a reasonably accurate assessment of initial fruit retention could be made and to identify a measurement method to allow early assessment of thinner efficacy. An estimate of initial fruit retention could be made when fruit enters the exponential growth stage, which is generally 6 to 9 days after petal fall and the receptacle has expanded to approximately 6 mm. There was a very good correlation between the slowing of fruit growth 4 to 7 days after thinner application and final fruit set, which led to the conclusion that following fruit growth during this critical time was a good and viable method to predict and quantify thinner response.