BACKGROUND: The productivity and quality of blueberry (Vaccinium ashei Reade) greatly depends on the number of fruits in a plant. Especially, fruit set more than appropriate number negatively affects productivity and marketability due to the increased number of small fruits and delayed harvest time. This study was conducted to investigate proper timing and concentration for applying chemical blossom thinners such as ammonium thiosulfate (ATS) and UREA.
METHODS AND RESULTS: ATS 1.25 and 1.50%, and UREA 6 and 8% were applied in four developmental stages, bud swell, pink bud, full bloom and petal fall. Fruit thinning rate was calculated based on the number of fruits harvested divided by that of flowers before applying blossom thinners. Leaf to flower or fruit ratio was calculated based on the number of fully developed leaves in 25 days after full blossom divided by that of flowers or fruits, respectively. Chemical injury of leaves was investigated by calculating the number of leaves with chemical injury divided by the total number of leaves. Fruit thinning rates were 48 and 66% for UREA 6 and 8% treatments, respectively, and 49 and 62% for ATS 1.25 and 1.50% treatments, respectively, in the full bloom stage. In the petal fall stage, fruit thinning rates were 18 and 24% for UREA 6 and 8%, respectively, and 49 and 35% for ATS 1.25 and 1.50%, respectively. Leaf to fruit ratio increased by 109 and 188% compared to leaf to flower ratio in ATS 1.25 and 1.50% treatments, respectively, and 93 and 196% in UREA 6 and 8% treatments, respectively, in the full bloom stage. In the petal fall stage, leaf to fruit ratio increased by 60 to 100% in ATS treatments but did not significantly differ from control in UREA treatments. Fruit harvest was delayed in all treatments of all developmental stages except for ATS 1.5% and UREA 6% treatments at the petal fall stage, whose fruit harvest was two or three days faster than control. CONCLUSION The application of ATS and UREA for blossom thinning should be in petal fall and full bloom stages for early and late harvest, respectively. Considering chemical injury, integrated harvesting and fruit size, however, it is appropriate to apply ATS 1.50% in the petal fall stage to increase fruit productivity and quality in blueberry.