THINNING ACTIVITY AND FRUIT QUALITY OF GALA AND FUJI APPLE VARIETIES AS AFFECTED BY CYTOKININS

Gala and Fuji apples are difficult to thin properly because of their high fertility and extended bloom period under the Italian climatic conditions. Since hand thinning is too expensive, two cytokinin-like compounds which are considered among the most effective chemical thinners, BA (benzylaminopurine) and CPPU (forchlorfenuron), were compared to NAA (-naphtaleneactic acid), hand thinning and untreated control. BA and CPPU were applied on Galaxy while BA and NAA on Fuji. In Galaxy, the two cytokinins induced abscission values similar to those obtained by control, and BA fruits were larger than the control ones. In Fuji, both NAA and BA application reduced fruit load. Average fruit weight was similar among all treatments; however, the average fruit weight of the NAA and BA treated trees tended to increase with increasing concentrations. BA fruits were heavier than NAA fruits, although the thinning effect of BA was slightly lower. As a consequence, yield per tree was highest in the untreated control, followed by BA trees. It is important to assess the effectiveness of a thinning agent at an early stage, since the strategies to enhance fruit size must be applied early in the season. A modeling approach has been adapted to assess the feasibility of using such a tool to evaluate the effectiveness of a thinning agent in the early stages, and to predict fruit size at harvest