Crop Regulation and Cytokinin Sprays to Improve 'Sweetheart' Sweet Cherry Fruit Size

The effect of crop regulation strategies of pruning severity or cytokinin sprays (CPPU or benzyl adenine, BA) on the fruit size of 'Sweetheart' cherry (Prunus avium) trees on Gisela 5 and Gisela 6 rootstocks, trained either as Vertical Axis or Central Leader, was evaluated at Geneva, NY, USA, in 2006 and 2007. Crop regulation treatments were: light pruning (control), hard pruning (stubbing back branches with less than 25 cm growth; HP), spur extinction (SP) or flower bud extinction (FE). Both extinction treatments were done on lightly pruned trees. In year one, CPPU at 10 mg.L-1 was sprayed on selected branches at balloon stage or twice at balloon stage plus petal fall or not sprayed at all. In year 2, BA was applied to entire trees at petal fall, or 10, 20 or 30 days after petal fall. Final fruit set in year 1 was around 70%, and was not affected by crop regulation or chemical treatments. In all experiments, crop load was affected by training system and crop regulation treatments. In year 1, FE affected crop load more than HP or SE; in year 2, there was an interaction between crop regulation treatment and rootstock on crop load. Fruit size was not affected by any cytokinin treatment. It was affected mainly by training system and crop regulation treatment. Fruit size was highly dependent on leaf area per fruit and maximum fruit size was obtained with 200 cm(2) leaf area per fruit. If fruit size was adjusted for leaf area per fruit, fruit size was not affected by any crop regulation or chemical treatment, which means that all observed differences due to block or treatment were explained by differential leaf area per fruit resulting from the different treatments. Yield efficiency (g fruit cm(-2) BCSA or TCSA) was reduced by crop regulation treatments; treatments with minimal pruning resulted in the greatest efficiency.