Development of a Fruitlet Growth Model to Predict Thinner Response on Apples

Previous reports have provided evidence that measuring fruit growth rate may be a viable method to predict if a fruit will abscise or persist through the June drop period. A series of experiments were carried out over several years to develop a procedure that could be used to predict the response to a chemical thinner application within 7 to 8 days after application and before thinners exhibit their final effect. The procedure developed involves tagging 105 spurs on seven individual trees distributed appropriately in the orchard. A minimum of two measurements must be made, one 3 to 4 days after application and again 7 to 8 days after application. This model requires that fruit measurement should not start before fruit grow to a diameter of 6 mm and individual fruit within a spur should be numbered and identified. The model is based on the assumption that if fruit growth rate of a particular fruit over the measurement period is less than 50% of the growth rate of the fastest growing fruit on the tree during the same growth period, it will abscise, whereas if fruit growth rate exceeds 50% of the growth rate of the fastest growing fruit, it will persist. All data can be entered into an Excel spreadsheet and the output in the summary page gives the predicted fruit set expressed as percentage of the total number of fruit present. The strategy for crop load adjustment with chemical thinners has evolved over the years to a point where most orchardists plan and are prepared to make two or more thinner applications. The dilemma associated with this approach is to determine if additional thinner applications are necessary. Up to this point a tool designed specifically to provide this information has not been developed.