The commercial adoption of the relatively new rootstock `Gisela 5' (Prunus cerasus L. × P. canescens L.) has been limited in the United States sweet cherry (P. avium L.) industry despite its ability to induce precocity and productivity and reduce scion vigor compared to the standard Mazzard (P. avium). This is due in large part to inadequate crop load management that has led to high yields of small fruit. This paper reports on sweet cherry chemical blossom thinning trials conducted in 2002 and 2003. Two percent ammonium thiosulphate (ATS), 3% to 4% vegetable oil emulsion (VOE), and tank mixes of 2% fish oil + 2.5% lime sulphur (FOLS) were applied to entire 8- and 9-year-old `Bing'/`Gisela 5' sweet cherry canopies at about 10% full bloom (FB) and again at about 90% FB. In both years, ATS and FOLS reduced fruit set by 66% to 33% compared to the control (C). VOE reduced fruit set by 50% compared to C in 2002 but had no effect in 2003. In 2002, fruit yield was 30% to 60% lower from thinned trees. In 2003, fruit yield was unaffected by thinning treatment. In 2002, ATS and FOLS improved fruit soluble solids but had no effect in 2003. VOE did not affect fruit soluble solids in 2002 and reduced fruit soluble solids by 12%, compared to C, in 2003. In 2002, each thinning treatment nearly eliminated the yield of the small fruit (≤21.5-mm diameter) and increased yield of large fruit (≥26.5 mm) by more than 400%, compared to C. In 2003, ATS and FOLS did not affect yield of small fruit but increased the yield of large fruit by 60%. In 2003, VOE-treated trees yielded 4.3 kg of small fruit per tree compared to about 0.15 kg from C, suggesting a phytotoxic response to VOE beyond that which may effect thinning. Compared to C, ATS and FOLS consistently reduced fruit set and improved fruit quality. We conclude that commercially acceptable yields of excellent quality `Bing' sweet cherries can be grown on size-controlling and precocious rootstocks.
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