Efficacy of ACC (1-aminocyclopropane-1-carboxylic acid) as a Chemical Thinner Alone or Combined with Mechanical Thinning for Japanese Plums (Prunus salacina)

Thinning is a labor-intensive and expensive, but important practice in japanese plum production requiring new thinning strategies. The purpose of this study was to evaluate a new chemical thinner 1-aminocyclopropane-1-carboxylic acid (ACC) on 'Laetitia', 'Fortune', and 'African Rose (TM)'. ACC was also combined with 6-benzyl adenine (6-BA) and in one season on 'African Rose (TM)' with mechanical thinning using the Darwin 300 (TM) or hand thinning during bloom. All the foliar applications were made when the average fruitlet diameter was 7-10 mm using a spray volume of 1000 L.ha(-1) under slow drying conditions. Significant thinning effects were found in all the trials conducted over the two seasons. ACC consistently reduced the hand-thinning requirement at commercial hand thinning in both seasons in 'African Rose (TM)'. In the second season there was a linear decrease in yield and an increase in fruit size as the ACC rate increased from the low to medium rate before flattening off. The combination treatment of ACC and the Darwin 300((TM)) used in the 'African Rose((TM))' trial thinned more aggressively, improved fruit size and shifted harvest earlier. The yield however was not lower than that of the control treatment. 6-BA was included in all trials to prevent ACC-induced leaf drop, and generally did not thin fruitlets, except in the case of 'Laetitia' where the combination with ACC resulted in stronger thinning. Cultivars differed in their sensitivity to ACC and the rate for each cultivar should be determined separately. The recommended ACC rate for 'African Rose (TM)' would be 600 mu L.L-1 and for 'Laetitia' 400 mu L.L-1. For 'Fortune' a rate recommendation is not possible at this stage, thus further trials should be conducted. No broken stones were observed in fruit in any trial. Also, no leaf drop/phytotoxicity was recorded in any trial when ACC was applied during cool, slow-drying conditions.