SEARCH FOR SAFE AND EFFICIENT METHODS OF CHEMICAL THINNING BY IMPROVING ABSORPTION OF PREPARATIONS AND THEIR SUBSEQUENT USE

The results of four experiments conducted in 2001-2002 with the apple cvs. ‘Jonagold’ and ‘Golden Delicious’ are presented. In two experiments, the preparation Genapol was used in mixtures with ethephon (Ethrel), NAA (Pomonit Super 050 SL) and BA (Paturyl 100 SL). The effects of using these mixtures were compared with the results of thinning with ethephon, NAA and BA used alone or in pairs: ethephon and NAA, ethephon and BA, NAA and BA. Genapol applied jointly with ethephon, markedly increased fruitlet abscission in ´Jonagold´ trees. When used jointly with BA, it caused overthinning in ‘Jonagold’ trees and a reduction in the total yield despite producing a large amount of oversized apples. However, in ‘Golden Delicious’, Genapol intensified the thinning effect (i.e. fruitlet abscission) only when used with NAA. In both apple cultivars, stronger russeting on fruits was observed when Genapol was used in a mixture with BA. In two other experiments, low doses of NAA and BA were used in conjunction with the following adjuvants/accelerators: Atpolan 80 EC, Tween 20, Bond, Li 700, Adbios 85 SL, Activator 90, Torpedo II. For comparison, NAA and BA were used alone at the same doses as in the mixtures, or at twice this strength. The adjuvants used with NAA intensified fruitlet abscission in comparison with NAA used alone at the same dose as in the mixture, but only the thinning effects achieved with addition of Atpolan 80 EC and Adbios 85 SL were significant. When Atpolan 80 EC and Adbios 85 SL were applied with NAA, the results were comparable with those produced by NAA alone at double the concentration. Similar effects were noticed following the use of Torpedo II with BA. The use of adjuvants/accelerators caused additional increases in fruit size. The advantageous effect of Activator 90 on fruit size in ‘Jonagold’ was noticeable, in spite of its rather unclear effect on fruit set. The effects of the use of ethephon at the beginning of bloom, and NAA or BA after bloom, and late thinning with the mixture of NAA and BA were also observed. It was revealed that in young trees of ‘Jonagold’ and ‘Golden Delicious’ the subsequent use of thinning agents, or thinning by means of a NAA and BA mixture, most often caused overthinning, and that intensive thinning did not always induce increases in fruit size.