Effects of different blossom thinning agents on fruit set, yield, and fruit quality of 'Jubileum' European plum in a Nordic climate

Abstract

European plums (Prunus domestica L.) blossom abundantly most years and often set too many flowers. If these excessive numbers of fruitlets remain on the trees until harvest, the crop would consist of small, unmarketable fruits of low fruit quality. Thinning agents like ammoniumthiosulphate (ATS), sulphur and soya oil desiccate flowers, especially stigma, which is the most sensitive tissue part of the flower. This way, the main effect of blossom thinning treatments is the disruption of pollination and fertilization. Thinning trials were conducted at a commercial orchard near the shore of the Hardangerfjord near Nibio Ullensvang, western Norway (60.2 degrees N) on mature 'Jubileum' trees, all grafted on 'St. Julien A' rootstock. The trees were treated with 0.4, 0.8, 2 and 4% sulphur; a mixture of 0.4% sulphur + 2% soya oil and 1.5% ATS (liquid and powder formulations), at full bloom. All treatments were compared with hand-thinned and unthinned trees which were used as a control. Experimental trees were sprayed to the point of run-off with a hand sprayer during May 2008 and 2009 at full bloom. Flower thinners were efficient at relatively low temperatures which is a benefit in a cooler climate. In 2008 all thinning treatments reduced fruit set compared to unthinned controls for all cultivars. Sulphur and soya oil, both alone and in combination, were significantly less effective than ATS. In 2009, fruit set was higher and the effects of all thinning agents were lower. Fruit set decreased with increasing sulphur concentrations, but fruit thinning was not sufficient, even at the highest concentration. Both the liquid and powder formulations of ATS gave the same thinning effects. For all thinning treatments, both significant yield reductions and fruit weight increment were noticed during the experimental period. Fruit over color and soluble solids were generally higher and increased significantly with lower crop load, while fruit firmness (Durofel) and total acidity were less affected. In conclusion, different concentrations of sulphur had a moderate thinning effect and are not recommended for use as plum thinners under these conditions. Instead, 1.5% ATS application, (liquid and powder) applied at full bloom, resulted in adequate thinning of 'Jubileum' plums under cool mesic northern climatic conditions.