Fruitlet growth model to predict chemical thinning response on apples in southern Brazil

Variability and unpredictability in chemical thinning has been a problem for apple growers for over 50 years. This situation motivated an intense work to identify factors related to fruit abscission. Among such factors, daily fruit measurement is the only practical method to assess early thinner response. Measuring fruit growth is useful to assess potential fruit drop after thinner application. The objective of this study was to evaluate the efficiency of fruitlet growth model for prediction of chemical thinning on apples in southern Brazil. The work was carried out in São Joaquim - Santa Catarina State, in the orchards of Epagri - São Joaquim Experimental Station (28°16’30”S; 49°56’09”W, altitude 1,400 m). The orchard was planted in 2006, the evaluated cultivars were ‘Fuji Suprema’ and ‘Maxi Gala’, grafted on M.9 with 1×4 m spacing. At 4 days after thinner application, 50 fruits tree^‑1 were measured. After 6 days new measurements were taken. The data obtained were inserted in the worksheet developed by Greene et al. (2013). Based on growth rate, the prediction of thinning efficiency and whether there was a need for new applications or not were determined. The following thinning treatments were compared: i) “conventional” chemical thinning – benzyladenine (10/14) + manual thinning (11/8); ii) chemical thinning based on the fruitlet growth model – 2 applications of benzyladenine (10/14 and 10/21) + metamitron (11/9). The number of fruit predicted by the model, the number of fruit manually thinned and the number of fruit harvested were evaluated. The fruitlet growth model to predict chemical thinning response was accurate when comparing the number of fruits predicted and number of fruits produced by a tree. The results of the first year are promising and indicate that the model can be useful on the decision making process of proper timing and need for additional chemical thinning application.