Apple thinning strategy based on a model predicting flower-bud formation

Abstract

Regular cropping is important for improving fruit quality and profitability of the apple (Malus x domestica Borkh.) orchards. To identify a thinning target that makes the rate of flower-bud formation constant every year, we investigated the relationship between crop load (CL) and the rate of flower-bud formation (RF) and developed a model to predict the change of the RF according to the CL. Because a Gompertz curve fitted well to the relationship between the RF of following year and the current year in both biennial and annual bearing cultivars, we developed a prediction model based on an equation of the Gompertz curve. The prediction model can clarify varietal differences in the ease of getting flower buds. When trees are 10 years old, the RF of the following year in ‘Morinokagayaki’ would be 0.9 or more, even though the CL is adjusted to 8 fruits per cm2 trunk cross-sectional area (TCA). On the other hand, trees of ‘Fuji’ and columnar selections are predicted to display a biennial bearing cycle under the same CL. When the CL is adjusted to 2 fruits per flower cluster at 30 days after bloom every year, the yield of ‘Morinokagayaki’ is predicted to be maintained at around 200 fruits per tree after 7 years. In the case of ‘Fuji’ and columnar selection, it would be necessary to restrict the CL to around 0.5 fruits per cluster every year to obtain a stable yield. The model can estimate the parameters by which the probability of biennial bearing can be compared between cultivars and predict the fluctuation of the RF depending on the thinning management. Therefore, the model would be very useful for growers to strategize for crop load management.