Excessive amount of fruits on a tree results in loss of their weight. An advantageous way of improving the quality of e.g. plums is thinning their fruitlets. The experiment involving this problem was conducted in the years 2003–2005 at the Fruit Experimental Station in Samotwór near Wrocław. The subject of investigation were 10-year-old plum trees of two cultivars – ‘Valor’ and ‘President’, growing on a ‘Myrobalan’ rootstock (Prunus divaricata) in the spacing of 4 × 3 m (833 trees./ha). In the course of three subsequent years fruitlets were hand thinned one and two month after full blooming, as well as chemically treated, using the following preparations: Pomonit R-10 (NAA) + Ethrel (ethephon) and exclusively with Ethrel (14–10 days after trees blooming respectively). As a control there served trees which did not undergo fruitlets thinning. Hand thinning increased mean weight of ‘President’ fruits, while in ‘Valor’ cv. the same dependence was observed only when the treatment took place later. Chemical thinning did not significantly affect on fruit yield quality. In both plum cultivars, fruits obtained from not thinned trees characterized similar content of soluble solids and mineral components to those produced after application of hand and chemical thinning.

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