Apple Cropload Adjustment with the Equillifruit Disk

Cropload adjustment via hand thinning is an effective means of increasing apple (Malus X domestica) fruit size. Unfortunately, reductions in yield can occur with excessive thinning. Growers utilize a variety of untested hand thinning heuristics to adjust final cropload after inadequate chemical thinning.  The Equilifruit disk, which was developed by the National Institute for Agricultural Research (INRA) in France, is a handheld tool that measures the fruiting branch cross-sectional area (FBCSA) and gives an estimate of the number of fruit that can best be supported. The accuracy and efficacy of the Equilifruit Disk was evaluated in the summer of 2009 at the Penn State Fruit Research and Extension Center in Biglerville, Pennsylvania. A randomized block experiment with five replications was performed on 6th leaf ‘Golden Delicious’ trees on Budagovski 9 rootstock. Despite two post-bloom applications of chemical thinners, hand thinning was necessary to optimize fruit size. Treatments were applied as follows: 1) no additional thinning (control), 2) hand thinned using the Equilifruit disk, and 3) hand thinned using a spacing heuristic of 7-8 inches between solitary fruits. All treatments and measurements were conducted after the June drop.  The treatments were compared by crop density (number of fruits per square centimeter of trunk cross-sectional area (TCSA)), yield efficiency (yield/TCSA), total yield, and fruit size distribution. The crop densities for each treatment were as follows: Control: 10.50 fruit/cm²; Equilifruit: 8.40 fruit/cm²; and the heuristic: 6.29 fruit/cm². Yield efficiency of the control and Equilifuit were similar; 1.82 and 1.78 kg/cm² TCSA respectively, while the spacing heuristic was 1.49 kg/cm² TCSA. Both the Equilifruit and the heuristic were effective in creating an upward shift in the fruit size distribution, however; the heuristic reduced yield by 5.40 kg per tree. The heuristic had the highest yield per tree of fruit greater than 76 mm (8.19 kg), followed by the Equilifruit (6.44 kg), and the control (2.76 kg).The results of this experiment indicate that the use of the Equilifruit disk has potential to increase fruit size in ‘Golden Delicious’ while maintaining yield. The Equilifruit disk can be used to train employees in hand thinning practices, and may be incorporated into future selective thinning technologies. Future studies are planned on small fruited cultivars such as ‘Gala’, and biennial cultivars, such as ‘HoneyCrisp’ and ‘Fuji’.